Glancing Angle Deposition of Silicon Nanostructures by Ion ... · Bibliographische Beschreibung...

143
Glancing Angle Deposition of Silicon Nanostructures by Ion Beam Sputtering Der Fakultät für Physik und Geowissenschaften der Universität Leipzig eingereichte DISSERTATION zur Erlangung des akademischen Grades Doctor rerum naturalium Dr. rer. nat. vorgelegt von Dipl. Phys. Christian Patzig geboren am 10. Oktober 1980 in Weida Leipzig, den 08.05.2009

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Page 1: Glancing Angle Deposition of Silicon Nanostructures by Ion ... · Bibliographische Beschreibung Patzig, Christian Glancing Angle Deposition of Silicon Nanostructures by Ion Beam Sputtering

Glancing Angle Deposition of Silicon

Nanostructures by Ion Beam Sputtering

Der Fakultät für Physik und Geowissenschaften

der Universität Leipzig

eingereichte

DISSERTATION

zur Erlangung des akademischen Grades

Doctor rerum naturalium

Dr. rer. nat.

vorgelegt

von Dipl. Phys. Christian Patzig

geboren am 10. Oktober 1980 in Weida

Leipzig, den 08.05.2009

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Bibliographische Beschreibung

Patzig, ChristianGlancing Angle Deposition of Silicon Nanostructures by Ion Beam SputteringUniversität Leipzig, Dissertation141 S., 156 Lit., 66 Abb., 4 Tab.

Referat:

Die vorliegende Arbeit beschäftigt sich mit der systematischen Untersuchung der Herstel-lung verschiedenartig geformter, nano- und mikroskaliger Strukturen aus Silizium mit derMethode der Glanzwinkeldeposition mittels Ionenstrahlzerstäubung. Basierend auf demPrinzip atomarer Selbstabschattung erlaubt dieses spezielle Verfahren der physikalischenGasphasenabscheidung das Wachstum poröser dünner Schichten, bestehend aus Struk-tureinheiten welche mittels geeigneter Rotation des Substrates unterschiedliche (zumBeispiel schrauben- oder säulenartige) Morphologien annehmen können.

Im Fokus der Arbeit steht zum Einen die Untersuchung des Einflusses diverser Ab-scheideparameter auf die Entwicklung der Strukturmorphologie auf nicht vorstrukturi-erten, planaren Substraten. Neben der Analyse des Einflusses von Prozessparameternwie Depositionszeit und Rotationsgeschwindigkeit des Substrates liegt hier das Hauptau-genmerk auf der Beschreibung des Einflusses der Substrattemperatur auf das Wachstumschrauben- und spiralartig geformter Strukturen. Die experimentellen Befunde werden miteinem qualitativen Modell erklärt und mit dreidimensionalen Monte-Carlo-Simulationendes Glanzwinkeldepositionsprozesses bei unterschiedlicher Substrattemperatur verglichenund diskutiert.

Ein zweiter Schwerpunkt der Arbeit ist die Betrachtung des Wachstums glanzwinkelde-ponierter Silizium-Strukturen auf mit definierten, periodisch angeordneten Keimpunk-ten vorstrukturierten Substraten. Ein Hauptaugenmerk liegt hier auf der Analyse desWachstums der Strukturen auf Substrattemplaten mit honigwabenartiger Anordnung.Neben dem Einfluss diverser Prozessparameter, wie zum Beispiel Depositionszeit undDepositionswinkel, wird hier auch der Einfluss der Templatgeometrie selbst auf dasWachstum und die Entwicklung der Strukturmorphologie untersucht, mit Monte-Carlo-Simulationen des Wachstums verglichen und diskutiert. Schließlich wird eine neue Meth-ode der Substratvorstrukturierung vorgestellt, welche den Selbstabschattungsprozess vonder Keimpunktgeometrie entkoppelt und somit die Deposition freistehender, nahezu be-liebig geformter Strukturen an definierbaren Plätzen erlaubt.

Der die Arbeit beschließende Schwerpunkt beschäftigt sich mit ersten (eigenen) Ap-plikationen der erzeugten Strukturen. Beschrieben wird hier die Möglichkeit, mit demVersuchsaufbau Si-SiOx-Heterostrukturen zu erzeugen. Des Weiteren werden Ergebnisseder mechanischen Charakterisierung spiralartiger Silizium-Dünnschichten vorgestellt.Schlussendlich wird aufgezeigt, dass sich die glanzwinkeldeponierten Strukturen aus Siliz-ium als Vorstruktur für die Ummantelung mit ferrimagnetischen, mittels atomic layerdeposition abgeschiedenen Fe3O4-Nanoröhrchen eignen, deren magnetische Eigenschaftenauch von der Form der Silizium-Strukturen abhängt.

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Contents

1 Introduction 3

2 Fundamentals of Glancing Angle Deposition 52.1 GLAD on flat substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Oblique angle deposition without substrate rotation . . . . . . . . . 5Influence of deposition angle on column tilt angle . . . . . . . . . . 6

2.1.2 Oblique angle deposition with substrate rotation . . . . . . . . . . . 72.2 GLAD on patterned substrates . . . . . . . . . . . . . . . . . . . . . . . . 102.3 GLAD at elevated temperatures . . . . . . . . . . . . . . . . . . . . . . . . 132.4 Deposition methods used for GLAD . . . . . . . . . . . . . . . . . . . . . . 142.5 MC - simulation of GLAD . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.6 Applications of GLAD-grown sculptured thin films . . . . . . . . . . . . . 16

3 Experimental Setup and Sample Analysis 173.1 Glancing angle ion beam sputter deposition of Si nanostructures . . . . . . 17

3.1.1 Deposition process . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.1.2 Deposition system setup and deposition parameters . . . . . . . . . 19

Broad beam ion source for sputter deposition . . . . . . . . . . . . 19Deposition chamber setup and deposition parameters . . . . . . . . 20

3.2 Substrate patterning methods prior to GLAD . . . . . . . . . . . . . . . . 233.3 Sample analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3.1 Scanning electron microscopy . . . . . . . . . . . . . . . . . . . . . 26Image analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.3.2 Time-of-flight secondary ion mass spectrometry . . . . . . . . . . . 283.3.3 Elastic recoil detection analysis . . . . . . . . . . . . . . . . . . . . 283.3.4 Indentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 GLAD of Si on Flat Substrates 314.1 Continuous substrate rotation at RT . . . . . . . . . . . . . . . . . . . . . 314.2 Influence of the substrate temperature . . . . . . . . . . . . . . . . . . . . 36

4.2.1 OAD without substrate rotation . . . . . . . . . . . . . . . . . . . . 364.2.2 GLAD with slow and intermediate continuous substrate rotation . . 384.2.3 GLAD with fast continuous substrate rotation . . . . . . . . . . . . 414.2.4 Growth model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.2.5 Comparison of experiment and MC simulation . . . . . . . . . . . . 46

4.3 Summary of results: GLAD on unpatterned substrates . . . . . . . . . . . 50

5 GLAD of Si on Patterned Substrates 535.1 Tetragonal template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.1.1 Influence of template period . . . . . . . . . . . . . . . . . . . . . . 55Four-fold spirals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Chevrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Columnar structures . . . . . . . . . . . . . . . . . . . . . . . . . . 60

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Contents

5.1.2 Influence of seed diameter and control of structure diameter . . . . 615.2 Honeycomb templates: experiments and MC simulations . . . . . . . . . . 64

5.2.1 Morphology evolution in case of fast substrate rotation . . . . . . . 645.2.2 Influence of deposition time . . . . . . . . . . . . . . . . . . . . . . 685.2.3 Influence of deposition angle . . . . . . . . . . . . . . . . . . . . . . 725.2.4 Series of different deposition angles . . . . . . . . . . . . . . . . . . 765.2.5 Influence of rotational speed . . . . . . . . . . . . . . . . . . . . . . 78

5.3 Nanospheres as hcp arranged seeds for GLAD . . . . . . . . . . . . . . . . 815.3.1 Influence of sphere diameter . . . . . . . . . . . . . . . . . . . . . . 815.3.2 Influence of substrate temperature . . . . . . . . . . . . . . . . . . 86

5.4 Periodically arranged structures with arbitrary periods . . . . . . . . . . . 895.4.1 Principle of two-step lithography patterning process . . . . . . . . . 905.4.2 Results and drawbacks . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.5 Summary of results: GLAD on patterned substrates . . . . . . . . . . . . . 96

6 Applications of Glancing Angle Deposited Si Nanostructures 1016.1 Si-SiOx-multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016.2 Spring constants of Si nanospiral arrays . . . . . . . . . . . . . . . . . . . . 1056.3 GLAD Si structures as templates for atomic layer deposition of magnetic

thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1086.3.1 Fe3O4 tubes with glancing angle deposited Si STF as substrate . . . 1086.3.2 Magnetization measurements . . . . . . . . . . . . . . . . . . . . . 109

6.4 Summary of results: applications of glancing angle deposited Si nanostruc-tures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

7 Summary and Conclusions 115

Bibliography 116

List of Acronyms 129

List of Variables 131

Acknowledgements 133

Curriculum Vitae 135

List of Publications 137

Selbstständigkeitserklärung 139

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1 Introduction

Since decades, the ongoing miniaturization in various fields of modern technology requiresprecise control on both deposition and structuring of thin films of manifold materials.Whereas the deposition of compact thin films with thicknesses in the nm- or µm- range canbe mastered with standard physical vapor deposition (PVD) or chemical vapor deposition(CVD) techniques, the formation of (regular) nanostructures can still be a challengingtask. A promising technique to achieve this goal is the glancing angle deposition (GLAD)technique, put forward by Robbie et al. in the mid-1990s [1–3].

GLAD offers the possibility to form (“sculpt”) the growing thin film during the PVDprocess. Therefore, GLAD-grown nanostructures are often referred to as “sculptured thinfilms” (STFs). Arrays of separated nanostructures with a variety of different shapes (suchas, for example, vertical columns, zig-zag shaped chevrons or spirals) can easily be grown,depending on the deposition conditions. As this technique only relies on self-shadowingprocesses and geometrical conditions on the substrate surface, it is in principle capable ofrealizing nanostructures of all standard materials used in common PVD processes. GLAD-grown STFs have already proven to be promising candidates for a variety of applications,the most recent being a graded-index antireflection layer with very low reflectance in abroad spectral range [4].

GLAD in its most fundamental form, performed on bare, unpatterned substrates, willmost likely result in randomly distributed STFs that comprise of single structures withnon-uniform diameters [5]. For some applications (for instance fully three-dimensionalphotonic crystals [6]), however, a lateral periodic ordering of the structures and precisecontrol of structure diameters will be needed. This can not easily be mastered on baresubstrates, but rather requires a patterning of the substrate prior to GLAD. Malac et al.[7] where the first to cover the substrate with hillock-like features that are supposed toact as seeds for the structures-to-grow in the following GLAD process. Since then, lotsof researchers used different patterning techniques in order to achieve regular arrays ofSTFs.

This work focuses on the growth of Si nanostructures by GLAD with ion beam sput-ter deposition (IBSD) on both unpatterned and patterned substrates, with the aim tocharacterize the morphological evolution of the structures (e.g. form, structure diameter,inter-structure-distance, and periodic arrangement), as function of various deposition pa-rameters (e.g. deposition angle, substrate rotation algorithm, deposition time, substratetemperature, and arrangement of template pattern). After a short introduction on GLADin general in chapter 2, and a short presentation of the experimental setup in chapter 3,the results of the influence of various deposition conditions on the evolution of Si nanos-tructures grown with ion beam sputter glancing angle deposition on bare, pattern-freesubstrates will be presented in chapter 4. The focus will lie on the influence of the sub-strate temperature TS on the evolution of helical Si nanostructures. The influence of anincreased substrate temperature TS (and, therefore, enhanced adatom diffusion length)is disregarded in most research work concerning GLAD so far, as it is mainly seen ascounteracting the desired self-shadowing process. Here, it will be shown that enhancing

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1 Introduction

TS has a large impact on the morphological evolution of the GLAD-grown structures.The experimental results will be compared with Monte Carlo simulations of the sputterGLAD process.

The influence of different template and deposition parameters on the growth and evolv-ing morphology of the Si structures deposited on patterned substrates will be presented inthe following chapter 5. Whereas most researchers so far have focused on highly symmetrictemplate patterns (mostly orthogonal or hexagonally closed packed arrays of seeds) whenperforming GLAD on patterned substrates, in this work a focus will lie on the growthof Si nanostructures on seeds arranged in a less symmetric, honeycomb-like pattern, andit will be shown that the arrangement of the pattern influences the morphology of thestructures deposited thereon. Again, the experimental results will be compared to MonteCarlo simulations of the sputter GLAD growth on patterned substrates. The chapter willbe concluded with the introduction of a new patterning scheme that enables the deposi-tion of free-standing structures in arbitrary distances and periods among the substrate,making use of a two-step lithography process.

Finally, some further aspects and applications of the glancing angle deposited Si nanos-tructures arisen from the present work will be introduced in chapter 6.

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2 Fundamentals of Glancing AngleDeposition

2.1 GLAD on flat substrates

2.1.1 Oblique angle deposition without substrate rotation

200 nm

particle flux

film

substrate

lhS

q

200 nm

(a) (b)

(d)(c) q

Figure 2.1: Comparison of thin film deposition (depo-sition angle θ = 0) resulting in dense layers (a) with theconcept of oblique angle deposition at large θ values: in earlygrowth stages (b) and later stages of growth (c). The seedheight hS and shadowing length l are sketched in (b). In (d),a scanning electron microscopy micrograph of an oblique an-gle deposited STF, comprising of Si needle-like structures, isshown.

Various PVD methods such as ther-mal or electron beam evaporation,magnetron sputtering and pulsedlaser deposition (PLD) are com-monly used to deposit thin films ofdifferent materials on substrates.Among the many variables that in-fluence the growth of the film interms of e.g. density, crystal struc-ture and morphology, the depositionangle θ between the direction of theincoming particle flux and the sub-strate normal plays a key role whenthe films morphology is to be influ-enced [8].In contrast to the dense thin film de-position as depicted in Fig. 2.1(a),where the flux of particles that con-tribute to the growth of the filmreaches the substrate parallel to thesubstrate normal, the concept ofoblique angle deposition (OAD) issuch that the particle flux will strike the substrate under a highly oblique angle θ to thesubstrate normal (usually, θ ≥ 80). If the nucleation process is governed by Volmer-Weber or Stranski-Krastanov like growth and the surface diffusion is kept low (therebyfavoring zone 1 of the structure zone model [9, 10]), nuclei on the substrate surface withdistinct heights hS will exist [11]. After nucleation, high θ values ensure that the nucleishadow the substrate region opposite to the direction of the incoming particle flux on ashadowing length l that is a function of hS and θ:

l = hS · tan(θ). (2.1)

Fig. 2.1(b) shows a sketch of the geometrical relations between incoming particle fluxand islands on the substrate surface. As a result of this atomic-scale self shadowing, thedeposition of further particles, as soon as first nuclei have formed on the substrate, is

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2 Fundamentals of Glancing Angle Deposition

restricted to the tops of these nuclei [8, 12]. Hence, instead of a compact thin film, sculp-tured thin films (STFs) comprising of columnar, needle-like structures, slanted towardsthe direction of the incoming particle flux, evolve, as sketched in Fig. 2(c). Exemplary,the scanning electron microscopy (SEM) cross-sectional micrograph in Fig. 2(d) showsthe real structure of a Si STF on a Si substrate that was grown by ion beam sputteroblique angle deposition.

First indirect observations of θ-dependent film microstructure date back to as early as1886 [13], when Kundt linked the optical anisotropic behavior of sputtered metallic filmsto the film microstructure, whereas the self-shadowing effect in OAD was recognized in1950 by Konig and Helwig [14].

Influence of deposition angle on column tilt angle

It is a common observation that in OAD films, the column tilt angle β (i.e. the anglebetween column length axis and substrate normal, as depicted in Fig.2.2) is different fromthe deposition angle θ.

qb

shadowedregion

substrate

b < q

particleflux

Figure 2.2: As a general rule, β < θ (sketch accordingto [5]).

The columnar structures evolvingwith OAD are slanted in the direc-tion of the incoming particle flux,but not directly towards the incidentvapor. As a general rule, based onobservations with numerous materi-als and a variety of deposition con-ditions, β has always been found tobe smaller than θ.

The empirical tangent rule is thefirst and probably most well-knownrelation of both, θ and β:

tan(θ) = 2 · tan(β) (2.2)

Proposed empirically in 1966 by Nieuwenhuizen and Haanstra [15], it is not derived frombasic physical principles, but was rather found to correlate well with experimental resultsfor low θ values. However, it usually fails if θ & 60 [5, 16, 17].

Tait et al. developed a ballistic model which is based on the fact that the surface ofa growing column that is exposed to a vapor flux is not symmetric about the depositionaxis once parts of the column are shadowed by an adjacent column [16]. The maingrowth direction is therefore shifted towards the substrate normal, leading to the followingrelation, known as cosine rule:

β = θ − arcsin

(

1 − cos(θ)

2

)

(2.3)

According to equation 2.3, the columnar tilt angle β will never exceed βmax = 60.A drawback of this model is that it is purely geometrical and disregards, for example,surface mobility of the adatoms, conservation of parallel momentum of the adatoms, andresidual gas pressure (which can get important for the case of sputter deposition).

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2.1 GLAD on flat substrates

Only few models take into account the effects of surface diffusion. In a qualitative way,Hara et al. [18] pointed out that a distinction between directional and random surfacediffusion has to be made. Whereas directional surface diffusion is mainly caused by theconservation of parallel momentum of the incoming particles, leading to a deviation ofthe column inclination away from the direction of the incoming flux to the substratenormal, thermally activated random surface diffusion fosters the opposite effect, tiltingthe columnar growth direction away from the substrate normal. Thus, increasing the selfdiffusion length Λ of the arriving particles on the substrate should lead to an increase ofβ under oblique angle deposition, if all other deposition parameters remain constant. As

Λ ∝√

exp

( −Eh

kB · TS

)

(2.4)

where Eh is the surface diffusion activation energy and kB is the Boltzmann constant[12], an increase of the substrate temperature TS should therefore increase β at a givendeposition angle θ.

Whereas both, Eqs. 2.2 and 2.3 disregard this surface diffusion influence, a continuummodel approach by Lichter and Chen [19] incorporates the effects of surface diffusion.They derived the expression

tan(β) =2

3

tan(θ)

(1 + Φ tan(θ) sin(θ))(2.5)

where the variable Φ is dependent on the magnitude of the beam flux J (in volume perunit beam area per time), on the height of the initial surface perturbation hi the columnstarts to grow from, and from the surface diffusion coefficient L:

Φ =4

27

hi · J

L∝ L−1 (2.6)

Therefore, trends on the column inclination angle regarding the influence of the substratesurface temperature TS can be treated with this model, as L ∝ exp(−Eh/kBTS), [5, 12].The model by Lichter and Chen predicts an increase of β with decreased Φ. A drawbackof this model is that it ignores shadowing.

All models have in common that they show and explain trends seen in the experiments,but exact predictions of β for a given set of deposition parameters and materials canhardly be given. Therefore, the tangent rule (for low θ values) and the cosine rule (forlarge θ values) which are the most widely used models to link θ and β have to be seen as”rules of thumb” for the prediction of β.

2.1.2 Oblique angle deposition with substrate rotation

In 1959, Young and Kowal were the first to combine OAD with azimuthal substraterotation under deposition angles 30 < θ < 60, thus realizing a fluorite polarisationfilter for visible light on glass [20].

Besides this particular experiment, the concept of altering the film morphology usingboth, OAD and substrate rotation was not used again until the mid-1990s, when thewidespread availability of analytical techniques such as SEM helped Robbie et al. to di-rectly prove the possibility to ”sculpt” the columnar OAD structures into almost arbitrary

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2 Fundamentals of Glancing Angle Deposition

0 15 30 45 60 75 90

0

100

200

300

400

500

600

l[

hS]

q[°]

q q

f

(a) (b)

depositionsource

depositionsourcesubstratesubstrate

OAD GLAD

hS

l

q

continuous rotationstepwise rotation

w slow w fast

(d)(c)

substrate substrate

Figure 2.3: In OAD, only the deposition angle θ is altered (a), whereas in GLAD, an azimuthalrotation is introduced (b). φ denotes an angular segment of the azimuthal rotation. For θ & 80,the shadowing length l increases strongly non-linear (c). The ”basic” types of structures that can bedeposited using GLAD are sketched in (d): n-fold structures with stepwise substrate rotation (exemplarilysketched is a two-fold chevron) and either helical (slow substrate rotational speed ω) or vertical, columnarstructures (fast substrate rotational speed ω) with continuous substrate rotation.

shapes with an appropriate substrate rotation [1–3]. They increased θ to ”glancing angle”values1 (with respect to the substrate surface) and suggested the term glancing angledeposition (GLAD) for this technique. Figs. 2.3(a) and (b) show the difference betweenOAD and GLAD.

If a narrow flux distribution of the incoming particles is considered, the shadowing lengthl increases strongly once θ ≥ 80, as shown in Fig. 2.3(c). Therefore, increasing 90 − θto ”glancing angles” fortifies the columnar growth and considerably decreases the densityof the growing film.

Using GLAD, various types of isolated nanostructures can be deposited and formed withsubstrate rotation. Generally, the two cases of continuous and stepwise substrate rotationhave to be distinguished [8]:

1Usually, the deposition angle in GLAD is set to θ ≥ 80.

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2.1 GLAD on flat substrates

• Stepwise substrate rotation, that is, a fast substrate rotation for a predefinedazimuthal angle φ, followed by a pausing cycle to grow an ”arm” of the desiredstructure. With this method, n-fold nanostructures (n being the number of pausingcycles in one full turn of the substrate), comprising of slanted column-like arms aregrown. For example, two-fold chevron-like structures can be achieved by keepingthe substrate motionless to grow the first arm, then rapidly rotating it around anazimuthal angle φ = 180, followed by keeping it motionless again to grow thesecond arm, then rotating it again around φ = 180, and so on, until the desirednumber of arms is deposited.

• Continuous substrate rotation, that is, rotating the substrate continously witha fixed rotational speed ω. In this case, three basic types of nanostructures can bedeposited, depending on the ratio ρ = r/ω of deposition rate r and substrate rota-tional speed ω. For large ρ values (slow rotation), the tops of the growing structures”follow” the slow change of the deposition direction. This leads to the growth ofnanospirals (helical structures with open core, i.e. the height c of the spiral gainedduring one full rotation, known as pitch, is less than the diameter of the spiral). Lowρ values (fast substrate rotation), on the other hand, simulate omnidirectional fluxas seen from the substrate, and therefore result in non-slanted, upright columnarstructures. Finally, intermediate ρ values (intermediate substrate rotational speed)give rise to the deposition of screw-like structures (helical structures with compactcore).

To highlight the influence of the substrate rotation on the growth morphology of glancingangle deposited films, in Fig. 2.3(d) the ”basic” types of nanostructures that can bedeposited with GLAD at different substrate rotation schemes are sketched.

A further degree of freedom can be introduced by not only rotating the substrate az-imuthally, but also changing the deposition angle θ during deposition as well, which altersthe density of the STF with film thickness [21, 22].

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2 Fundamentals of Glancing Angle Deposition

2.2 GLAD on patterned substrates

A wide morphological variation of nanostructures can be achieved using GLAD2 in an easybottom-up process. However, if GLAD is performed on bare planar substrates, severalgrowth phenomena do occur that might be a drawback for possible applications, such as:

• Random arrangement and non-uniform column size of the single nanos-tructures, as they evolve out of seeds that nucleate statistically distributed on thesubstrate surface during the first stages of growth [23, 24].

• Competitive growth, meaning that a number of structures cease growing on ac-count of neighbored ones that (for example due to temporarily existing local fluxinhomogeneities) grow faster and eventually overgrow adjacent structures that fallin the shadows cast by the higher features on the surface. This extinction processgoes along with a diameter increase of the surviving columns in order to keep theplanar density constant [5].

• Scaling behavior of the above discussed diameter increase. The structure diameterw increases with increasing structure height h. For vertical column-like structures(deposited with fast substrate rotation) this increase has been found to obey a powerlaw with growth exponent p:

w ∝ hp (2.7)

with 0.3 < p < 0.5, depending on the strength of the surface diffusion [25].

• Second Anisotropy if the nanostructures are not circular symmetric around thesubstrate normal, as it is the case for vertical columns, but have an oblique growthaxis, as it is the case for OAD-grown slanted columns. As self-shadowing is restrictedto the plane of incoming particle flux, the columns broaden perpendicular to thatplane, until they are large enough in that growth direction to finally form chainsperpendicular to the incoming vapor flux [26].

One approach to control both, placement and uniformity of glancing angle depositednanostructures is the introduction of substrates that were patterned previous to depo-sition. To avoid the randomness of nucleation, the substrate has to be covered with(periodically arranged) features that are supposed to act as artificial seeds for the parti-cles striking the substrate under high θ values.To suppress aperiodic film growth and gain the formation of structures that are uniformin morphology and diameter via the implementation of a suitably pre-patterned substrate,different pattern design considerations have to be encountered [27, 28]:

• To circumvent condensation between the seeds, the inter-seed spacing dS of seedswith height hS should obey

dS ≤ hS · tan(θ) (2.8)

under a given deposition angle θ [29]. A highly directional flux with low deviation ∆θshould prevent growth between the seeds, if surface diffusion is negligible. However,as GLAD is usually performed with azimuthal substrate rotation, the direction ofthe incoming particle flux alters, and so does the distance between two artificialseeds that are in line-of-sight with the incoming particles. Therefore, the term”inter-seed distance” is somewhat questionable.

2GLAD is generally material-independent, as long as the element or compound can be deposited withPVD methods.

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2.2 GLAD on patterned substrates

• Besides the prevention of condensation between seeds, which is fulfilled with Eq.2.8, the planar density of the seeds must be taken into consideration, which (ontetragonal lattices) is defined as

π(wS/2)2

P 2

where P is the lattice period (center-to-center seed spacing). In order to preventa change of the aspired morphology3 of the structures that grow on the artificialseeds, the planar density of the seed pattern should match with the volume density ofthe STF [5, 28]. Otherwise, the structures will broaden their diameters and mightdegenerate into pillar-like entities in their attempt to ”fill” the offered inter-seedspacing to reach a quasiequilibrium volume density. If the growing structures are tokeep the morphology intended with the applied rotation mechanism, the relation

π(wS/2)2

P 2≥ π

((P − hS · tan(θ))/2)2

P 2⇒ wS ≥ P − hS · tan(θ) (2.9)

should hold. In other words: Even if the seed heights hS are sufficient enough toshadow the whole inter-seed-region to prevent inter-seed-growth, if the period P ofthe pattern gets too large, the structures growing on the seeds suffer from insufficientshadowing by adjacent structures and tend to broadening and bifurcation in anattempt to fill the inter-seed-space.

• If multi-structure growth on a single seed is to be avoided, the seed diameter wS

should be less than the equilibrium structure diameter [27].

qws

dS

hs

P

PdS2

Figure 2.4: Artificially seeded substrate:seed diameter wS , seed height hS , seed periodP , inter-seed-spacing dS . Inset: tetragonal lat-tice (after Ref. [7]).

Tetragonal lattices are the most commonlyused template patterns for GLAD [7, 29–32],especially because the proposed square spiralarchitecture for large three-dimensional pho-tonic band gap crystals by Toader and John [6]inspired some research concerning the GLADrealization thereof [33–36].

For a tetragonal lattice of artificial nuclei(Fig. 2.4), it is assumed that shadowing bythe next nearest neighbors will be sufficientto suppress random growth between the pat-terned seeds [7]. Thus, Eq. 2.8 changes to4

√2 · dS ≤ hS · tan(θ) (2.10)

Besides tetragonal lattices, other seed ar-rangements have been applied for GLAD aswell, for instance triangular lattices [30] or quasi- onedimensional line arrays real-ized with photoresist lines on a substrate [37]. If in the latter case the planar fill factor isdesigned to be less than the volume fill factor of the initially separated column-like struc-tures grown on it (i.e. Eq. 2.9 is not fulfilled), it was shown that column broadening and

3The morphology is defined by the applied rotation scheme.4It should be noted that the expression given in ref. [7],

√2 · P ≤ hS · tan(θ) is slightly incorrect in

so far as it links the period P , not the inter-seed-spacing dS to the shadowing length.

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2 Fundamentals of Glancing Angle Deposition

merging can lead to the growth of continuous nano-ribbons in that case [38]. Hexago-nally closed packed (hcp) [39–41] and honeycomb-like [42] arranged seed layers havealso been used as substrates for GLAD. In principal, the same design considerations asin the case of the tetragonal lattice, concerning the prevention of inter-seed-condensationand loss of intended structure morphology, can be applied in those cases. As in particularthe honeycomb-like pattern will be a subject of consideration in this work, some furtheraspects concerning patterning and inter-seed distances in this case will be discussed inchapter 3.

Different patterning principles have been used so far to produce template substrates forGLAD, such as patterning the substrate surface with polystyrene (PS) spheres [39, 40]or SiO2 nanospheres [43–45], electron beam lithography (EBL) [29, 36, 46], nanospherelithography (NSL) [42], and optical lithography [30, 32, 37, 41].

The main patterning principles used for this work are EBL and NSL. They will beexplained further in chapter 3.

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2.3 GLAD at elevated temperatures

2.3 GLAD at elevated temperatures

GLAD is commonly performed at low substrate temperatures TS, as it is based on self-shadowing effects that are counteracted by temperature-driven surface diffusion of theadatoms [27]. In terms of the reduced temperature TS/TM , where TM is the melting pointof the material to be deposited, as a rule of thumb surface diffusion becomes importantonce TS/TM & 0.3 [10–12, 27]5.

Zhou et al. reported on the growth of magnetron sputtered, columnar Ta structureson substrates pre-patterned with hcp arranged silica nanospheres (nanosphere diameterD = 260 nm) with GLAD at elevated substrate temperatures TS [43, 44, 47]. They ob-served changes in the branching behavior of single columns into subcolumns at differentsubstrate temperatures in a range 200C . TS . 700C [43, 44]. Furthermore, theyreport on a degradation and loss of the hcp arrangement of single columns at substratetemperatures TS & 900C [42]. The morphological changes observed are attributed tothe surface diffusion length dependency on the substrate temperature, arguing that atelevated temperatures, the increase in adatom mobility fosters the growth competition ofneighbored columns. However, no data is provided on the effects of elevated substratetemperature TS on the glancing angle deposition of (columnar) Ta structures on unpat-terned substrates, where the column-column separation is not governed by the distancebetween artificial seeds and therefore by the diameter D of the nanospheres.

Suzuki et al. observed the growth of rod- and wirelike6 Al whiskers under obliqueangle electron beam evaporation conditions on surface-oxidized Si substrates at elevatedTS [48, 49]. They found that at TS ≈ 180C, the columnar morphology disappearsand a dense thin film is deposited, which is attributed to the surface diffusion lengthovercoming the self shadowing distance, thereby outweighing the self-shadowing effect.At even increased temperatures, between 180C . TS . 290C, the morphology isfound to be shifting from a dense thin film to a substrate-near dense film that exhibitssingle crystalline, whisker-like features with diameters ranging in-between approximately50 nm and 1200 nm, that can reach remarkable lengths on the µm-scale7. The detailedmechanism of this temperature-mediated OAD nanowire growth is yet to be understood.

Schubert et al. characterized the in situ and ex situ temperature influence on thecrystal structure of ion beam sputter deposited, four-fold Si spirals on surface-oxidized Sisubstrates [50, 51]. They found a recrystallization of the Si spirals (that are amorphousafter deposition for substrate temperatures TS . 475C) with post-deposition annealing,the onset of this behavior at an annealing temperature of approximately 800C. Post-deposition experiments on glancing angle deposited chiral TiO2 structures performed byPursel et al. indicate a recrystallization at annealing temperatures of approximately500C, which is accompanied by a reduction of the helical pitch c of the structures,thereby changing the optical properties of the STF [52].

5According to that, for the deposition of Si, surface diffusion should play no dominant role in thetemperature range TS . 330C.

6With diameters less than 100 nm, an Al whisker is referred to as nanowire and with diameters exceeding100 nm, it is referred to as nanorod [48].

7As an example, for a nominal Al film thickness of 16 nm, wire-like, crystalline whiskers with diametersof approximately 50 nm and lengths of approximately 5.4 µm are found [48].

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2 Fundamentals of Glancing Angle Deposition

2.4 Deposition methods used for GLAD

Different PVD methods have been used so far for GLAD. A directed particle flux witha narrow angular distribution of the emitted particles is considered to be important fora reduced columnar expansion [53], and as GLAD is based on ballistic shadowing, theangular flux deviation ∆θ from the deposition angle θ should be as low as possible [8].

In evaporation methods, the angular flux distribution of the particles is usually ofthe over-cosine type, meaning that the particles are emitted mainly along the directionof the source normal [8]. Additionally, evaporation methods usually enable high growthrates r up to (nm/s) values, as the relation between evaporation pressure pev and sourcetemperature T is exponential, pev ∝ eT , and r ∝ pev. As in evaporation there is noneed for process gases, low working pressures pdep can be applied, and gas-phase scatteringof evaporated particles can be minimized. Thus, many research into GLAD is based onthermal or electron beam evaporation ([1–3, 7, 35–37, 39, 41, 49, 54–56], to cite only afew publications). However, there are some disadvantages of evaporation, including thechallenge to deposit high melting point materials like molybdenum or tungsten. Addi-tionally, the deposition of compounds and alloys with exact stoichiometry is problematicdue to the differences of vapor pressures in the elements.

Magnetron sputtering has been used for GLAD as well, although in sputter processesthe angular flux distribution is broad (typically of the cosine type), and the need for aprocess gas (usually Ar) increases pdep to typical values in the range of pdep ≈ 10−3 mbar,thus lowering the mean free path of particles and therefore increasing the probability ofgas-particle-collisions. However, the use of flux collimating systems can limit the angularflux distribution [29]. Therefore, the advantages of sputtering (such as, for example, thepossibility to deposit high melting point and compound materials) can be incorporatedfor GLAD of different material systems as well [29, 43, 57–61].

In ion beam sputter deposition (IBSD), the energetic ions used to sputter a target(and thereby generating a particle flux via a momentum-driven process) are generated inand extracted out of an ion beam source. In comparison to magnetron sputtering, thetarget is not needed as cathode anymore and is therefore decoupled from the ion gener-ating process. The distances and angles between ion source, target and substrate can bechosen independently in this setup. Another advantage is that the working pressures ofmost ion beam sources are in the range of pdep ≈ 10−5 mbar. Therefore, the particle fluxgenerated by ion beam sputtering is transported through a plasma-free, lower pressureregion as compared to magnetron sputtering [8]. However, only few reports concerningGLAD with IBSD of Si and SiO2 exist [46, 50, 51, 62, 63].As IBSD is the deposition method chosen for this work, it will be more thoroughly ex-plained in chapter 3.

Pulsed laser deposition (PLD) is a deposition method based on the ablation of atarget under laser irradiation and condensation of the so-generated particle flux on asubstrate. An advantage of PLD is the ability to deposit stoichiometric films from asingle target. The angular distribution of ablated particles concentrates strongly aroundthe target normal (over-cosine flux distribution) [64]. So far, PLD-based GLAD has beenused to deposit porous carbon- [65] and ZnO- [66] STFs.

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2.5 MC - simulation of GLAD

2.5 MC - simulation of GLAD

For the computational simulation of growth dynamics in OAD and GLAD, ballistic depo-sition models in form of Monte Carlo (MC) simulations are the most used algorithms sofar. Besides a few research examples where aspects of thin film deposition under obliqueparticle incidence were simulated with molecular dynamics models [67] or with continuummodel approaches [19, 68], most simulations base on ballistic deposition in either two orthree dimensions [1, 25, 29, 65, 69–72].

Figure 2.5: Principle of MC simulation ofGLAD (taken from Ref. [25]).

The three-dimensional (3D) MC simulationused here to compare with some of the experi-mentally gained results in chapters 4 and 5 wasintroduced by Karabacak et al. [25, 73]. Thesimulations include an oblique deposition angleθ, substrate rotation (having the possibility toalter the azimuthal angle φ) and surface diffu-sion. A square lattice with continuous bound-ary conditions and a size of 512 × 512 latticeunits (lu) simulates the substrate surface. Ineach simulation step, a particle in form of a cu-bicle with the dimension of one lattice point issend towards a randomly chosen lattice pointon the substrate under a pre-defined depositionangle θ. To take into account the angular distribution of the particles in the experimentalsputter deposition process, the simulated flux has an angular spread according to thedistribution probability function

dP (θ, φ)

dΩ=

2 cos(θ)

π sin(θ)(2.11)

where Ω is the solid angle, θ is the polar (deposition) angle and φ is the azimuthal angle.Furthermore, θ is restricted to values 0 ≤ θ ≤ 88. The model assumes negligibleparticle energies (no momentum transfer). The substrate rotation is simulated with achange of the azimuthal angle, ∆φ, between two following particles. As shown in Fig.2.5, the model allows sidewall sticking to simulate truly 3D structures. Surface diffusionis simulated by randomly choosing a surface or bulk atom within the vicinity of a radiusof 5 lu surrounding the impact point of the deposited particle and letting this atom diffuseto another nearest neighbor location by ”hopping” from lattice point to lattice point. Thehopping probability is related to the number of nearest neighbors the atom has beforehopping8. When the diffusing atom comes to rest, this diffusion step is repeated witha new atom until an assigned number of diffusion steps DS is made9. An alteration ofthe ratio ρ = r/ω of vertical deposition rate r to substrate rotational speed ω (whichinfluences the morphology of the glancing angle deposited nanostructures, as described insection 2.1.2) can be incorporated in the simulations by changing the number of particlesper rotation (NPR). Additionally, different film heights (and therefore deposition times)can be simulated by changing the number of deposited particles (NOP ).

8The more nearest neighbors the atom has, the stronger it is bound to its location and the less is theprobability for diffusing.

9That means, increased diffusion is simulated by increasing the number of diffusion steps DS.

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2 Fundamentals of Glancing Angle Deposition

2.6 Applications of GLAD-grown sculptured thin films

Functional thin films for various applications can be deposited with the GLAD approach.On the one hand, as STFs deposited with GLAD show an internal microstructure, thesurface-to-volume ratio of such a thin film can be highly increased (several tens of cm2 ofsurface area per one cm2 of substrate surface area can be reached under glancing angledeposition conditions [74–76]). On the other hand, the unique possibility to directly influ-ence the structure form with a specific substrate rotation algorithm offers the possibilityto alter several physical properties which are dependent on the structure morphology [5].

In the field of optics, the promising approach of a square spiral photonic crystal structuremade of amorphous Si (a-Si) as proposed by Toader and John [6] inspired a lot of researchregarding the experimental realization and morphological optimization thereof [34–36, 77,78]. As helical STFs can exhibit the circular Bragg phenomenon10, applications such aspolarization filters [79] or polarized light emitters [80] are possible. As the porosity of aSTF can be engineered to a certain degree by adjusting the deposition angle θ [81], and therefractive index is controlled by the porosity, the refractive index within one film can bealtered. The deposition of highly reflecting films [82] as well as graded index antireflectioncoatings [83] or narrow bandpass optical filters [84] is therefore possible.

Changes in the transmittance spectrum of nanostructured gradient index optical filtersof TiO2 under varying humidity conditions can be used for high-speed humidity sensing[22]. Additionally, glancing angle deposited Cr nanostructures were shown to react aspressure sensors, making use of changes in the cross-film resistivity when an externalpressure load forces the initially separated structures to touch each other [85]. Anotherfield of sensing applications for GLAD-grown nanostructures, especially if the STFs consistof noble metals such as Au and Ag, is their use as high sensitivity surface-enhanced Ramanspectroscopy substrates with potential for biosensing [86–90].

Other applications of glancing angle deposited thin films make use of the increasedsurface area of a STF in comparison to a compact thin film. For example, Karabacak etal. could show that the photoluminescence of a polymeric thin film deposited with CVDon a Si substrate is enhanced when the substrate is coated with a GLAD-grown STF of Wnanocolumns [91]. This enhancement is linked to the increased effective substrate surfaceafter GLAD in comparison to a bare, planar substrate. A comparable approach withglancing angle deposited Si nanocolumns on Si substrates showed a higher yield for enzymeimmobilisation in comparison to a reference sample without the columnar STF [92]. Thepossibility of stress reduction in compact W thin films by using a thin, glancing angledeposited compliant layer between substrate and film has been demonstrated as well [93].Finally, it has been shown that oblique angle deposition can be used for biomimetizationpurposes, by replicating biological probes such as fly eyes and butterfly wings [55].

10Circularly polarized light with handedness equal to the handedness of the structure will be reflected,and the opposite handedness will be unaffected [5].

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3 Experimental Setup and SampleAnalysis

3.1 Glancing angle ion beam sputter deposition of Si

nanostructures

3.1.1 Deposition process

The deposition method used for GLAD of Si nanostructures in the present study is ionbeam sputter deposition.

The erosion of surfaces under particle bombardment is generally known as sputtering[94]. Usually, energetic ions are accelerated towards a target, where surface erosion occursdue to energy transfer from ion to target atom in atomic collisions. When energetic enoughto overcome the surface binding forces, atoms can leave the target surface. If a significantnumber of emitted target species are generated via this momentum-driven process, theso-generated particle flux can be used for thin film deposition on a substrate [8].

If electronic excitation is left out of consideration, the theory of knockon sputtering(sputtering by elastic collisions) [94] describes three different sputtering regimes that aremainly influenced by the kinetic energy of the incident ion and the mass ratio of targetatom and bombarding ion: the single-knockon regime, the linear cascade regime and thespike regime.

In the single-knockon regime, the energy E of the incident ions is low (typically, Efalls in the lower and medium eV region), and the recoil atoms from ion-target collisionsreceive sufficient energy to get sputtered, but not enough to generate recoil cascades. Inthe linear cascade regime (typically, E ranges up to a few hundred keV), recoil atomsreceive enough energy from the incident ions to produce higher-generation recoils, whichin turn may reach the target surface and overcome the binding energy barrier. In thespike regime (E in the MeV range, large ion masses), the density of recoil atom cascadesis high, and the majority of atoms in the spike volume are in motion [94].

The sputtering process can be characterized by the sputtering yield Y (average numberof emitted atoms per incident ion). Following Sigmund [95], depending on different pa-rameters (e.g. ratio of target atom mass to incident ion mass MT /MI , ion energy E, ionincidence angle αS−T with respect to the target normal), Y is approximated as

Y (E,αS−T ) ≈ 4.2 · FD(E,αS−T )

NU0

(3.1)

where FD(E,αS−T ) is the depth distribution of the deposited energy, and N and U0 are theatomic density of the target material and the average surface-binding energy, respectively.Y can reach values between 10−5 . Y . 103 [94].

17

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3 Experimental Setup and Sample Analysis

Concerning the yield dependency on the ion energy E, for the case of Si as targetmaterial sputtered with Ar ions as in this study, it is found that for energies E . 20 keV,Y increases approximately with

√E and reaches maximum values of Ymax ≈ 2 under

normal incidence [96]. At high ion energies the sputtering yield Y will decrease, as theions loose their energy further away from the substrate surface (their penetration depthincreases with increasing E). FD(E,αS−T ) has a maximum at the maximum of the nuclearenergy loss curve. For higher ion energies, the decrease of the nuclear energy loss leads toa decrease of the sputtering yield. It follows that energies of the incident ions E . 20 keVare necessary to gain a high yield and therefore high deposition rate of the sputtered Siatoms. For the case of sputtering Si with Ar ions, the Y (E)-dependency for normal ionincidence is shown in Fig. 3.1(a).

Concerning the yield dependency on the incidence angle αS−T of the incomingions with respect to the target normal, the ratio of the angle-dependent sputtering yieldto the yield at normal incidence, Y (αS−T )/Y (0), is found to be a function of the cosineof αS−T :

Y (αS−T )/Y (0) ≈ cos−f (αS−T ) (3.2)

where f is dependent on the mass ratio of target atom and ion MT /MI and is approxi-mately f ≈ 5/3 if MT /MI . 3 according to Sigmund [95]1, but other f values have beenreported as well, depending on the particular experimental setup and combination of ionspecies, ion energy and target material [96]. Experimental data is often compared with acos−1(αS−T )-dependency. According to that, with increasing αS−T , the sputtering yieldY increases, as the energy is deposited closer to the surface then. However, this relation isonly valid for not too large angles αS−T . For very large angles (i.e. grazing incidence withrespect to the target surface), more and more ions are reflected and Y decreases again.Independent of the ion/target material combination, the yield usually reaches a maximumfor αS−T values between 60 and 80 [96]. Typical examples for Y (αS−T )-dependenciesare depicted in Fig. 3.1(b).

1 2 5 10 20 50 100 200 500

E [keV]

0.05

2.00

1.50

1.00Y

(a)

1.50

1.00

Y

2.00

0 30 60 90

aS-T [°]

Ti

Zr

W

1/cos( )aS-T(b)

Figure 3.1: (a): Sputtering yield Y as function of the primary ion energy E for the case of sputteringSi with Ar ions under normal incidence (data taken from Ref. [96]). (b): Dependency of sputtering yieldY on the angle between incident ions and target for three different metals (ion species: Ar+ with E =1.05 keV energy, data taken from Ref. [97]). Dashed line : cos−1(αS−T )-dependency.

Concerning the polar angular distribution of the particle flux, for the case ofsputtering this is generally described to be of the cosine type [94]. Under a certain angle

1For the case of sputtering a Si target with Ar ions, MT /MI ≈ 0.63.

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3.1 Glancing angle ion beam sputter deposition of Si nanostructures

of particle emission ξ (with respect to the target normal), the emission intensity I(ξ) ofsputtered particles with respect to the emission intensity along the target normal I0 isI(ξ)/I0 = cos(ξ) in this case. Many experiments, however, suggest a deviation fromthe cosine distribution [98]. For the case of Si sputtered with Ar ions2, an overcosine fluxdistribution I(ξ)/I0 = cos2(ξ) was found [99].

The energy distribution of the sputtered atoms is strongly dependent on the ionincidence angle αS−T and the atom emission angle ξ. Generally, the average energy ofthe sputtered atoms is lowest for atoms sputtered in normal direction. According toThompson [100], in the spectrum of the energy of emitted atoms, the probability p(EA)of an atom to have an energy EA is highest at

EmaxA =

U0

2(3.3)

after which the probability decreases like 1/E2A. If the surface-binding energy U0 is ap-

proximated with the sublimation energy of Si which is about (4.3 ± 0.3) eV [101, 102],it follows that the majority of the sputtered Si atoms emitted in normal direction shouldhave an energy of ESi ≈ 2.2 eV. As often deviations from Eq. 3.3 are found [94], it hasto be seen as a rule of thumb. The energy of the sputtered particles can rapidly increasewith increased ξ: For Si atoms sputtered with Kr ions at E = 1 keV and αS−T = 60,in Ref. [103] at ξ ≈ 5, the average energy 〈 ESi(ξ) 〉 of the sputtered Si particles isapproximately 〈 ESi(5

) 〉 ≈ 13 eV, whereas at increased ξ values, 〈 ESi(ξ) 〉 is found toexhibit larger values: 〈 ESi(45) 〉 ≈ 34 eV.

3.1.2 Deposition system setup and deposition parameters

Broad beam ion source for sputter deposition

screen grid

acceleration grid

deceleration grid

gas inlet

plasma graphiteanode

HF coil

dischargehousing

~~~ HF

UB

UA

ion beam

Figure 3.2: Sketch of the HF ion source.

The generation of the Ar ion beam that isused for the sputtering process of the Si tar-get takes place in a high-frequency (HF) ionbeam source [104]. Fig. 3.2 shows the prin-ciple setup of the source. The Ar plasma isgenerated in a ceramics discharge housing.The gas inlet is used to provide the processgas (Ar). A 5-turned coil surrounding thehousing that is energized by a HF supply(13.56 MHz) provides the inductively cou-pled HF field in the discharge housing whichis used to ionize the inert process gas. Theenergy of the Ar ions is determined by the(positive) beam voltage UB that is suppliedto the graphite anode: E = e · UB, wheree is the elementary charge. To extract theions out of the discharge chamber in form ofan ion beam, a set of three multiaperture grids opposite to the anode is used. Each grid ismade of graphite, has a diameter of 40 mm and consists of several apertures that allow forthe extraction of the ion beam. The screen grid (closest to the discharge chamber) floats

2The ion energy studied in the cited work was 10 keV . E . 20 keV, and the ion incidence angleαS−T was set to 45.

19

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3 Experimental Setup and Sample Analysis

electrically at UB and is therefore biased positively with respect to the ground. The accel-eration grid is contacted with the (negative) acceleration potential UA and establishes anelectric field between screen and acceleration grid. Positive ions drifting close to that fieldare accelerated and extracted from the discharge chamber, whereas electrons are repelledby the negative potential, thus leading to the separation of electrons and ions and theextraction of an ion beam. The deceleration grid is at ground potential, thus deceleratingthe extracted ions, leaving them with a net ion energy E of approximately E = e · UB.Individual beamlets extracted out of each aperture combine to form the broad ion beamthat is directed towards the target, where it leads to sputtering of the target surface andthe establishment of a Si particle flux.

Deposition chamber setup and deposition parameters

All Si GLAD structures were deposited in a high vacuum load-locked deposition chamberas sketched in Fig. 3.3.

5

64

43

107

81

12

9

2

11

11

13

sputtertarget

HF ion source

substrateholder

qa

S-T

(a) (b)y

z

x sputteredparticle flux

Ar ionbeam

+

Figure 3.3: (a) Sketch of the deposition system (top view). 1: deposition chamber. 2: load-lockchamber for sample transfer and change. 3: gate valve. 4: pumping system (turbomolecular pump,backed by a scroll pump). 5: substrate manipulator, transferable in x-, y- and z-direction and rotatablearound the z-axis, with feedthroughs for thermocouple and tantalum wire resistance heater. 6: substrateholder, rotatable around its normal. 7: HF ion beam source. 8: Si sputter target. 9: control window.10: gas inlet. 11: pressure gauges. 12: substrate handler. 13: flux aperture.(b) Sketch of the deposition principle.

The base pressure p0 of the system is better than 3 × 10−8 mbar. During deposition,the Ar gas flow fAr was set to (3.5 < fAr < 6.2) sccm (standard cubic centimeters perminute), thus resulting in working pressures (7.0×10−5 < pdep < 1.0×10−4) mbar. Theion beam source-target distance measures approximately 15 cm and the distance betweentarget and substrate is approximately 12 cm. The mean free path of the sputtered particles(l) can be estimated as

l =kB · T√

2 · π · d2RG · pdep

(3.4)

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3.1 Glancing angle ion beam sputter deposition of Si nanostructures

where kB is the Boltzmann constant, T the temperature, and dRG the atomic diameterof the residual gas (or, in this case, working gas Ar) [105]. At pdep ≈ 1.0 × 10−4 mbar,T = 300 K and dRG ≈ 0.32 nm, l ≈ 0.7 m. Thus, l is about 6 times larger thanthe target-substrate distance, and, consequently, the probability of scattering of sputteredparticles at residual gas particles is low.

The deposition chamber is equipped with a substrate manipulator that can be movedin x-, y- and z-direction. It also allows for a rotation around the z-axis (to adjust thedeposition angle θ). Additionally, the substrate holder can be rotated azimuthally aroundthe substrate normal with rotational speeds (0.01 < ω < 0.2) rev/min. The substraterotation is computer controlled and can be either continuous or stepwise. When stepwise,the increment is (360/100) = 3.6. A tantalum wire resistance heater and a type Kthermocouple are both situated under the substrate holder and can be used to adjustand control the substrate temperature TS. According to [106], the dependence of thesubstrate temperature TS on the temperature measured with the thermocouple TT islinear and obeys the following relation: TS = 0.8 · TT − 6.5 K. TS can be adjusted inthe range RT . TS . 400C.

The sputter target, a sintered disc of amorphous Si with a diameter of 15 cm isfreely rotatable around its x-axis. As the sputtering yield Y reaches a maximum for60 . αS−T . 80 [96], the angle between ion beam source and target was set toαS−T ≈ (70±5), where experimentally a maximum of the deposition rate for otherwiseconstant deposition parameters was found.

60 mm

o90° - q

Siflux

substrate

aperture

Figure 3.4: Sketch of the aperture between targetand substrate to minimize the angular spread of thedeposition angle θ.

The sample preparation was done asfollows: The substrate3 was attached tothe substrate holder with double-sided ad-hesive carbon tape and attached to thesubstrate manipulator via the load-lockchamber. Before the substrate manipu-lator was turned to attain the θ valuewanted for the specific experiment, thetarget was sputtered clean for approxi-mately 2 min. For the experiments, typi-cal ion beam source parameters were: HFgenerator power PHF ≈ (120 ± 10) W,beam voltage 1100 V < UB < 1250 V,screen grid current IB ≈ 65 mA, acceler-ation voltage UA ≈ 350 mA, accelerationgrid current IA ≈ 2 mA. By this, ion energies E of 1100 eV . E . 1250 eV andstable ion beam source working conditions could be reached. The ion beam current den-sity at UB = 1200 V was approximately j ≈ 3.1 mA/cm2 at those conditions [106].Although not determined experimentally here, the applied ion energy of E ≈ 1.1 keVin combination with an angle between ion beam source and target αS−T ≈ (70 ± 5)

should result in a sputtering yield Y (70) ≈ 6, if the sputtering yield at normal incidenceis set to Y (0) ≈ 1 and Eq. 3.2 is applied. As the substrates were placed directly infront of the center of the target area sputtered by the incident ion beam, the majority ofthe Si particles contributing to the STF growth left the target under normal incidence,thus leading to average Si atom energies in the range of 10 eV . 〈 ESi 〉 . 15 eVat the substrate [103]. Without aperture between target and substrate, these parameters

3If unpatterned, substrates of Si(100), freshly broken out of a Si wafer with typical sample sizes of1 cm × 1 cm were used.

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3 Experimental Setup and Sample Analysis

resulted in deposition rates r ≈ (7± 0.5) nm/min in the glancing angle case (θ ≈ 85)and r ≈ (21.5 ± 2.0) nm/min in the case of normal deposition (θ = 0). However,GLAD needs a highly directed flux in order to secure that the self-shadowing effect is notcounteracted by atoms reaching the substrate under incoming angles significantly largerthan the aspired deposition angle θ. As not all Si atoms are emitted in normal directionfrom the target4, and as the target itself is not to be treated as a point source (the sput-tered area has an elliptical shape and extends over a few cm2), a significant fraction ofthe Si atoms is likely to reach the substrate under deposition angles different from the θvalue as adjusted with the tilt of the substrate manipulator. Thus, for most experimentsin this research, a slit aperture between substrate and target was incorporated to blockatoms from reaching the substrate under low deposition angles. As Fig. 3.4 shows, theopening o (in mm) of this aperture defines the minimum deposition angle θmin (with re-spect to the substrate normal) according to θmin ≈ 90 − arctan(o/60). A typical valueof o is (15 ± 3) mm, thus leading to θmin ≈ (76 ± 3), i.e. an angular divergence of∆θ ≈ (9 ± 3) for the “typical” glancing angle value of the deposition angle, θ = 85.As, on the other hand, the incorporation of the aperture lowers the deposition rate sig-nificantly (r ≈ (3.5 ± 0.3) nm/min at θ = 85, if o ≈ 15 mm), values of o ≤ 15 mmhave not been used.

Without additional heating, the substrate remained at TS ≈ RT. Depending on thedeposition time, an increase of TS due to heat radiation of the ion beam source could beobserved. However, the maximum temperature in this case did not exceed TS ≈ 55C,and therefore no additional cooling of the substrate holder was supplied.

4As described in the previous chapter, the angular distribution of the sputtered particles can be describedto be of the overcosine type.

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3.2 Substrate patterning methods prior to GLAD

3.2 Substrate patterning methods prior to GLAD

Electron beam lithography (EBL) is a widespread method used since roughly threedecades [107, 108] as tool to form patterns of various shapes with resolutions that arenowadays better than 20 nm.The basic principle is to first ”write” patterns on a substrate or film that is covered with aresist by exposition to an electron beam that is moved over the substrate in a controlledway. In the following step,either exposed (when using positive resist) or unexposed (whenusing negative resist) parts of the resist are removed by developing. Then, the remainingresist pattern is transfered into the substrate, for example by ion beam etching, if adifferent etching sensitivity of substrate and resist allows the remaining resist patterns tocover the underlying substrate regions. In a last step, the remaining resist is removed,and the pattern is successfully transferred.

When EBL is used to pattern templates for subsequent GLAD growth, most researchwork so far concentrated on dot-like seeds forming tetragonal arrays with specific periods(nearest-neighbor-distance) P [7, 22, 29, 30, 32, 35, 36, 46, 77].

Although EBL allows the fabrication of sub-µm structures in a controlled manner, it istime consuming (several fabrication steps, serial processing) and therefore unsuitable topattern large areas of a substrate.

For this work, fields of tetragonally arranged seeds with different periods P , seed heightshS and seed diameters wS have been patterned using EBL and pattern transfer by reactiveion beam etching in Si(100) covered with a thermally oxidized SiO2 layer.

Nanosphere lithography (NSL), in contrast to EBL, is a patterning principle thatallows to fabricate large arrays of regular nanostructure templates [109]. Polystyrene(PS) or SiO2 nanospheres with diameters D in the nm range are dispersed over thesubstrate, for example by spin-coating of nanosphere suspensions [109], but also by othermethods [110]. With the help of surfactants, the primarily unordered spheres arrange inhexagonally closed packed (hcp) assemblies on the substrate surface.

Depending on the nanosphere density in the suspension, not only hcp single monolayers(SL), but also double layers (DL) of the spheres can be formed. Besides the use of SLof spheres as artificial, hcp-arranged seeds for GLAD, it is also possible to use those SL-or DL- arranged spheres as masks for a subseqent evaporation step [109]. The evaporantwill pass through the holes in-between the spheres, covering the substrate only at thosepredefined spots. If the spheres are removed afterwards5, the evaporant remains on thesubstrate, forming periodically arranged patterns of nanostructures that can serve aspre-defined seeds for GLAD. A scheme of the NSL process is shown in Fig. 3.5.

With a change of the sphere size, the distances between the seeds can be altered. WhenGLAD is performed on the hcp arranged nanospheres as seeds directly, the seed periodP equals the nanosphere diameter D. When, on the other hand, the hillock-like featuresthat remain after NSL with seed material evaporation and nanosphere liftoff are used asseeds for GLAD, two cases have to be distinguished (see Fig. 3.5(b)):

• DL - case: hcp patterns of evaporant material remain on the substrate, with everyseed point having six equidistant nearest neighbors with inter-seed distance

d = D (3.5)

5The sphere liftoff can for example be done by chemical etching, but also mechanically with adhesivetape.

23

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3 Experimental Setup and Sample Analysis

substrate

SL

DL

evaporant

DD

d1

d2

1 µm 1 µm

dis

db

(b)D

(d)(c)

(a)

Figure 3.5: (a): Principle of NSL. (b): In the SL areas, a honeycomb pattern remains after evaporationand nanosphere (sphere diameter D) liftoff, whereas in the DL areas, a hcp pattern remains. d = D,d1 and d2 indicate the different inter-seed distances of the seeds after sphere removal. (c) Top-view SEMmicrograph of honeycomb seed pattern after sphere removal in the case of SL, and (d) hcp seed patternafter sphere removal in the case of DL. dis indicates a dislocation, and db indicates a domain boundaryin the hcp pattern.

• SL - case: honeycomb patterns of evaporant material will remain on the substrate.In this geometry, three different inter-seed distances d can occur as function of thenanosphere diameter D, as shown in Fig. 3.5(b): The nearest-neighbor-distance is

d1 = D · tan(30) =D√3

(3.6)

Every seed has 3 nearest neighbors. They are symmetrically arranged around the”central seed”, each shifted by 120 to the former. The six second nearest neighborsto every seed can be found in a distance d = D, as they equal the nearest neighborsin the DL- induced hcp seed array. Finally, the three third nearest neighbors thatare symmetrically distributed around every seed have inter-seed distances

d2 =D

cos(30)=

2D√3

= 2 · d1 (3.7)

Only few publications so far deal with the use of self-assembled nanospheres as templatesfor GLAD [39, 40, 43, 47, 111, 112] and even less with the use of honeycomb-arrangedhillocks patterned with NSL as seeds for GLAD [42, 113].

As will be shown in chapter 5 of this work, the different periodic arrangements intro-duced here (tetragonal, honeycomb and hcp pattern) have a profound influence on the

24

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3.2 Substrate patterning methods prior to GLAD

development of the morphology of nanostructures grown with GLAD on artificially seededsubstrates.

For this work, Si(111) and Si(100) substrates as well as fused silica substrates were cov-ered with PS spheres and SiO2 spheres of different sphere diameters D ranging from 260nm to 3550 nm. For some experiments, the spheres themselves served as seeds for theglancing angle deposition. For other experiments, the PS spheres served as shadow maskfor Au evaporation. After a subsequent sphere removal, the Au dots that formed aftercondensation of the evaporated Au flux that passed through the holes between the PSspheres remained on the substrate, thus forming either a honeycomb (SL - case) or hcp(DL - case) arranged pattern of artificial seeds, as shown in Figs. 3.5(c) and (d).

25

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3 Experimental Setup and Sample Analysis

3.3 Sample analysis

3.3.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is a technique used for the investigation and visu-alization of surface structures. The surface of the sample under observation is scannedwith a primary electron beam. The electrons are emitted out of a cathode and accel-erated towards the sample surface. Typically, the beam voltage used to accelerate theprimary electrons lies in-between approximately (0.1 − 30) kV. Depending on the beamvoltage (and the sample material), in the excitation volume, different secondary signalsare generated. For visualization purposes, the most important secondary products aresecondary electrons that are generated surface-near and backscattered electrons that aregenerated deeper in the excitation volume: secondary electrons leave the sample fromdepths down to approximately 50 nm from the surface, whereas backscattered electronscan be generated in depths down to approximately 100 nm6.

Secondary electrons are excited by inelastic scattering of the primary electrons at theelectron shell of the atoms of the sample. They are of low energy (less than 50 eV)and originate in the surface-near region, in the upper part of the excitation volume.Backscattered electrons, on the other hand, are electrons with energies of more than ap-proximately 50 eV. They are generated by elastic scattering of the primary electrons.Whereas secondary electrons mainly contain information concerning the surface topogra-phy and structure, the intensity of the backscattered electrons signal is a function of theatomic mass of the sample elements. Therefore, backscattered electrons carry informationabout the distribution of different elements in the sample. Typical detectors for secondaryand backscattered electrons are Everhart-Thornley detectors (scintillator-photomultipliersystem) and semiconductor detectors.

All samples within the frame of this work have been analyzed with a scanning electronmicroscope equipped with a tungsten Schottky field emission cathode (Carl Zeiss SMTAG, Ultra 55 with Gemini column) capable of beam voltages in the range of (0.5−50) kVat the sample surface. Out of the field emission tip, the primary electrons are emittedand accelerated along the electron tube, thereby passing several electromagnetic lenses,until they are focused on the sample surface where secondary signals are excited. Theresolution limit of the system is approximately 3 nm.

The Scanning electron microscope has two secondary electron detectors, one ring-shapedwithin the electron tube7 and another one positioned outside the electron column, pointingtowards the sample stage. Furthermore, the microscope is equipped with two backscat-tered electron detectors that can be used for elemental discrimination (in a qualitativeway).

For the cross-section micrographs that were done to analyze the morphology of theGLAD-grown Si STFs, the samples were cut right before analyzing. The beam voltagewas set to relatively low values between (1.5 − 2.5) kV in order to prevent charging ofthe sample surfaces, and the working distance between detector and sample was set to beapproximately 5 mm in all cases.

6Besides that, x-rays, Auger electrons and visible light (cathodoluminescence) can be excited by theprimary electrons as well.

7This so-called in-lens detector was mainly used for analyzing the samples within the frame of this work.

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3.3 Sample analysis

Image analysis

The data analysis of the SEM micrographs and the characterization of the sample pa-rameters in terms of e.g. structure height h or structure diameter w was done withthe computer programs ImageJ (version 1.35) [114] and SPIP (version 3.2.6.0) [115]. Insome cases, in order to obtain information about the positional correlation of (neighbored)structures deposited with GLAD on patterned and unpatterned substrates, top-view SEMmicrographs were analyzed in the reciprocal space with a two-dimensional fast Fouriertransformation (FFT) using SPIP. For a square top-view SEM micrograph with N pixelsin each dimension, the FFT is given by

FFT (fx, fy) =1

N2

N∑

x=1

N∑

y=1

I(x, y)e−2π i (xfx+yfy)/N

where fx and fy are the spatial frequency coordinates in x- and y- direction, and I(x, y)is the greyscale intensity of the SEM micrograph at position (x, y). With the FFT, thedominating spatial frequencies of the structures can be deduced.

Fig. 3.6(a) shows an example of a top-view SEM micrograph of Si nanostructures de-posited with GLAD on rapidly rotating substrates in the early stages of growth (structureheight h ≈ 13 nm). The respective two-dimensional FFT as shown in Fig. 3.6(b) shows aring-like pattern, indicating the existence of a preferred frequency range in reciprocal spacefor those structures. To quantify this, a power spectral density (PSD) function evaluationcan be done with the FFT pattern. Especially for structures with isotropic distribution(for example vertical, columnar structures deposited with GLAD and fast substrate rota-tion), it is useful to perform an angular averaged PSD(f) calculation [116]. By performingangular averaging over all spatial frequencies with constant distance f 2 = f 2

x + f 2y to

the center of the FFT image (fx = fy = 0), the PSD(f) function is

PSD(f) =1

Nf

Nf∑

n=0

(FFT (fx, fy))2

where Nf is the number n of points (pixels) at a constant distance f [117]. When the PSDfunction graph is drawn versus the spatial frequency f , the column-column separation λcan be deduced from the maximum frequency fmax.

100 nm

(a)

100 µm-1

(b)

10-3

10-2

10-1

106

107

108

PSD

[nm

4]

f [nm-1]

fmax

(c)

( = 22 nm)l

Figure 3.6: (a): Top view SEM micrograph of glancing angle deposited columnar Si structures witha column height of approximately h = 13 nm. (b): The corresponding FFT spectrum. (c): Angularaveraged PSD function graph of the FFT pattern in (b).

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3 Experimental Setup and Sample Analysis

3.3.2 Time-of-flight secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is an analyzing technique used for the detectionof secondary ions that are sputtered from the surface of the examined sample with afocused primary ion beam of some keV energy. Most of the emitted particles from thesamples surface are neutral. However, a small fraction of the particles that leave thesample surface is charged. Those ejected ions can then be mass analyzed, providinginformation on the elemental or molecular composition of the sample, as the measuredintensity corresponds to specific elements, compounds and their charge states. The massanalysis of the secondary ions can be done with different analyzers, for example quadrupoleanalyzers or - as it is the case for the present system from IONTOF - time-of-flightanalyzers.

In time-of-flight secondary ion mass spectrometry (TOF-SIMS), the secondary ions thatleave the sample are accelerated by an electric field towards the detector. As the velocityof the ion depends on its mass-to-charge ratio, different ions will need different times toreach the detector: lighter ions ejected at the same time than heavier ones will arrive atthe detector earlier, thus giving the possibility to record a mass spectrum. This techniquerequires pulsed secondary ion generation in order to have exact “starting times” of thesecondary ions at the sample surface.

If a depth profile of a sample is to be recorded, the so-called dual beam mode can beused, in which one ion beam (usually Cs+ or O+

2 ) is used to sputter a crater into thesurface of the sample, whereas the second beam (the analysing beam, for example Ga+)is used to analyze the bottom of this crater.

In the scope of this work, 2 keV Cs+ ions were used to sputter the surface of the sampleson an area of 300 µm × 300 µm, and 15 keV Ga+ ions were used for analyzing the sampleon an area of 75 µm × 75 µm in the center of the sputtered crater. When the samplesshowed charging effects, charge compensation by means of an electron flood gun was usedto suppress this effect.

3.3.3 Elastic recoil detection analysis

In the elastic recoil detection analysis (ERDA) technique, a collimated beam of highenergetic heavy ions (e.g. Au15+ with 200 MeV kinetic energy) is used to irradiate thesample under observation at grazing incidence. By this, recoil atoms are generated inelastic collision events that can leave the sample8. They can be detected energy-sensitivelyin reflection geometry, for example with ionization chamber detectors, that allow for amass separated and depth sensitive detection [118].

ERDA is a very sensitive, quantitative and non-ambiguous elemental detection method,that allows for element depth profiling of a wide range of elements [119]. Depending onthe ion mass and energy, the depth of analysis, the sensitivity and the depth resolutioncan vary for different experimental setups. All elements lighter than the projectile ioncan be detected. Using heavy ion ERDA, an increase in the ion energy leads to a largermaximum analysis depth.

8Thus, ERDA is the “complementary” analysis method to Rutherford backscattering spectrometry(RBS), a method used for elemental detection by analysis of elastically backscattered light ions (usu-ally He+).

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3.3 Sample analysis

The ERDA measurements done in the frame of this work were performed at the tandemion accelerator laboratory of the Ludwig-Maximilians-Universität München in Garching,using 200 MeV Au15+ ions with an incidence angle of 19 with respect to the samplesurface, and a detector angle of 37.4. An ionization chamber (filled with isobutane) wasused as detector. Using the computer code KonzERD [120], the energy spectra gainedwith ERDA were converted into elemental concentration depth profiles.

3.3.4 Indentation

Indentation methods are commonly used to determine mechanical properties of a sample.A tip with well-defined shape and mechanical properties, a so-called indenter, is pressedinto the sample under observation with increasing load. After reaching a maximum load-ing force Fmax

i , the force is reduced again. During this indentation process, the depth ofpenetration, i.e. the displacement hdis, as a function of the applied force Fi is recorded.The load-displacement curve gained this way can then be used to extract different me-chanical properties of the material, such as, for example, the hardness of the sample (orthin film), or the Young’s modulus E.

0.0 0.1 0.2 0.3 0.4

0

100

200

300

400

F[m

N]

hdis

[µm]

loadingcurve

unloadingcurve

S =dF

i

dhdis

(a) (b)

20 µm

100 µm

i

Figure 3.7: (a) Example of a typical load-displacement curve of a 4-turned spiral-like Si STF (structureheight h ≈ 2850 nm) with a maximum applied load Fmax

i = 400 mN (measurement done by A. Mießler[121]). (b) SEM micrographs of two different indenter types: Berkovich (top) and flat punch (bottom).

In the scope of this work, a flat punch indenter was used to record load-displacementcurves of a multitude of helical Si structures in the quasi-elastic regime (only little plasticdeformation of the STF).

In Fig. 3.7(a), an example of such a measurement on a 4-turn, spiral-like Si STF isshown. The loading curve depicts the Fi(hdis) - dependency when the indenter is pressedon the sample with increasing load until Fmax

i is reached, whereas the unloading curveshows the relaxation of the sample (decrease of hdis when Fi is decreased from Fmax

i to

29

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3 Experimental Setup and Sample Analysis

zero). The contact stiffness S, which is defined as the slope of the unloading curve at thepoint of maximum displacement [122],

S =dFi

dhdis

hmaxdis

(3.8)

can be used to extract the averaged spring constant of the nano-spirals that were com-pressed with the indenter. According to Ref. [123], if the structure number density n ofthe glancing angle deposited spirals is known (i.e. the number of spirals per µm2), thespring constant k can be determined as

k =S

Ac · n(3.9)

with Ac as contact area of indenter and STF.

The flat punch indenter used for the experiments with an UNAT9 nanoindentationmeasuring setup (asmec GmbH) has the shape of a truncated cone with an opening angleof 29.7, and a spherical surface with a diameter of 45.65 µm (and therefore a contactarea Ac ≈ 1637 µm2). This indenter geometry, as shown in Fig.3.7(b) on the bottom,has a large contact area (in contrast to more common indenter forms like, for example,the pyramidal-shaped Berkovich indenter as shown in Fig. 3.7(b) on the top) and ensuresthat a number of spiral-like nanostructures is compressed by the indenter. Using, forexample, a Berkovich indenter could lead to ambiguous results, as the tip diameter ofsuch an indenter typically extends over only (300-500) nm, and thus can easily be on therange of the structure diameters and inter-structure distances (depending on the heightof the STF under observation).

The computer-controlled indentation experiments were performed in dynamic mode,meaning that the displacement hdis is recorded during the loading and unloading cy-cle. The spiral-like STFs exhibit a quasi-elastical behavior up to a maximum forceFmax

i ≈ 400 mN10. Therefore, the contact stiffness S was determined from record-ings of force-displacement curves with different maximum applied forces Fmax

i = 100,200, 300 and 400 mN. At every value of Fmax

i , 80 different measuring points were usedfor indentation, each point in a distance of (150-200) µm from the preceding one. Asthe indentations were performed in the quasi-elastic regime, the deviations of the contactstiffnesses S for all applied forces Fmax

i did not exceed approximately 5% of the averagedvalue of S.

9Universeller Nanomechanischer Tester (universal nanomechanical testing device).10Previous experiments showed that multiple loading and unloading experiments with the flat punch

indenter at the same spot on the sample resulted in load-displacement curves with no significantdifferences, as long as Fmax

i . 400 mN.

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4 GLAD of Si on Flat Substrates

In this chapter, the growth of Si nanostructures with ion beam sputter OAD and GLADon bare planar substrates will be examined.

First, some general aspects of GLAD at TS = RT, in particular the dependency ofthe structure morphology on both, deposition time tdep and ratio ρ = r/ω of depositionrate r to substrate rotational speed ω will be shown and discussed. After those moregeneral aspects, the influence of the substrate temperature TS in the temperature rangeRT . TS . 360C on the growth of oblique angle deposited slanted columns andglancing angle deposited structures at different ρ values will be examined and discussed.Most GLAD experiments described in literature so far were done at TS = RT, arguingthat high TS values would increase the surface diffusion length of the incoming particles,which is undesired for GLAD as it counteracts the self-shadowing process1. Only fewreports concerning the influence of elevated TS on glancing angle deposited nanostructuresexist so far, for example examining the growth of Ta columnar structures on substratespre-patterned with hcp arranged nanospheres of SiO2 [43, 44, 47], or describing atypicalbehavior as the growth of Al nanowhiskers on substrates heated to TS & 290C [48, 49].In the present study, however,the influence of surface diffusivity on the growth of helicalstructures grown at low and intermediate substrate rotational speed ω (with respect tothe deposition rate) is evaluated, and it will be shown that TS influences the growth ofthe structures in terms of merging behavior, film density and diameter of the structures.

4.1 Continuous substrate rotation at RT

The deposition experiments in this section were done with a slit aperture between targetand substrate, with an opening of (15±3) mm and situated 60 mm away from the substratecenter. Therefore, the minimum angle of incidence for the incoming particle flux withrespect to the substrate normal was restricted to θmin ≈ (76 ± 3). With the nominaldeposition angle θ = 85, the resulting angular spread was ∆θ ≈ (9 ± 3). At RT,these deposition conditions resulted in vertical deposition rates r ≈ (3.5± 0.3) nm/min.

As already described in section 2.1.2, the ratio ρ = r/ω of deposition rate r to substraterotational speed ω defines the morphology of the growing structure at TS = RT. Fig.4.1illustrates the three different structure morphologies that are obtainable at different ρ -values when the substrate is continuously rotated in contrast to a two-fold chevron struc-ture that is deposited with stepwise substrate rotation2. The growth of the structures,regardless of the ρ - dependent morphology, will always start on seeds that nucleate inthe beginning of the growth process with diameters within the (20-30) nm-range [46].Keeping r constant, ρ was adjusted by changing ω. For the ion beam sputter glancing

1As a rule of thumb, the ratio of substrate temperature TS to melting point TM of the material to bedeposited should be TS/TM < 0.3 for negligible surface diffusivity [11, 27].

2Chevrons are deposited by keeping the substrate motionless while depositing each structure arm andrapidly rotating the substrate azimuthally for 180 between the pausing cycles.

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4 GLAD of Si on Flat Substrates

500 nm500 nm500 nm500 nm

(a) (b)

pausesubstrate

pause

particleflux

chevrons spirals screws columns

increase of substrate rotational speed w

increase of vertical deposition rate r

increase of = r/r w

r = 175 nm/rev r = 100 nm/rev r = 16 nm/rev

Figure 4.1: Si nanostructures grown with GLAD by ion beam sputter deposition: examples forachievable structure types. Two-fold chevrons with 8 arms grown with stepwise substrate rotation (a).Continuous substrate rotation (b): obtainable structure types according to ρ.

angle deposited Si structures studied here, it was found that a column-like morphology isobtained with ρ . 20 nm/rev. Likewise, with ρ & 120 nm/rev, spiral-like structuresare obtained. For intermediate ρ cases, screw-like structures that quickly evolve out ofseparated, single spiral-like fibers are obtained.

However, this rather broad classification can only be seen as rule of thumb, and is onlyvalid for structure heights h . 800 nm, for the following reason: during the early phasesof growth, the rate of structure extinction due to self-shadowing of adjacent structures ishigh, until a stable ratio of average column separation to average column thickness forevery film height h is established [69].

Especially for vertical, column-like structures grown with GLAD and fast substrate rota-tion, it is known that as an effect of both, column extinction and column coalescence in theearly stages of growth, the average column diameters w and column-column separationsλ increase [8, 62].

200 nm

t =10 mindept =200 mindep

200 nm

(a) (b)

Figure 4.2: Top-view SEM micrographs of glancing angle deposited films with ρ ≈ 7 nm/rev aftertdep = 10 min (a) and tdep = 200 min (b).

Under low surface diffusion conditions, after a transition from two dimensional to threedimensional island growth in the very early stages of growth (h . 1 nm) with glancing

32

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4.1 Continuous substrate rotation at RT

angle conditions, the shadowing instabilities that are a result of the highly oblique deposi-tion angle θ overpower diffusion-mediated smoothing processes [124]. This self shadowingfavors the growth of larger structures over smaller, adjacent structures in the beginning ofgrowth. Thus, the number of columns is continuously reduced as the deposition goes on,as is clearly observable in Fig. 4.2. When the surviving structures increase their lateralsize as the incoming particle flux is kept constant, the column-column separation λ in-creases as well [62]. At each growth stage, there exists a favored column-column distance.This quasi-periodic nature of the surface morphology evolution is reflected in the powerspectral density evolution of STFs that consist of vertical Si columns with increasing tdep

(and therefore increasing film height h) [125].

0 50 100 150 200 250 300

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

f max

[nm

-1]

h [nm]

0.01 0.1 110

4

105

106

107

108

109

1010

1011

10 min20 min30 min40 min50 min70 min140 min200 min

PS

D[n

m4]

f [nm-1]

f (200 min)

[ = 84 nm]max

l

f (10 min)

[ = 22 nm]max

l

Figure 4.3: (a) Power spectral density curves calculated from top view SEM micrographs of glancingangle deposited vertical Si columns with different deposition times tdep (i.e. film height h). A shift of themaximum peak positions fmax to lower frequencies with increasing h is clearly visible (inset).

Fig. 4.3 shows that from the beginning of growth (tdep = 10 min) to tdep = 200 min,the maximum peaks positions fmax in the PSD curves decrease, resulting in a shift ofthe column-column separation λ from λ ≈ 22 nm to λ ≈ 84 nm, respectively. Thisindicates that the growth of the glancing angle deposited Si nanostructures starts onseeds with lateral dimensions in the range of approximately 20 nm, and that columncompetition, extinction and merging as the deposition goes on reduces the number ofsurviving structures which in turn increase their diameters and column-column separation.

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4 GLAD of Si on Flat Substrates

The inset of Fig. 4.3 indicates that the change of the maximum peak position fmax with hhas an exponential decay form, which is consistent with similar experiments on tungstencolumns [125]. This implies an increase of λ with tdep and therefore h, that eventuallystabilizes once λ reaches the shadowing length l [62].

200 nm

column

screw

spiral

400 nm(a)

(b)

Figure 4.4: (a) ρ = 44 nm/rev: Structuremerging in the initial growth stages. Dotted cir-cle: originally separated spirals merge togetherto form screws. Ellipse: structure that stoppedgrowing due to enhanced shadowing by adjacentstructures.(b) ρ = 100 nm/rev: Structure evolution.

Although most research so far concentratedon the case of sufficiently fast substrate rota-tion (leading to columnar structures [8]), thesame effects of structure broadening and merg-ing take place under slow substrate rotationcircumstances where the evolving structuresare not vertical columns, but rather exhibit he-lical morphologies. In Ref. [126], this broad-ening effect of non-columnar, helical structures(though not sputtered, but evaporated in thecited study) is discussed in terms of a mi-croscopic growth mechanism: As each struc-ture consists of several fibers with diametersw . 10 nm, self-shadowing of fibers withinone helix is made responsible for fiber bifur-cation and, therefore, an increasing number offibers in one evolving structure during growth.The increase of the overall structure diameteris then related to this increase in the numberof fibers within one structure.

As already mentioned in section 2.2, thestructure diameter w of the surviving struc-tures increases according to a scaling relation-ship w ∝ hp, (Eq. 2.7) where p is a scalingexponent [25]. MC Simulations have shownthat this broadening effect is correlated withsurface diffusion. Low adatom-mobility con-ditions, that usually exist if TS/TM < 0.3,where TM is the melting point of the depositedelement [11, 12, 38], tend to exacerbate theincrease of w with increasing h [25]. For thecase of Si, at TS = RT, TS/TM ≈ 0.18.Therefore, the surface diffusion length Λ canbe considered to be insufficient to allow con-siderable mass transport away from the impactpoint of the particle if the experiments are donewithout extra substrate heating [124]. The in-coming particles stick close to the tops of thestructures, which in turn increase their diame-ters as the deposition continues, until neigh-bored fibers touch each other and merge toform broader structures with increased diam-eters. Especially for the case of non-columnarstructures (slow and intermediate ρ values),where the growth front of a growing structureoverlaps with already existing parts of adjacent

34

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4.1 Continuous substrate rotation at RT

structures, this diameter increase can lead to a touching of neighbored, originally sepa-rated structures and a subsequent structure merging. Here, it was found that this mergingtakes place after the first approximately 200 nm of film growth, as can be seen in Fig.4.4.

The merging effect exacerbates the overall structure broadening. Further structure ex-tinction due to the competitive growth mode in GLAD even at late growth stages canoccur as well, since slight differences in structure heights can amplify the self shadowingprocess, thereby favoring the ”survival” of some of the structures over its neighbors [69].The extinction of some of the structures is compensated by an increase of w of the sur-viving structures in order to keep the lateral density constant. Thus, as the structurediameter w increases, the ratio of the pitch c (height of the structure during one fullrotation) to w decreases, leading to a shift of the structures morphology from spiral-liketo screw-like, as can be seen in Fig. 4.4(a).

A further structure diameter increase (due to both, merging and structure extinction)leads to a further decrease of the pitch-to-diameter ratio, until finally the screw-like char-acter of the structure diminishes and a column-like structure type evolves.Therefore, it is important to notice that the morphology of the growing structures on flatsubstrates at TS = RT is not to be considered constant (except for the case of a fastsubstrate rotation with ρ . 20 nm/rev that leads to column-like structures right fromthe beginning of growth), but rather always changes from spiral-like over screw-like tocolumn-like, as can be seen in Fig. 4.4(b).

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4 GLAD of Si on Flat Substrates

4.2 Influence of the substrate temperature

For the experiments in this section, the substrate temperature TS was varied betweenRT . TS . 360C. As described in section 4.1, a slow continuous substrate rota-tion during GLAD at TS = RT will either induce the growth of spiral-like structures(ρ & 120 nm/rev) or the growth of screw-like structures that evolve out of single spiral-like fibers by diameter increase and structure merging (20 nm/rev . ρ . 120 nm/rev).In contrast to that, as will be shown in the following subsections, increasing TS will signif-icantly alter the structure morphology, as the influence of diffusion-driven processes willincrease when the ratio TS/TM of the growing film increases [9, 10, 12, 27].

4.2.1 OAD without substrate rotation

0 30 60 90

0

30

60

90tangentcosineLichter/Chenexperiment

b[°

]

q [°]

Figure 4.5: Comparison of three analytical modelslinking the column inclination angle β to the deposi-tion angle θ with the experimental β(θ) data.

As described in Chapter 2, different mod-els predict the column inclination angleβ as function of the deposition angle θfor obliquely deposited STFs. Fig. 4.5shows the experimentally gained values atTS = RT in comparison to the the-oretically obtained β(θ) dependencies ofthe tangent rule (Eq. 2.2), the cosinerule (Eq. 2.3) and the model of Lichterand Chen (Eq. 2.5). For the latter, thediffusion coefficient- dependent constantΦ was calculated from Eq. 2.5 to beΦ(RT) = 0.263 with the experimentallyobtained value β = 62 for θ = 85

and then used as constant to calculate theβ(θ) values according to Eq. 2.5. As canbe seen in Fig. 4.5, the experimentallygained values for Si follow the trends pre-dicted by the cosine rule (Eq. 2.3) withinthe range 0 ≤ θ ≤ 90. Especially for

the glancing angle (θ = 85) case, the deviation from the empirical tangent rule, Eq.2.2, is obvious, and the model of Tait [16] that predicts the cosine rule leads to morereliable results. The cosine rule is derived for the assumption of ballistic deposition un-der low adatom mobility. Thus, the slight deviation of the experimentally gained result,βexp(85) = (62±3) from the material- independent theoretical result βtheo(85) = 58

of the cosine rule is probably due to the fact that on the one hand, the case of a ballisticdeposition is not given in the experimental setup, as the sputtered particle flux has acertain angular distribution. On the other hand, the sputter deposition at TS = RTis not resembling the case of limited surface diffusion, as even at RT, under the givenexperimental parameters with UB ≈ 1200 V and αS−T ≈ 60, the Si particles reachingthe substrate have a most probable energy in the range of 10 eV . 〈ESi〉 . 15 eV [103],thus exceeding the activation energy for kinetic surface diffusion Eh of Si on a-Si, whichin Ref. [127] is found to be Eh = (2.38 ± 0.05) eV. It follows that a certain amount ofthe Si atoms that reach the substrate bring along enough energy to overcome the energybarrier for surface diffusion and can diffuse small distances away from the impact point.

3Following [5], Φ ≈ 0.2 is ”[...]representative of typical experimental conditions[...]”.

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4.2 Influence of the substrate temperature

As this diffusion is not directed, this can lead to an increase of the column inclinationβ away from the substrate normal, as predicted in the models of Hara [18] and Lichterand Chen [19]. However, as the deviation of βexp from the predicted value is small, theinfluence of surface diffusion at TS = RT is certainly not large.

Other researchers found values of β(85) = 60 for electron-beam-evaporated Si [30],and β(86) = 58 for electron-beam-evaporated SiO [128]. The experimentally gained βvalue for the oblique angle deposition case at TS = RT is therefore in good accordancewith both the model of Tait and published data.

The influence of TS on the growth of columnar Si structures with ion beam sputteroblique angle deposition was investigated in the temperature range RT . TS . 360C.

0 50 100 150 200 250 300 350 400

60

62

64

66

68

70

72

74

76

bhh

col

T [°C]

b[°

]

300

400

500

600

1000120014001600

han

dh

col[n

m]

500 nm

500 nm

1 µm

1 µm

(e)

bh hcol

(c)

(a)

(d)

(b)

Figure 4.6: Columnar Si structures deposited at TS = RT (a),(b) and at TS = 360C (c), (d). Inclination angle β, film heighth and column length hcol as function of TS (e).

Figs. 4.6(a)-(d) show thedifference between columnsgrown at TS = RT and360C. The total film height hdecreases, whereas the columninclination angle β increaseswith elevated TS. As increas-ing TS fosters the surface dif-fusion, this is in accordance toboth, the models of Hara andLichter and Chen as explainedabove. Using Eq. 2.5 it fol-lows4 that the constant Φ getsbisected from Φ(RT) = 0.26to Φ(360) = 0.13. Thus,with Eq. 2.6 the ratio of thesurface diffusion coefficients atTS = RT and TS = 360C,as L ∝ Φ−1, gets bisected aswell:

L(RT )

L(360C)≈ 0.5

This is, of course, only a roughestimation, but it clearly in-dicates that surface diffusionhas to be considered as in-fluencing factor in the OADand GLAD of Si nanostruc-tures in the temperature rangebetween RT and 360C.

The decrease of the filmheight h with increasing TS isa consequence of the increase of β. As shown in Fig. 4.6(e), the column lengthhcol = h/ cos(β) ≈ (1300± 100) nm stays nearly constant for all temperatures. There-fore, the effect of re-evaporation in the examined temperature range RT . TS . 360C

4If the initial surface perturbation h1 and the beam flux J are considered constant for all temperaturesin the investigated TS range.

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4 GLAD of Si on Flat Substrates

can be considered negligible. With the same deposition time tdep, the same amount ofmaterial is deposited in the columnar thin films.

4.2.2 GLAD with slow and intermediate continuous substrate

rotation

For the following experiments, a slow continuous substrate rotation was gained withρ = 175 nm/rev5. With this ρ value, the glancing angle deposited Si structures atTS = RT have a spiral-like morphology.

Likewise, to resemble a continuous substrate rotation with ”intermediate” substraterotational speed, ρ = 100 nm/rev6 was chosen, thus leading to screw-like structures atRT. In both cases, at TS = RT the growth starts with the evolution of spiral fibers,having diameters of approximately 20 nm. At a certain critical height, a merging oforiginally separated fibers sets in, leading to spiral-like structures with increased spiralarm diameters in the case of ρ = 175 nm/rev or to screw-like structures in the case ofρ = 100 nm/rev. As discussed in the previous section, increasing TS leads to enhancedsurface diffusion and influences the growth and morphology of oblique angle depositedcolumnar Si structures. It is therefore legitimate to assume that an enhanced TS willinfluence the growth of glancing angle deposited Si nanostructures with helical and verticalcolumn-like morphology as well.

Fig. 4.7 shows the morphological evolution of Si nanostructures with increasing TS andthe same deposition time tdep = 155 min for the cases of slow and intermediate substraterotational speed. In the following, the term hcrit will be used to indicate the critical heightat which single, non-merged spiral fibers are still observable. The trends for both, hcrit

and h with increasing TS are shown in Fig. 4.8.

Two TS regions have to be distinguished:

• In the TS range from RT to 300C, an increase of the critical height of merg-ing hcrit is clearly visible for both ρ values. At ρ = 175 nm/rev, hcrit increasesfrom hcrit(RT) ≈ 150 nm to hcrit(300C) ≈ 300 nm, and at ρ = 100 nm/rev,hcrit increases from hcrit(RT) ≈ 150 nm to hcrit(300C) ≈ 350 nm. The heli-cal fibers with diameters within the range of w ≈ (20-30) nm that emerge in thebeginning of growth stay separated over a larger thickness range with increasingTS. Likewise, with increasing TS the film height h decreases. Since the amountof deposited material is the same for all examined substrate temperatures, and asre-evaporation should be negligible even at TS = 300C as discussed for the caseof OAD in the precedent section, this film thickness decrease indicates an increaseof the films overall density. Thus, the morphology changes from broad, mergedstructures (spiral-like for ρ = 175 nm/min and screw-like for ρ = 100 nm/min)with diameters in the range of w & 100 nm7 (being a result of merging of single,adjoining spirals) with large interstructure distances at TS = RT to nonmerged,densely packed spirals at TS = 300C for thicknesses h < hcrit.

It is observable that the critical height at TS = 300C is different for both ρ values.With hcrit(300C) ≈ 300 nm, with the slow substrate rotation (ρ = 175 nm/rev)

5This value is valid for TS = RT, by using ω = 0.02 rev/min at r = 3.5 nm/min.6This value is valid for TS = RT, by using ω = 0.035 rev/min at r = 3.5 nm/min.7At the top of the structure, at structure heights h ≈ 540 nm.

38

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4.2 Influence of the substrate temperature

250 nm

500 nm

500 nm

r = 175 nm/rev

r = 100 nm/rev

TS = RT 300°C 360°C

(a)

(b)

(c)

(d)

250 nm

Figure 4.7: Temperature effect on the growth of Si nanostructures grown at ρ = 175 nm/rev(a,b) and at ρ = 100 nm/rev (c,d). (a) and (c): Cross-sectional micrographs. (b) and (d): Top-view micrographs. The dashed lines in (a) and (c) indicate the critical height hcrit for each substratetemperature TS .

only 85% of the hcrit value at ρ = 100 nm/rev are obtained. Comparing thetop-view images in Figs. 4.7(b) and (d) at TS = 300C, a remarkable differencefor both ρ cases can be seen. Whereas in the case of slow rotation, only mergedstructures and no single spiral fibers are observable at h ≈ 400 nm, in the caseof the intermediately fast substrate rotation, besides islandlike clusters of mergedspirals with diameters8 100 nm < w < 400 nm, the tops of close-packed spiralswith w ≈ (20−40) nm are clearly visible. The difference in hcrit for both ρ values isbelieved to be a consequence of the different amount of adjacent structures a single

8Here, the term diameter refers to the cluster diameter at the top of the film.

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4 GLAD of Si on Flat Substrates

0 50 100 150 200 250 300 350 400100

150

200

250

300

350

T [°C]

hcr

it[n

m]

380

400

420

440

460

480

500

520

540

560

hcrit

h

h[n

m]

(a) (b)

0 50 100 150 200 250 300 350 400100

150

200

250

300

hcrit

h

T [°C]

hcr

it[n

m]

380

400

420

440

460

480

500

520

540

h[n

m]

Figure 4.8: Influence of TS on the critical height of merging hcrit and the total film height h for (a) slowsubstrate rotation (ρ = 175 nm/rev) and (b) intermediate substrate rotational speed (ρ = 100 nm/rev).

fiber is directly able to interfere with for both, slow and intermediate substraterotational speeds and will be discussed later.Nevertheless, for both ρ cases it can be concluded that in the TS range between RTand approximately 300C, the merging of primarily separated, fibrous nanospiralsto broader structures is delayed and the critical height of merging hcrit is increased,thus leading to the growth of STFs that consist of separated, densely packed, spiral-like fibers.

• A further increase of TS from 300C to 360C results again in a significantchange of the structure morphology, accompanied by a drastic drop of hcrit: asFig. 4.7 shows, at TS = 360C, broad, screwlike structures with values ofhcrit(360C) ≈ 115 nm and diameters w between 250 nm and 500 nm do existfor both ρ cases.

Obviously it is possible to grow separated, helical nanostructures of Si with diameters inthe (20-40) nm- range using GLAD at elevated temperatures, as long as the total filmthickness h is kept below hcrit, for the cases of slow and intermediately fast substraterotations.

500 nm

(a)

(b)

t =dep 85 min 155 min 175 min

500 nm

Figure 4.9: Effect of film thickness on the GLAD growth of Si nanostructures deposited atρ = 100 nm/rev at TS = 300C. (a) Cross-sectional SEM micrographs, (b) top-view SEM micrographs.

40

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4.2 Influence of the substrate temperature

To evaluate the changes during film growth at constant TS, a set of films was grown atTS = 300C for different deposition times tdep to examine the film morphology beforeand after reaching hcrit. As in principle the effects are the same for both ρ values, this wasdone with ρ = 100 nm/rev only. The results are shown in Fig. 4.9. As can be seen, ifh < hcrit (tdep = 85 min, h ≈ 220 nm), truly separated spiral fibers without structurebroadening and merging are deposited. The onset of merging is reached for h ≈ hcrit

(tdep = 155 min, h ≈ 400 nm) and is characterized by the existence of merged clustersthat are embedded in-between some still separated spiral fibers. Finally, when h > hcrit

(tdep = 175 min, h ≈ 450 nm), the formerly separated spiral fibers are merged intoscrew-like structures with increased diameters.

4.2.3 GLAD with fast continuous substrate rotation

Similar experiments with the same deposition parameters except for ρ which is decreasedto ρ ≈ 17 nm/rev9, lead to the growth of vertical Si columns.

(a)

500 nm

10-3

10-2

10-1

109

1010

1011

RT100°C200°C300°C360°C

f [nm-1]

PSD

[nm

4 ]

0 50 100150200250300350400

115

120

125

130

135

140

145

<w

>[n

m]

TS

[°C]

500 nm

D fmax

0 50 100 150 200 250 300 350 400

360

400

440

480

h[n

m]

TS

[°C]

500 nm

(b) (c)

0 50 100 150 200 250 300 350 40020

25

30

35

40

45

n[µ

m-2]

TS

[°C]

(d) (e) (f)

(g) (h)

500 nm

Figure 4.10: ρ = 17 nm/rev: Si columns deposited at TS = RT (a,c) and at TS = 360C (b,d).Structure number density n (e), film height h (f), and average structure diameter at structure top 〈w〉(g) as function of TS . PSD function curves (h) for different values of TS , indicating the shift of fmax withincreasing TS .

9This value is valid for TS = RT, by using ω = 0.2 rev/min at r = 3.5 nm/min.

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4 GLAD of Si on Flat Substrates

Figs. 4.10 (a) and (c) show cross-sectional and top view SEM micrographs of columnsdeposited at TS = RT and Figs. 4.10(b) and (d) of columns grown at TS = 360C,respectively. It is clearly observable that the film height h decreases with increasing TS.The data given in Fig. 4.10(f) shows that with increasing TS from RT to 360C, the filmthickness decreases from h(RT) = 490 nm to 73% of this value: h(360C) = 360 nm.Moreover, the structure number density n [µm−2] decreases with increasing TS, whereasthe structure average diameter 〈w〉 (at the structures top) increases if the substrate tem-perature is raised10. Obviously, at ρ ≈ 17 nm/rev an increase of TS from RT to360C changes the structure morphology from separated columns with average diameters〈w〉 ≈ 115 nm to densely-packed, thick columns with 〈w〉 ≈ 140 nm. Evaluatingthe PSD function curves for the columnar films at each TS shows that the maximumpeak position of the spatial frequency fmax shifts to larger values with increasing TS, ascan be seen in Fig. 4.10(h). This means that the spatial wavelength λ (the column-column separation) decreases with increasing temperature, from λ(RT) ≈ 179 nm toλ(360C) ≈ 156 nm. This result is in accordance with MC simulations of the growth ofcolumnar nanostructures deposited with θ = 85 and with different numbers of diffusionsteps DS, which indicate an increase of fmax with increased diffusivity as well [23, 125].

0 100 200 300 400 5000

20

40

60

80

100

120

140

160

180

w[n

m]

h[nm]

RT100°C200°C300°C360°C

p(RT) = 0.71p(100°C) = 0.71p(200°C) = 0.70p(300°C) = 0.67p(360°C) = 0.67

Figure 4.11: ρ = 17 nm/rev: w(h) at different TS .

To evaluate the scaling exponentsp of the scaling law w = Z ·hp (Eq. 2.7), the w(h) data forall temperatures used is plotted inFig. 4.11 (the w(h) data is theaverage value of 6-10 columnar Sistructures) and fitted to the scal-ing law. The results show that theexperimentally gained p values de-viate from the theoretical predic-tions. Following the scaling growthmodel predicted in [25], p shouldspan the range between 0.3 and0.5, and should decrease with in-creasing diffusion strength. Here,it is found that at TS = RT,w(RT ) ≈ 1.52 · h0.71, whereasw(360C) ≈ 2.64 · h0.66. Thus,the p values decrease only slightly

with increasing TS (and therefore enhancing the surface diffusion), whereas the constantZ increases with increasing TS, indicating that at elevated temperatures, the structurebroadening in the early stages of growth is fostered. The discrepancies between the mea-sured p values and their theoretical counterparts might be explained taking into accounttwo effects. On the one hand, the ρ values to gain columnar structures used for the exper-iments here11 are very large compared with published data. For example, in [25] or [129],the ρ values used to deposit columnar structures are about an order of magnitude lowerthan in the present study. On the other hand, due to the principle of sputter deposition,there will be a certain deviation in the angular distribution of the flux of Si particles thatcontribute to the STF deposition. As described earlier, although the deposition angle isset to θ = 85 in the experiments, the aperture allows particles with deposition angles as

10Both, n and 〈w〉 were calculated with statistical analyses of large scale top view SEM micrographsusing SPIP [115].

11The experimental conditions do not allow for larger rotational speeds than ω = 0.2 rev/min.

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4.2 Influence of the substrate temperature

low as θ ≈ 76 to reach the substrate. Non-glancing (90 − θ) values, however, certainlyincrease p to values above p = 0.5 [129].

4.2.4 Growth model

The temperature-driven morphological changes for slow (ρ = 175 nm/rev), intermediate(ρ = 100 nm/rev) and fast (ρ = 17 nm/rev) substrate rotation described here can beunderstood considering the effects of surface diffusion.

TStdep

RT

500 nm

(a)

8 min

30 min

(d)

(b)

0.01 0.1

106

107

108

109

1010

RT, 8 minRT, 30 min300°C, 8 min300°C, 30 min

PSD

[nm

4]

f [nm-1]

(c)300°C

tdeptdep

Figure 4.12: Illustration of the surface diffusion along the sides of the single fibers and the evolutionof the fiber diameter with time at (a) low and (b) high TS . Top-view micrographs of experimentalresults at ρ = 100 nm/rev (c). Highlighted in white are already merged structures at TS = RT aftertdep = 30 min. In (d), the PSD function curves calculated from the SEM micrographs in (c) are shown.Whereas at TS = 300C, the maximum peak position remains roughly constant with tdep (dashed lines),at TS = RT there is a clear shift towards lower frequencies with increasing tdep.

In the cases of slow and intermediate substrate rotation, in the beginning ofgrowth the film consists of separated, spiral-like fibers. The top of one growing spiral-likestructure that is ”seen” by the incoming particles resembles a slanted rod. At TS = RT,as discussed above, the adatom mobility is low. The particles that reach the top of thegrowing spirals will not travel long distances along the sides of the already existing fibers,but will rather stick in close vicinity to their impact points, thus gradually increasing thespirals’ diameter. Finally, neighbored structures are broad enough to touch and mergetogether, as sketched in Fig. 4.12(a). Increasing TS to 300C enhances the adatom

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4 GLAD of Si on Flat Substrates

mobility, giving the incoming particles the possibility to travel along the sides of thespirals for a certain distance and preventing a broadening and early merging of the spiralfibers, as shown in Fig. 4.12(b).

Fig. 4.12(c) underlines this point: whereas for the case of TS = 300C, increasing tdep

from 8 min to 30 min at ρ = 100 nm/rev leads to no significant merging of the singlespiral fibers that are observable in the beginning of the growth process, at TS = RTafter tdep = 30 min merged structures with diameters in the range of w . 100 nm havedeveloped out of single spirals that exist in the beginning of the growth. According tothis, the average inter-structure separation λ stays about constant at TS = 300C withincreasing tdep from 8 min to 30 min, whereas λ increases for the same variation of tdep atTS = RT. In Fig. 4.12(d), the PSD function graphs of the top-view SEM micrographsof Fig. 4.12(c) indicate that at elevated temperatures, with increasing tdep from 8 minto 30 min the peak maxima of the spatial frequency fmax show only a slight decreaseaccording to a change in λ from λ(8min) ≈ 34 nm to λ(30min) ≈ 42 nm. Differentfrom that, for TS = RT, λ increases from λ(8min) ≈ 34 nm to λ(30min) ≈ 114 nm(although a clear PSD peak at TS = RT and tdep = 30 min is difficult to observe, theshift of the peak to lower frequencies with increasing deposition time is obvious).

The densification of the films with increasing TS, visible through the decrease of theoverall film thickness h, is also a consequence of the enhanced surface diffusion. AtTS = RT, shadowing effects are dominant compared to diffusion-driven mass transport.Thus, slight height differences of neighbored spirals and merged screw-like structuresexacerbate the shadowing effect, which in turn leads to the extinction of smaller structuresthat are overgrown by adjacent ones. The surviving structures capture all the incomingparticle flux and the resulting, underdense film consists of structures with large inter-structure-distances.

At increased temperatures, however, the diffusion-driven mass transport along the spiralfibers sides favors the growth of densely packed spirals with equal diameters. As structuremerging and extinction is delayed in this case, the STF remains dense, thus having anoverall film height h being less than in the RT case.

The fact that at elevated temperatures, the single fibers still merge together to formscrews, although only after an increased critical height hcrit as compared to the structuresgrown at RT, can be attributed to heat losses during the growth process. As the substrateis heated from the backside, the actual surface temperature of the growing film duringthe deposition process will probably be less than the substrate surface temperature itself,due to poor heat conduction along the spirals (the thermal conductivity of Si nanowireshas been found to be as much as two orders of magnitude lower than the correspondingbulk value [130, 131]) and over the interface of the substrate and the Si spirals grownon it. When the substrate temperature is increased in steps during deposition of onesample, the hcrit values can be increased as well, as is illustrated in Fig. 4.13: 60 min ofdeposition at TS = 300C, followed by deposition cycles of 30 min each at TS = 320C,340C, 360C and 380C, respectively, results in hcrit = 450 nm, which is approximately1.3 · hcrit(300C) = 350 nm. It is therefore likely that a constant temperature ofapproximately 300C at the top of the growing film during deposition would suppress themerging of the spirals to screws, thus making it possible to grow separated spirals withdiameters within the (30-50) nm range with heights only determined by the depositiontime and growth rate.

If TS is increased to values at which the adatom mobility becomes sufficiently large,enabling the diffusing particles to overcome the distances between single fibers, early

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4.2 Influence of the substrate temperature

(a) (b)

(c) (d)

500 nm 500 nm

200 nm 200 nm

Figure 4.13: Cross-sectional (a,b) and top view (c.d) SEM micrographs of glancing angle depositedSi STFs at ρ = 100 nm/rev. TS = 300C, tdep = 175 min (a,c). TS = 300C, tdep = 60 min,followed by deposition cycles at 30 min each for TS = 320C, 340C, 360C, 380C (b,d).

merging results in broad, screw-like structures. Unlike as in the case of TS = RT, wherethe shadowing effect is dominant, at elevated temperatures (TS = 360C in this case)mass transport by surface diffusion plays a significant role. Screw-like structures depositedat TS = 360C are therefore broader and denser than screws (at ρ = 100 nm/rev) andmerged spirals (at ρ = 175 nm/rev) grown at TS = RT.

The differences concerning hcrit between slow and intermediate substrate rotation canbe explained with a simple geometrical consideration: in the case of a slow substraterotation, the growing spiral fiber, if not merging to broader structures, circumscribes acylinder with a diameter of (150 ± 15) nm, as shown in Fig. 4.14(a). In the case ofintermediate substrate rotational speed, the imaginary cylinder has a smaller diameter of(85±10) nm (Fig. 4.14(b)). If the clusters the fibers start to grow off are considered to behemispherical with a diameter of approximately 30 nm, it follows that one spiral arm in thecase of the slow substrate rotation interferes in the growth of approximately 25 adjacentstructures, whereas one spiral arm in the case of the intermediate substrate rotationdirectly intercepts the growth direction of only approximately 8 neighbored structures.Thus, it seems possible that there is greater inter-structure competition in the case of theslow substrate rotation (ρ = 175 nm/rev), which can be an explanation for the earlierstructure merging as compared to the intermediate substrate rotation (ρ = 100 nm/rev)case.

It is worth noticing that a delay of structure broadening, as is reported here for thespecial case of Si deposited in a certain TS window with defined ρ values, has not beenreported yet for glancing angle deposited anorganic materials. In Ref. [126] it is stated

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4 GLAD of Si on Flat Substrates

that there were no major differences between helical Si films deposited at stationaryTS = RT and films where TS was increased between RT and 300C during deposition.However, those researchers did not perform depositions at elevated temperatures from thebeginning of growth, but gradually increased the substrate temperature from TS = RTto TS ≈ 300C. Additionally, their deposition setup consisted of an evaporation systemand larger ρ values (ρ ≈ 550 nm/rev) than applied here, making a direct comparisondifficult.

100 nm

(a) (b) (c)

Figure 4.14: Different structure diameters for (a) slow, (b)intermediate and (c) fast substrate rotation. The scale in (a) isvalid for for all micrographs.

A delay or even completeabsence of structure broaden-ing and merging effects duringGLAD has so far only been re-ported for the deposition of or-ganic films consisting of tris(8-hydroxyquinoline) aluminium(Alq3) at TS = RT [80, 132].For Alq3, no remarkable struc-ture broadening and secondanisotropy effects seem to takeplace, independent of ρ. How-ever, the researchers there fo-cused on the optical charac-terization of those STFs anddid not try to explain the ab-sence of structure broadeningand merging for glancing an-gle deposited Alq3.

Comparing slow and in-termediate substrate rotationwith fast substrate rota-tion, one finds that in the lat-

ter case, broadening and densification of the vertical columns with increasing TS can beattributed to surface diffusion effects as well. On the one hand, an increase of TS re-duces the island number density in the beginning of the growth process, especially forTS > 300C were island coarsening becomes important [133]. Thus, with fewer islandsto start growing off, the number density of the columns decreases as well. On the otherhand, less columns that start to grow will have more space to fill before they reach theadjacent structures. This broadening of the single columns is assisted by diffusion-drivenmass transport. Additionally, an increased surface diffusion at higher substrate tempera-tures increases the density within the individual columns, as intracolumnar voids will befilled by diffusing adatoms [42]. Therefore, increasing TS at the glancing angle depositionon rapidly rotating substrates will lead to the growth of densely-packed, broad, verticalnanocolums.

4.2.5 Comparison of experiment and MC simulation

Few work has been done so far on the simulation of GLAD with ρ values sufficientlyhigh to gain helical structures (slow substrate rotation). Smy et al. [69] introduced a 3Dballistic deposition simulator that allows the incoming particles to have a certain angularflux distribution ∆θ. They simulated different structure morphologies that qualitatively

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4.2 Influence of the substrate temperature

reflect the morphology of experimentally glancing angle sputter deposited structures. Thissimulation model was also used by Dick et al. [29] to simulate columnar and helical sputterdeposited GLAD films on pre-defined seed arrays. However, although this model allowssurface diffusion in form of hopping of particles from the impact point on the substrateto nearby lattice sites, in those studies the influence of the surface diffusion was limitedto fixed diffusion lengths.

As shown in the previous sections, the surface diffusion, determined by the substratetemperature TS, influences the growth of ion beam sputter glancing angle deposited Sistructures in terms of structure morphology, diameter and film density. It is thereforeinteresting to compare those experimental results with GLAD simulations that incorporatethe effect of variable surface diffusion.

Concerning the influence of TS on GLAD of Si nanostructures with continuous substraterotation and different substrate rotational velocities, the most outstanding morphologicalchanges occur at low and intermediate ρ values, such as the transit from films consisting ofmerged, broad structures to films of densely packed, separated, fibrous spirals with delayedmerging at increased TS. Simulating the influence of surface diffusion on the glancingangle deposition of STFs at slow substrate rotation ω with the MC simulation code ofKarabacak et al. [25] (as described in section 2.5) partially reflects the experimentallygained results in a qualitative way.

DS: 10 50 300 1000

Figure 4.15: Change of structure morphology for GLAD simulations with total number of depositedparticles NOP = 5 × 107 and number of particles per rotation NPR = 1 × 107 and different numberof diffusion steps DS. Top row: top view, bottom row: cross-section.

Fig. 4.15 shows the simulated evolution of helical glancing angle deposited nanostruc-tures with a deposition angle θ = 85 and different numbers of diffusion steps DS. It isobservable that similar morphological changes as seen in the experiment take place in thesimulations. Low surface diffusion (DS = 10) leads to the growth of bundled structureswithout clear boundaries, whereas high surface diffusion (DS = 300 , DS = 1000) fosters

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4 GLAD of Si on Flat Substrates

the growth of sculptured thin films that consist of separated spirals with smaller diametersas compared to the low surface diffusion case.

Just as in the experiment, the overall density of the film decreases with increasing DS,which is reflected in a decrease of the film height of approximately 28% from DS = 10to DS = 300 for otherwise constant deposition parameters12. Therefore, the simulationssupport the assumptions of the simple growth model developed in the previous sectionat least partly. Both, simulations and experiments indicate a surface-diffusion drivenmorphological change of the growing nanostructures from broad, merged structures to aSTF of densely packed, smooth spirals with equal diameters. However, the comparisonbetween experiment and simulation provides similar results only for a certain range ofTS or, respectively, DS. The experiments show that for both, slow (ρ = 175 nm/rev)and intermediate (ρ = 100 nm/rev) substrate rotational speeds, an increase of TS totemperatures above approximately 300C results in the formation of merged, broad screw-like structures with low hcrit values being in the range of hcrit ≈ 115 nm, as can be seenin Fig. 4.7. This early broadening effect, being attributed to the surface diffusion lengthbecoming sufficiently large to overcome the distances between single spiral fibers right inthe beginning of the growth process, can not be seen in the simulations. Fig. 4.15 showsthat if DS is increased from 300 to 1000 diffusion steps, the film thickness and thereforefilm density remains constant, whereas the film itself remains consisting of separated,fibrous spirals. Obviously, other effects not taken into account in the simulation butexistent in the experiments, such as the incident kinetic energy of the sputtered particles,influence the growth of the structures as well.

The experiments show the existence of a critical height hcrit(TS) at which the mergingof originally separated spiral fibers to broad, bundled structures sets in. The simulationsat high diffusion rates (DS = 300, DS = 1000) do not display those bundlingeffects in the STFs. However, for the experiments it was assumed that the existence ofa temperature gradient along the growing spirals finally leads to the merging at hcrit, ifthe real temperature at the top of the growing structure becomes insufficient to generatesurface diffusion efficient enough to prevent structure broadening. It was shown that withgradually increasing TS from TS = 300C to TS = 380C (thereby even surpassingTS = 360C, a temperature at which - if existent right from the beginning of growth -a change of the structure morphology to broad, screw-like structures can be expected),hcrit can be increased as well. If in the simulations a temperature gradient is incorporatedby gradually reducing the number of diffusion steps, a gradual change of the structuremorphology can be found, just as in the experiments.

Fig. 4.16 shows a simulation of GLAD at slow substrate rotation where the numberof diffusion steps D gradually changes from DS = 300 in the beginning of growth toDS = 10 at the end of the simulated deposition. The morphological evolution of thefilm gradually changes from isolated spirals to the onset of bundling at a certain hcrit

to a film consisting of rugged features as would be obtained if the simulation startedwith insignificant diffusion at DS = 10. The simulation with a ”diffusion gradient”qualitatively causes comparable results as the experiments do in terms of the existence ofa critical height of merging hcrit. Therefore, it seems likely that a temperature gradientalong the growing structures exists and influences the existence (and magnitude) of hcrit.

Comparing experiment and simulation, it can be concluded that the used simulationmodel with different diffusion steps, differing by one order of magnitude, fits well with

12In the experiments with ρ = 100 nm/rev, the film height decreases by approximately 30% fromTS = RT to TS = 300C.

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4.2 Influence of the substrate temperature

hcrit

Figure 4.16: Change of structure morphology for GLAD simulations with total number of depositedparticles NOP = 5 × 107 and number of particles per rotation NPR = 1 × 107 with graduallydecreasing the number of diffusion steps DS from 300 to 10. Highlighted in white: onset of bundling athcrit.

the experimental results. It has to be added, though, that the model does not allow toassign fixed DS values to fixed TS values. As will be shown in section 5.2.1 of the nextchapter, on patterned substrates, DS = 300 diffusion steps is found to resemble theexperimental results better. Nonetheless, at least the trends of the influence of surfacediffusion on the growth of helical nanostructures with GLAD are correctly reflected withthe MC simulation approach.

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4 GLAD of Si on Flat Substrates

4.3 Summary of results: GLAD on unpatterned

substrates

GLAD on planar substrates at RT

The following main results concerning the glancing angle deposition of Si with differentrotational speeds on flat substrates at TS = RT by ion beam sputter deposition canbe drawn:

• STFs comprising vertical, columnar Si structures are grown with ρ . 20 nm/rev.

– Inter-column-competition due to the shadowing effect leads to the effects ofstructure merging and structure extinction. The surviving structures increasetheir diameters according to a power law relationship. For every stage of growththere exists a distinct column-column separation λ which increases with depo-sition time. This increase of λ with tdep follows an exponential decay law withdeposition time. Those findings are in good accordance with the existing lit-erature [5, 8, 25, 62, 125].

.

• If ρ is increased to values ρ & 20 nm/rev, the resulting structure has a screw-likemorphology (helical with no open core, pitch c smaller than the structure diame-ter) that gradually develops into spiral-like once ρ approaches approximately 120nm/rev.

• However, this classification is only valid for a certain STF height region h . 800 nm,as

– for ρ & 20 nm/rev, the structure growth starts with the evolution of spiral-likefibers out of seeds with lateral sizes within the range of (20-30) nm, but

– inter-structure-competition and merging effects lead to a gradual increase ofthe structure diameter w with film height h. This w increase induces a shift ofthe structure morphology from spiral-like over screw-like to finally column-like.

OAD and GLAD on planar substrates at elevated temperatures

The following main results concerning the temperature influence on ion beam sputterglancing angle deposition of Si on flat substrates can be drawn:

• For the case of OAD (no substrate rotation):

– at TS = RT, the column inclination angle β = (62 ± 3) is explained bestwith the model of Tait et al. (Eq. 2.3);

– increasing TS from RT to 360C is followed by an increase of β from β ≈ 62

to β ≈ 72, which is in accordance with the model of Lichter and Chen;

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4.3 Summary of results: GLAD on unpatterned substrates

– although the film height h decreases with increasing TS, the column length hcol

stays approximately constant, indicating that the sticking coefficients for Si onSi are not changing significantly in the considered temperature range.

• For the case of slow (ρ ≈ 175 nm/rev) and intermediate (ρ ≈ 100 nm/rev)substrate rotation :

– the merging of adjacent, fibrous spirals with fiber diameters within the rangeof w ≈ (20− 50) nm to broader structures with w & 100 nm is delayed withincreasing TS from TS = RT to TS ≈ 300C;

– the critical height of merging hcrit thereby increases from hcrit(RT) = 150 nmto hcrit(300C) = 300 nm (ρ = 175 nm/rev) or to hcrit(300C) = 350 nm(ρ = 100 nm/rev), respectively;

– surpassing TS ≈ 300C, the critical height is reduced tohcrit(360C) ≈ 110 nm for both ρ cases;

– hcrit can be further increased by gradually increasing TS during deposition;

– the total film height h decreases by approximately 30% from TS = RT toTS = 360C, thus indicating an increase in film density.

• For the case of fast substrate rotation (ρ ≈ 17 nm/rev):

– increasing TS from RT to 360C at otherwise constant deposition parametersmonotonically decreases the film height h and areal structure number density n,which is accompanied by an increase of the average diameter (at the structurestop) 〈w〉;

– the temperature-dependent scaling exponents p of the scaling law w ∝ hp

(Eq. 2.7) were found to lie in the range of 0.67 < p < 0.71 and to deviatefrom the theoretical prediction 0.3 < p < 0.5.

• A simple growth model to qualitatively explain the experimental results concerningGLAD with different rotational speeds at elevated TS was given, based on the as-sumption that at enhanced TS diffusion-driven mass transport away from the topsof the growing spiral-like fibers delays structure broadening and subsequent merg-ing, thus explaining the growth of separated nanospirals at slow substrate rotationalspeeds at increased substrate temperatures. At high TS & 300C, the early broad-ening can be seen as a result of the adatom diffusion length becoming sufficientlylarge to overcome the inter-structure-distances.

• A comparison of the experimental results for the cases of slow and intermediatesubstrate rotation with 3D MC simulations of GLAD at slow substrate rotationwith different numbers of simulated diffusion steps DS supports the assumed growthmodel qualitatively in a certain range of diffusion steps (i.e. substrate temperatureTS), but fails to explain the experimental results at slow and intermediate substraterotation with very high TS.

The results show that on bare planar substrates, it is possible to tailor the architectureof Si nanostructures with different deposition parameters for OAD and GLAD ion beamsputter deposition setups.

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4 GLAD of Si on Flat Substrates

In OAD without substrate rotation, the column inclination angle β can be controlledwith both, deposition angle θ and substrate temperature TS.

In GLAD with substrate rotation, the substrate rotation mechanism (stepwise vs. con-tinuously) and, in case of continuous substrate rotation, the ratio ρ = r/ω of depositionrate to substrate rotational speed allow for the deposition of STFs that comprise of nanos-tructures with complex shapes. Additionally, controlling the substrate temperature TS

can result in a delay of structure diameter broadening and structure merging for helicalstructures, thus giving the possibility to control the diameter of the structures to be onthe range of a few nm.

However, the arrangement of the structures on bare substrates will always be random,and the inter-structure-distances can not be controlled easily. Therefore, in the followingchapter the influence of pre-patterned substrates on the glancing angle deposition of Sinanostructures will be studied.

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5 GLAD of Si on PatternedSubstrates

As described in chapters 2 and 4, if GLAD is performed on planar substrates, due tothe statistical nature of the deposition process the location of the growing structures onthe surface will be random [23, 24, 65], and the structure diameter and size can not beprecisely controlled [5]. There are some applications, however, that require an exact con-trol over uniformity and placement of the deposited structures, including photonic crystalapplications [6, 34, 35], or applications as artificial separation matrices for microfluidic de-vices with defined pore sizes [134–136]. It is therefore interesting to analyze how structureform, size and lateral placement can be influenced by a given substrate pattern, therebyintroducing seeds for the incoming particle flux to condense on.

Besides the model case of a tetragonal square lattice, which is well-established in GLADresearch so far [7, 28, 41], the less symmetric case of a honeycomb-like patterned substrate1

will be treated in this chapter, regarding the influences of several deposition and patternparameters (for example deposition angle θ, substrate temperature TS and seed height hS)on the morphological evolution of glancing angle deposited Si nanostructures. After dis-cussing the growth of columnar Si nanostructures on self-assembled colloidal nanospheres,finally a novel approach for substrate patterning prior to GLAD will be given, that in prin-ciple allows the glancing angle deposition of nanostructures with designed morphologies,placed with arbitrary periods and inter-seed distances on the substrate without having totake into account the design considerations described in section 2.2 of chapter 2.

5.1 Tetragonal template

The experiments in this section were done with a slit aperture between source and target,with an opening of (15±3) mm and situated 60 mm away from the center of the substrate.Therefore, the particles contributing to the growth of the STFs were centered aroundthe main deposition direction under a deposition angle θ = 85, with a deviation ofapproximately ∆θ ≈ (9 ± 3). The vertical growth rate was adjusted to r ≈ (2.5 ±0.3) nm/min. As substrates, Si wafers that were partly patterned with tetragonal seedarrays by EBL (as described in section 3.2) with different seed diameters wS and periodsP were used. The experiments were done at TS = RT.

Three basic forms of structure morphologies were deposited onto Si substrates that arepatterned with fields consisting of tetragonally arranged, artificial seeds: Chevrons, four-fold spirals and vertical columnar structures. As described in section 2.1.2, the latter isachieved with continuously rotating the substrate during deposition, while n-fold struc-tures, consisting of slanted-column-like arms are deposited using stepwise substrate rota-tion, with a motionless substrate during the actual deposition of every arm and a fast sub-

1To date, to the best of the author’s knowledge only two reports on the use of such a template for GLADare known [42, 113].

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5 GLAD of Si on Patterned Substrates

12

2

3

(a) (b) (c)

1 µm1 µm

Figure 5.1: Sketch of the three-phase rotation algorithm (a). The arrow in zone 1 indicates thedirection of columnar growth. The top-view SEM micrographs of 5-armed chevrons (h ≈ 500 nm)grown without (b) and with (c) three-phase deposition cycle show that if the substrate is kept motionlessduring the deposition of the arms (b), lateral broadening perpendicular to the direction of the incomingparticle flux leads to chain-like agglomerations of adjacent structures (highlighted in white), whereaswhen the three-phase rotation method is applied (c), this broadening and merging is less severe.

strate rotation around (360/n) between the deposition cycles. However, for the growthof chevrons and four-fold spirals, to prevent a lateral broadening (second anisotropy, asdescribed in section 2.2 [26, 137]) of the structures due to the anisotropic shadowing effect,the substrate should be rotated during the deposition of each arm in order to decouplethe direction of structure growth and the direction of incoming Si flux [138]. Severalrotation schemes for the growth of structures comprising of slanted, columnar arms thatare designed to prevent the lateral broadening phenomenon are described in literature,such as the substrate swing rotation mechanism [30], the PhiSweep scheme [139] and thetwo-phase substrate rotation [32]. All have in common that the anisotropy in the self-shadowing is broken by constantly rotating the substrate or altering the substrate posi-tion, thereby delaying the broadening effect of the single arms2. The substrate motioncycle applied here is a three-phase substrate rotation, closely related to the two-phasescheme proposed in Ref. [32].

As Fig. 5.1(a) shows, the substrate is divided in three zones and rotated continuously,but with different rotational speeds ω in each zone. Zone 1 stretches over approximately60, each part of zone 2 over approximately 30 and zone 3 over approximately 2403.For the growth of each single arm of the structures, the deposition started when thecenter of zone 1 was aligned parallel to the direction of the incoming flux. The substratethen was rotated in the shown direction, with different rotational speeds ω for each zone:ω1 = 0.01 rev/min for zone 1, ω2 = 0.04 rev/min for the zones labeled ”2” andω3 = 0.2 rev/min for zone 3. Four full revolutions have been performed for each armof the chevrons and six full revolutions for each arm of the four-fold spirals. Betweenthe deposition of each arm of the structure, the substrate was rapidly rotated with 0.2rev/min around φ = 180 for the chevron structures or, respectively, φ = 90 for thefour-fold spirals.

Using this highly asymmetric rotation scheme, the growing arms effectively receivedparticle flux from all angles due to the constant movement of the substrate. However,according to the extremely different ω values for each zone ( ω3 = 20 ω1, ω2 = 4 ω1),

2Additionally, due to the fact that the direction of the particle flux, as seen by the substrate, changescontinuously, the cited rotation schemes also alter the column inclination angle β by shifting it towardsthe direction of the substrate normal. Therefore, they also offer a possibility to tailor this structureparameter as well.

3As the increment of the stepwise rotation is 3.6, the exact angular values for zones 1, 2 and 3 are28.8, 57.6 and 244.8.

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5.1 Tetragonal template

the effective growth took place when zone 1 faced the incoming particle stream, while theconstant rotation decoupled the direction of the incoming flux from that of the structuregrowth, leading to well-separated, slanted structure arms without too much lateral broad-ening. Comparing Figs. 5.1(b) and (c), one can see that that the three-phase substraterotation cycle lowers the effect of lateral structure broadening.

5.1.1 Influence of template period

Four-fold spirals

1 µm 500 nm

w

W

1 µm

Figure 5.2: SEM micrographs of four-fold spirals deposited on a planar substrate in cross-section(left) and top view (right). The micrograph in the middle shows the difference between (arm) diameterw and lateral expansion W (diameter of imaginary cylinder circumscribing the whole structure).

Four-fold spirals with a total number of 8 arms were deposited with the three-phasesubstrate rotation cycle as explained above. Six full substrate revolutions were appliedto grow each single arm, gaining a total structure height h ≈ 2600 nm after 8 depositedarms4. Fig. 5.2 shows that if deposited on planar substrates, the typical features of GLAD-grown nanostructures can be found, such as merging and bundling of single structures tobroader entities, an overall increase of the structure (arms) diameter w and non-periodicstructure arrangement due to random nucleation. As is illustrated in the figure, dueto the fact that the structure is non-columnar, a difference has to be made concerningthe structure arms diameter w and the lateral expansion of the whole structure W , whichwould be the diameter of an imaginary cylinder that circumscribes the whole structure, or,in other words, the length of the projection of each single structure arm on the substratesurface. At the top of the structures, the arm diameter is spanning a wide range ofw ≈ (400± 150) nm in the case of an unpatterned substrate. For the applied depositionconditions, the lateral expansion of the structure is found to be W ≈ (800 ± 100) nm.

To investigate the influence of an underlying tetragonal pattern on the growth of thefour-fold spirals, patterns with seed diameters wS ≈ 260 nm, seed height hS ≈ 100 nmand different periods P = 2200 nm, 800 nm and 500 nm were used as substrates. For thegiven hS and θ values, the shadowing length caused by each artificial seed is, accordingto Eq. 2.1: l = hs · tan(θ) ≈ 1145 nm. Thus, noteworthy inter-seed-condensation andgrowth has to be expected for the case of P = 2200 nm. Likewise, as described in section

4The pitch c of the four-fold spiral, defined by the structure height after one full spiral turn, is thereforec = h/2 ≈ 1300 nm.

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5 GLAD of Si on Patterned Substrates

2.2, the design consideration for tetragonal patterns, Eq. 2.9: wS ≥ P −hS · tan(θ), thatlinks the planar seed density to the equilibrium volume density of the structures [28] is onlyfulfilled for the periods P = 800 nm and 500 nm, but not for P = 2200 nm. Thus, forthe large seed period P = 2200 nm (large in comparison to the seed height and thereforeshadowing length) one can expect a significant change in the structure morphology, asthe growing structures will broaden in an attempt to fill the space between the seeds [5].

As Fig. 5.3 shows, introducing a periodically patterned substrate for the glancing angleion beam sputter deposition of Si nanostructures with a substrate rotation scheme that isdesigned to result in a four-fold structure morphology influences the growth behavior ofsuch structures in terms of structure diameter, structure shape and periodic arrangement.Obviously, the pattern period P has a major influence on those three characteristics.

P = 2200nm

P = 800nm

P = 490nm

5 µm 5 µm-1 5 µm 5 µm

-15 µm

(a)

(b)

(c)

(d)

Figure 5.3: Four-fold spirals deposited on tetragonal patterned substrates with (a) P = 2200 nm,(b) P = 800 nm and (c) P = 500 nm, in comparison to (d) deposition on planar substrates. The scalebars in (a) are valid for (b), (c) and (d) as well. The first column shows a top view SEM micrograph of thetemplate, the second column shows the respective FFT pattern, the third column shows the structuresafter deposition in cross-section, the fourth column shows the pattern after deposition in top-view, andthe fifth column shows the respective FFT pattern after deposition.

On the one hand, as suggested in ref. [28], two seed design considerations have to bemet in order to grow structures with distinct morphologies that depend on the appliedrotation algorithm. If the pattern period P gets larger than the sum of shadowing length

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5.1 Tetragonal template

l and the seed diameter ws (Eq. 2.9), severe structure broadening sets in, resulting in anoverall structure morphology that disregards the structure shape information transferredwith the substrate rotation scheme. Additionally, if P &

√2l (Eq. 2.10), the shadowing

of the inter-seed spacing is insufficient, resulting in noticeable inter-seed growth. Botheffects can be seen in Fig. 5.3(a). As P = 2200 nm exceeds both,

√2l and l + wS by

far, the shadowing effect caused by one structure with respect to an adjacent structure isinsufficient. Therefore, as the growth continues, not only the top regions of the growingstructures are exposed to the particle flux (as would be the case for an unpatternedsubstrate), but also a large part of the side walls of the structures are constantly gatheringSi particles, thus increasing their diameter as the deposition continues. As a result, insteadof four-fold spirals, broad, pillar-like entities are growing on the seeds, intersected by inter-seed-grown structures. Decreasing the template period P , however, results in structureswhose four-fold spiral morphology is clearly visible, as can be seen in Figs. 5.3(b) and (c).

On the other hand, concerning the adaptation of the periodicity of the underlying seedpattern by the four-fold spiral structures deposited on it, not only the design consider-ations explained above have to be taken into account, but also the ratio of the patternperiod P to the lateral expansion of the four-fold spirals W . In the present case, asW ≈ 800 nm, those ratios are P/W > 1, ≈ 1 and < 1, for P = 2200 nm, 800 nm and500 nm, respectively. Comparing the two-dimensional FFT patterns5 of the patternedregions of the substrate before and after deposition, it can be seen that if P/W > 1(Fig. 5.3(a)), the tetragonal period is perfectly adopted. If P/W < 1, however, theFFT pattern after deposition (Fig. 5.3(c)) is similar to the FFT pattern that results fromfour-fold spirals grown on a template-free, bare substrate, resembling a diffuse ring (Fig.5.3(d)). For the intermediate case, (P/W ≈ 1, Fig. 5.3(b)) although the FFT patterndoes not as clearly exhibit only the spots indicating the tetragonal pattern underneathas for the case of P/W > 1, it is obvious that after deposition on the seeds, the four-fold spirals are arranged tetragonally. The evaluation of the PSD data of the ring-likeFFT patterns for the case of the flat substrate and the substrate with the small patternperiod P = 500 nm (P/W < 1) leads in both cases to maximum frequencies fmax

that indicate inter-structure-distances λ ≈ 840 nm. Although this evaluation can onlybe an estimation of the preferred inter-structure distance here (as the structures are notcolumnar and deposited with fast substrate rotation), it indicates that the preferred inter-structure distance6 lies within the range of the lateral expansion W of the structure andis therefore dependent on the deposition conditions. As W ≈ 800 nm for the four-foldspirals, artificial seeds with a period P = 500 nm do not prevent the growth competitionof the spirals grown on them, resulting in a non-periodic growth mode, disregarding theunderlying, periodically arranged seeds.

Chevrons

Similar results are gained when two-fold chevron-like structures are deposited on tetrag-onally patterned substrates. Chevrons with a total number of 8 arms were depositedwith the three-phase substrate rotation cycle as explained in the previous section. Fourfull substrate revolutions were done for each arm except for the first, were only two fullrevolutions were done and therefore half the amount of Si was deposited, in order to

5The FFT patterns are obtained with SPIP [115].6After the early stages of growth that are governed by a competitive growth of adjacent structures,

accompanied by structure merging and the cease of growth of some of the structures that are overgrownby neighbored ones.

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5 GLAD of Si on Patterned Substrates

grow structures that are symmetric with respect to their seed point. The total structureheight accumulates to h ≈ 1700 nm. On planar substrates, the lateral expansion isW ≈ (550 ± 50) nm, as depicted in Fig. 5.4(a). Again, typical growth phenomena thatoccur if GLAD is done on unpatterned substrates can be found here, such as structurebroadening and merging.

(c)

w

W

500 nm0 500 1000 1500

0

50

100

150

200

250

300

w[n

m]

h [nm]

planar substratepatterned substrate

(b)(a)

Figure 5.4: Chevrons deposited on a planar substrate in cross-section (a) and on a tetragonallypatterned substrate with period P = 500 nm and seed diameter wS = 250 nm (b). The scale in (a) isvalid for (b) as well. The slightly asymmetric growth is probably due to a misalignment of the substratemanipulator with respect to the Si target in the experiment, resulting in slightly different depositionrates r for arms pointing in opposite directions. The graph in (c) shows the development of the structurediameter w with increasing structure height h for both cases, exemplary for the highlighted structures.

On the patterned substrates, however, if the template period P is adjusted to beP ≈ W , the chevrons that evolve off the seeds are not as affected by growth com-petition, and the arm diameters w do not increase with increasing film height h, but arerather determined by both, template period P and seed diameter wS.

Fig. 5.4 exemplarily shows two chevrons, each grown on planar and, respectively, pat-terned substrate. While on the planar substrate, the diameter of the chevron arm wincreases with increasing height h, this is not the case for the chevrons grown on thepatterned substrate, where the lack of inter-structure-competition and structure mergingleads to the growth of structures that have more uniform diameters. The decrease inw for the uppermost arm of the highlighted chevron in Fig. 5.4(a) as compared to theprevious arms is a fact of the competitive growth on an unpatterned substrate: as can beseen in the figure, the chevron to the right of the highlighted one already grew to a largerheight h and has larger arm diameters w. If the deposition would not have been stoppedat this point, it seems likely that the highlighted structure would have ceased growingon account of the the adjacent chevron growing larger in all dimensions, thus graduallygathering more flux than the surrounding chevrons.

For the eight-armed chevrons investigated here, the influence of an underlying tetrag-onal template pattern is the same as for the four-fold spirals as described previously.Substrates with pattern periods P = 2200 nm, 800 nm and 500 nm were used, withseed diameters wS ≈ 250 nm and seed heights hS ≈ 130 nm, resulting in a shadowinglength l ≈ 1485 nm. Just as for the case of the four-fold spirals, if wS & P − h · tan(θ)(Eq. 2.9), which is the case here for P = 2200 nm, the resulting structures suffer severe

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5.1 Tetragonal template

broadening effects and show no distinguishable two-fold shape anymore, as can be seen inFig. 5.5(a). This is accompanied by inter-seed-condensation, as can be observed as well.Just as in the four-fold-spiral case described earlier, reducing P leads to less inter-seed-growth and a distinct (in this case, chevronic) structure shape.

2 µm 5 µm 5 µm-1

(a)

(b)

(c)

(d)

Figure 5.5: Chevrons deposited on tetragonallypatterned substrates with (a) P = 2200 nm, (b)P = 800 nm and (c) P = 500 nm, in comparison with(d) deposition on planar substrates. The scale bars in (a)are valid for (b), (c) and (d) as well. The first columnshows the structures after deposition in cross-section, thesecond column shows the pattern after deposition in top-view, and the third column shows the respective FFTpattern after deposition.

Concerning the adaptation of the pe-riodicity of the underlying templatepattern, here again the ratio P/W ap-pears to be dominating. As P/W > 1for P = 2200 nm and 800 nm, re-spectively, the period of the underly-ing pattern is preserved and the two-dimensional FFT pattern after deposi-tion indicates the existence of a tetrag-onal symmetry (Figs. 5.5(a) and (b)).For P/W ≈ 1, as it is the casefor P = 500 nm (Fig. 5.5(c)),although the FFT pattern is not asclearly indicating a tetragonal symme-try as for the cases were P > W , thechevrons arrange in a periodic man-ner on the artificial seeds of the pat-tern as well. However, with decreasingperiod from P > W to P ≈ W(Figs. 5.5(a-c)), the periodic arrange-ment of the chevrons on the seeds be-comes less distinct. In fact, for the caseof P ≈ W (Fig. 5.5(c)), the FFT pat-tern resembles a superposition of thering-like features typical for the case ofthe growth on a planar substrate (Fig.5.5(d)) and the equidistant spots indi-cating the growth on the tetragonallyarranged seeds.

Therefore, although not done in thisstudy, it seems likely that the growthof chevrons on tetragonally patternedsubstrates with P/W < 1 would re-sult in an aperiodic arrangement of thestructures, as it is the case for the four-fold spirals (Fig. 5.3(c)).

For the growth of non-columnar structures that consist of slanted arms, such as chevronsand four-fold spirals, the conclusion can therefore be drawn that not only the design rulesgiven in Ref. [28] for GLAD pattern design (Eqs. 2.8 and 2.9) have to be fulfilled in orderto deposit structures with distinct shapes and insignificant inter-seed condensation, butalso the period P of the underlying pattern has to be matched with the lateral expansionW of the as-grown structures (P/W & 1) in order to gain a periodic arrangementof the structures after deposition. Tab. 5.1 summarizes the design considerations for

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5 GLAD of Si on Patterned Substrates

P/W Recognizable periodicity Recognizable spiral/chevron structure wS ≥ P − hS · tan(θ)after deposition? after deposition? (No inter-seed-condensation?)

> 1 yes no no≈ 1 yes yes yes< 1 no yes yes

Table 5.1: Influence of template period P and lateral structure expansion W on the structure peri-odicity of four-fold spiral and chevron structures grown on the template. Additionally, the verification ofthe design rule Eq. 2.9 for the structures grown here is shown (last column).

tetragonal lattices that have to be taken into account when non-columnar structures areto be deposited with distinct morphologies in a periodic array.

Columnar structures

2 µm 5 µm 5 µm-1

(a)

(b)

(c)

(d)

Figure 5.6: Columnar structures depositedon tetragonally patterned substrates with (a)P = 1100 nm, (b) P = 800 nm and (c)P = 490 nm, (d) deposition on planar substrates.The scale bars in (a) are valid for (b), (c) and (d)as well. First column: structures after deposition incross-section, second column: pattern after depositionin top-view, third column: respective FFT pattern.

To evaluate the influence of a tetrago-nal pattern on the growth of rotationalsymmetric, vertical columnar structures,a continuous substrate rotation was usedwith ρ ≈ 100 nm/rev on tetrago-nally patterned substrates with periodsP = 1100 nm, 800 nm and 490 nm andseed diameters wS ≈ 250 nm. With aseed height of hS ≈ 100 nm, the shadow-ing length is l ≈ 1145 nm. The structureheight accumulated to h ≈ 2100 nm.

On planar substrates, as explained insection 4.1 and shown in Fig. 4.4, thesedeposition conditions result in the de-velopment of primarily spiral-like fibersin the early stages of film growth, thateventually merge to form broader, screw-like entities which in even later stages ofgrowth evolve into columnar structures,once the structure diameter w approachesvalues of w ≈ 200 nm, which is the caseafter a film height h ≈ 800 nm. As theseed diameter is wS ≈ 250 nm, on thepatterned substrates the structures there-fore develop into columnar shapes rightfrom the beginning of growth.

Analyzing the PSD function graphthat can be calculated from the two-dimensional FFT pattern of the top-viewmicrographs from the columnar struc-tures deposited onto planar substrates(Fig. 5.6(d)), a maximum frequency fmax

is gained that shows a preferred column-column-separation of λ0 ≈ 850 nm (for

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5.1 Tetragonal template

the given deposition conditions). This value is almost twice the value of the smallestperiod, P = 490 nm, for the patterned substrates used here. Consequently, althoughthe growth starts on the artificial seeds, in an attempt to reach the equilibrium stage ofthe planar substrate, severe growth competition sets in at P = 490 nm, leading to theextinction of some of the structures, accompanied by structure broadening of adjacentones, and finally to a non-periodic arrangement of the surviving columns despite the un-derlying periodic seed pattern, as can be seen in Fig. 5.6(c). The PSD analysis of thering-like FFT pattern indicates an average column-column-separation λ ≈ 1050 nm inthis case, showing that the underlying periodic seed arrangement is not adopted. If, how-ever, the template period P is increased to values P & λ0, the inter-column-competitionis reduced and the columns adopt the period of the underlying template pattern, as canbe seen in Figs. 5.6(a) and (b) for P = 1100 nm and 800 nm.

Therefore, it has to be concluded that even for the growth of circular symmetric glancingangle deposited structures, a (tetragonal) periodic arrangement (without growth compe-tition of neighbored columns) with any period P can not be obtained by introducing atemplate pattern with seeds arranged with P . If the template period is less than the inter-structure-distance that would evolve on an unpatterned substrate for the given depositionconditions, column competition will set in, resulting in structure extinction and thereforeno periodic arrangement of the surviving columns. This finding is in good agreementwith similar results of Dick et al. [140] for sputtered W films on tetragonal templateswith seed periods P being less than the average inter-structure separation of columns onplanar substrates.

5.1.2 Influence of seed diameter and control of structure diameter

The growth of glancing angle deposited Si structures on planar substrates will start withthe nucleation of seeds having diameters within the range of (20-30) nm (section 4.1, [46]).Thus, if artificial seeds are incorporated prior to GLAD whose dimensions exceed this size,the condensation and growth of multiple sub-structures on each seed can be expected. IfwS is oversized in relation to the equilibrium structure diameter found on unpatternedsubstrates, multi-structure growth on every artificial seed is likely to occur [29, 46]. If,however, the seed diameter wS is chosen to be within the range of the equilibrium structurediameter7, multi-columnar growth (i.e. the survival of more than one structure per seed)on single seeds can be avoided, and only one structure will grow on every seed [27, 28].Apart from that, wS influences the diameter w of the structures growing on the artificialseeds by setting a downward limit for w8.

Figs. 5.7(a) and (b) show four-fold spirals grown on tetragonally patterned substrateswith comparable template periods P = 690 nm and 800 nm, but different seed diameterswS = 155 nm and 260 nm. Albeit the seed diameters differ by a factor of approximately1.7, the structure diameters w (measured at the top of the structure) are comparable forboth cases.

For wS = 155 nm, a structure diameter w ≈ (550 ± 100) nm develops, and forwS = 260 nm, the resulting structure diameter is w ≈ (600 ± 100) nm. In both cases,the resulting structure diameter is larger than the diameter of the underlying seed: as

7It has to be taken in to account that especially for columnar structures, w is dependent on the actualheight of the structure [25].

8w refers to the column diameter for the case of fast continuous substrate rotation and to the diameterof the structures arm for the case of the deposition of n-fold structures, respectively.

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5 GLAD of Si on Patterned Substrates

200 nm

2 µm 2 µm

(a) (b)

(c)

0 500 1000 1500 2000 25000

400

800

1200

1600

2000

2400

wS

= 150nm

wS

= 260nm

w[n

m]

P[nm]

(d)

Figure 5.7: Four-fold spirals deposited on tetragonally patterned substrates with comparable periodsP , but different seed diameters wS : P = 690 nm, wS = 155 nm (a) and P = 800 nm, wS = 260 nm(b). Close-up micrograph of the seed region with wS = 260 nm (c). Dependency of structure diameterw on template period P for two seed diameters wS (d).

the equilibrium volume density of the respective STF on planar substrates is approachedby the structures that grow on the patterned substrate, these structures increase theirdiameters in order to ”fill” the empty volume in-between the artificial seeds [28]. Therefore,the influence of wS on the diameter w of the spirals is not as prominent as the influence ofP on w: Fig. 5.7(d) shows that for both wS values, comparable template periods P leadto comparable structure diameters w, and w always increases with increasing P : the ratioof template period to according structure diameter9 is found to be P/w ≈ (1.25 ± 0.4),although this is just a very rough approximation due to the limitation of data sets.

Although the influence of the seed diameter wS on the structure diameter w is insignif-icant in comparison to the strong influence of the template period P on w, the seed sizeinfluences the resulting structure especially in the beginning of growth. Comparing Figs.5.7(a) and (b), one can observe that for the case of the larger seed (wS = 260 nm, Fig.5.7(b)), the resulting four-fold spirals show branching and bifurcation to a higher degreeas in the case of the smaller seed (wS = 155 nm, Fig. 5.7(a)). Fig. 5.7(c) shows that theseeding zone for wS = 260 nm comprises multiple sub-structures that eventually mergeto form the actual four-fold spiral structures. Clearly, a multi-columnar growth sets inon the plateau-like seed point. After the nucleation of a number of several nuclei withdiameters within the (20-30) nm range on each seed, fiber-like structures start to grow on

9The diameter w is measured at the top of the four-fold spirals for all cases except P = 2200 nm, werethe obtained structures resemble a pillar-like morphology and the maximum ”pillar” diameter is takenas w.

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5.1 Tetragonal template

those nuclei due to the self-shadowing effect, and inter-fiber competition and merging setsin, which finally results in the singular, large structure that then incorporates multiplesub-branches and fibers. The smaller the seed, the less branching will occur, as then theinter-seed-competition is less likely to end in a significant number of surviving branchesafter the beginning of the deposition process.

Thus, although the diameters of the structures that grow on the seeds withwS = 155 nm (Fig. 5.7(a)) and wS = 260 nm (Fig. 5.7(b)) show no big difference10, inthe case of wS = 155 nm, less inter-seed branching is observable.

To control the lateral dimensions of glancing angle deposited nanostructures on tetrag-onally patterned substrates, the conclusion can be drawn that the template period P hasa major influence on the diameter w of the growing structures, whereas the seed diame-ter wS influences the inner structure of the deposited entities. The smaller wS, the lessbranching and bifurcation can be expected [27]. The diameter w of the deposited struc-tures is not limited by the seed diameter wS, but is governed by the inter-seed-spacingand therefore the pattern period P .

10The structure diameters w are mainly controlled by the template period P which is comparable forboth cases.

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5 GLAD of Si on Patterned Substrates

5.2 Honeycomb templates: experiments and MC

simulations

Tetragonally arranged seeds (as studied in the previous section) for subsequent glancingangle deposition are well-established in literature [7, 29–32, 34–36, 140–142]. In contrast tothat, the use of a honeycomb-like seed arrangement that results from the NSL patterningapproach for subsequent glancing angle deposition is not as well described and examinedto date [42, 113]. In this section, the use of such a pattern geometry for the growthof Si nanostructures with ion beam sputter glancing angle deposition will be examinedconsidering the influence of different template and deposition parameters. The mainfocus will lie on the growth evolution of columnar Si structures with respect to differentdeposition parameters (e.g. deposition time tdep or deposition angle θ) in comparison toMC simulations thereof.

Most of the experiments were done using a slit aperture between target and sub-strate, thereby allowing deviations ∆θ ≈ (9 ± 3) from the average deposition angleθ that was set to θ = 85 for the glancing angle experiments, with deposition ratesof r ≈ (3.5 ± 0.2) nm/min at TS = RT. The deposition experiments performedto study the influence of the deposition angle on the STF formation by GLAD (Sec-tions 5.2.3 and 5.2.4), however, were done without flux aperture, resulting in θ-dependentdeposition rates of r(0) = (21.5 ± 2.0) nm/min, r(35) = (17.3 ± 0.5) nm/min,r(70) = (9.6 ± 0.3) nm/min and r(85) = (5.3 ± 0.2) nm/min.

5.2.1 Morphology evolution in case of fast substrate rotation

If GLAD is done with a sufficiently fast substrate rotation (i.e. a low ratio ρ = r/ω,where r is the deposition rate and ω is the substrate rotational speed), the particle fluxthat reaches the substrate appears to be coming from every azimuthal direction [8]. Onplanar substrates, the resulting structures are rotational symmetric with respect to thesubstrate normal, and have an upright (vertical), columnar shape, as already describedin section 2.1.2. For the case of Si GLAD with ion beam sputter deposition, it was foundthat ρ . 20 nm/rev will result in such columnar structures (section 4.1). However, thesestructures are not of cylindrical shape, as their diameters w (or radii R, respectively)are not constant, but increase with increasing structure height h according to Eq. 2.7:R ∝ hp, where p is a growth exponent [143]. On patterned substrates, the evolutionof R obeys the same scaling prediction in the beginning of growth. However, R quicklysaturates into a saturation value Rsat that for tetragonal lattices is a function of the inter-seed distance d (i.e. period P ) between nearest neighbored seeds: Rsat ∝ dq, with qbeing a growth exponent [68].

The difference in the seed arrangement of both, tetragonal and honeycomb pattern,in terms of number, distance and relative position of seeds to each other, has a largeinfluence on the development of the structure morphology during GLAD with fast sub-strate rotation. Fig. 5.8(a) shows that in the case of the tetragonal pattern, if GLADis done with sufficiently fast substrate rotation and the design considerations concerningseed height hS and inter-seed distance d (Eqs. 2.10 and 2.9) are fulfilled11, the evolvingstructures exhibit a columnar shape with an almost circular cross-section. For the caseof the honeycomb-like patterned substrate, however, the situation is different, as can be

11The seed heights must be large enough to supress inter-seed-condensation, and the inter-seed distancesd must not overcome a certain value to prevent a pillar-like growth on the seeds.

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5.2 Honeycomb templates: experiments and MC simulations

seen in Fig. 5.8(b). Clearly, GLAD with fast substrate rotation leads to the evolution ofcolumnar structures with a cross-section that is best described to be triangular12.

d

d2

d1

Dd d2d

D

1 µm 500 nm 500 nm

(a) (b) (c)

Figure 5.8: Top-view SEM micrographs of (a) tetragonal seed pattern before (top) and after (bottom)deposition. The white circle and arrow indicate the saturation radius Rsat. (b): Honeycomb seed patternbefore (top) and after (bottom) deposition. The full circle and arrow indicate the saturation radius Rsat

in direction d1 of the nearest neighbor, and the dotted circle and arrow indicate the different Rsat value indirection d2. (c) Hcp seed pattern before (top) and after (bottom) deposition. Again, the white circle andarrow indicate the saturation radius Rsat. The tetragonal template (a) has a nearest neighbor distance d(i.e. period P ) of 1100 nm, with seed heights hS ≈ 100 nm. The column height is h ≈ 2100 nm. Thenanosphere diameter for the honeycomb pattern in (b) and the hcp pattern in (c) is D = 508 nm, andthe column height is h ≈ 700 nm in both cases. The seeds are Au dots with seed heights hS ≈ 50 nm.

This difference can be explained in terms of the seed layer symmetry: according to Ref.[68], in the case of the tetragonal seed design, the saturation radius Rsat of the nearlycylindrically shaped structures grows exponentially with the nearest-neighbor-distance d:Rsat ∝ dq. In the case of a tetragonal pattern, every seed is surrounded by eight adjacentseeds (four nearest and four second nearest neighbors) in a circle with a radius of

√2d,

where d is the nearest-neighbor-distance13. Thus, the growth front of the evolving struc-ture encounters nearly the same “free volume” in every growth direction. Consequently,the final structure (after reaching Rsat) has a close-to-circular cross-section.

In contrast to that, on honeycomb-like patterned substrates, every seed has onlythree nearest neighbors in a distance d1 = D/

√3, where D describes the diameter of the

nanosphere used for NSL. Unlike as in the case of the tetragonal pattern, for any single

12As will be described later, the triangular shape of the underlying seeds has no influence on the evolutionof the triangular cross-section of the columns.

13An angle of 45 is spanned between the line-of-sights from central seed to two subsequent seeds of theeight surrounding seeds in the tetragonal case.

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5 GLAD of Si on Patterned Substrates

seed point in the center of a circle with radius√

2d1, only three instead of eight seeds14

(the nearest neighbors) can be found in the honeycomb case. Thus, the shadowing effect ofthe surrounding seeds (and columns evolving of them) is not close to be multi-directionalas in the tetragonal case. On the contrary, the growing structures will encounter a strongshadowing effect in the direction towards the nearest neighbors, but will have more spaceto fill in the other directions.

d1 d2

Figure 5.9: Sketch to illustrate the devel-opment of triangular cross-sectioned columnarstructures on honeycomb patterns due to differ-ent inter-seed distances d1 and d2 = 2d1 indifferent growth directions.

The second nearest neighbors will be of mi-nor importance for the growth of the columnarstructure on the examined seed. Once the de-position starts, the line-of-sight from the con-sidered seed to a second nearest neighbor (ina distance d = D) is almost immediatelyblocked by the column that grows off the near-est neighbor, as can be seen in Fig. 5.8(b).Thus, besides the nearest neighbors, the onlysignificant shadowing effect a growing columnwill experience is given by the three third near-est neighbors. The distance from the “central”seed under consideration to those three thirdnearest neighbors is twice the distance to thethree nearest neighbors: d2 = 2d1. As a con-

sequence of these strongly differing distances from one seed to its adjacent ones in thecase of a honeycomb template, two growth directions will occur. One will be towards thenearest neighbor, in which due to Rsat,1 ∝ dq

1 [68] the growth quickly saturates, and theother in the direction of the third nearest neighbor, which is twice as far away, as depictedin Fig. 5.9. In this direction, another saturation radius Rsat,2 ∝ dq

2 [68] will develop,and consequently, the growth in this direction will saturate later. In the end, the finalcolumnar structure has a cross-section that resembles an equilateral triangle, the sidesfacing in the direction of the nearest neighbors and the pikes pointing towards the thirdnearest neighbors.

Fig. 5.10 shows cross-sectional micrographs of two different growth directions of Sinanostructures deposited with GLAD on honeycomb-like patterned seed arrays in com-parison to MC simulations run on a height matrix that resembles the honeycomb pattern.The different lateral development of the columnar structures in both cases with increasingheight h that is due to the different seed distances in different growth directions is clearlyvisible for both, experiment and simulation. Obviously, in the first direction (solid line inFig. 5.10(c)) the seeds are equally spaced (with an inter-seed distance d = D), resultingin symmetric structure growth (Fig. 5.10(a)). In the other direction (dashed line in Fig.5.10(c)), the seeds have alternating inter-seed distances d1 = D/

√3 and d2 = 2d1,

resulting in asymmetric structure growth, which is observable in Fig. 5.10(b).

A pattern arrangement with hexagonal-closed-packed arrays of seeds, that can beachieved with a double-layer NSL approach, is shown in Fig. 5.8(c). In this patterngeometry, six equidistant nearest neighbors in an inter-seed distance d = D surroundeach seed. In this highly symmetric case, with even more nearest neighbors than in thecase of the tetragonal pattern, every single seed is effectively shadowed by its nearestneighbors. Just as in the case of the tetragonal seed pattern (Fig. 5.8(a)), this results inrotationally symmetric, columnar structures with close-to-circular cross-sections.

14An angle of 120 is spanned between the line-of-sights from central seed to two subsequent ones of thethree surrounding seeds in the honeycomb case.

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5.2 Honeycomb templates: experiments and MC simulations

1 µm 1 µm 1 µm

d d2d1 d1

(a) (b) (c)

Figure 5.10: Cross-sectional SEM micrographs of Si columns deposited with GLAD and fast substraterotation on honeycomb-like arranged patterns of artificial seeds in experiment (top row) and simulation(bottom row). In (a), a cross-section that is approximately following the solid line depicted in (c) isshown, whereas in (b), the cross-section approximately follows the dashed line marked in (c).The experiments were done with the following parameters: nanosphere diameter D = 517 nm, heightof the Au seed hs ≈ 35 nm, height of the Si column h ≈ 820 nm, ratio of deposition rate to substraterotational speed ρ = r/ω ≈ 26 nm/rev, substrate temperature TS = RT. No flux aperture wasapplied. The simulations were run under the following parameter set: sphere diameter D = 70 lu, seedheight h = 15 lu, number of particles per rotation NPR = 1 × 106, number of particles depositedNOP = 1.5 × 107, and DS = 300 diffusion steps.

Thus, it can be concluded that the design of the pre-pattern has a huge influence on themorphology of glancing angle deposited structures grown on artificial seeds with fast sub-strate rotation. Albeit the structures are always growing parallel to the substrate normal,the cross-section of the columns is dependent on the symmetry of the the seed pattern.As a rule of thumb, the more nearest neighbors are equidistantly surrounding each seedpoint, the more likely is the evolving structure to be of rotationally symmetric shape.On the other hand, complex patterns like the honeycomb case studied here can evokeother shapes. When one seed point is encircled by adjacent seeds with altering inter-seeddistances, the development of different growth directions can occur. As the saturationradius Rsat of the columns is dependent on the inter-seed distance d with Rsat ∝ dq

[68], different saturation radii in different growth directions, depending on the inter-seeddistance in the respective direction, are possible. For the honeycomb-like pattern case, asconsidered here, this effect leads to the development of a triangular cross-section of thecolumnar Si structures deposited with GLAD. To underline the impact of the seed patterndesign on the cross-sectional shape of vertical, columnar structures that are deposited onthe artificial seeds with glancing angle deposition, in Fig. 5.11, structures grown on both,honeycomb and hcp patterned substrates are compared in experiment and simulation.Obviously, both, experiment and simulation corroborate the assumption that the struc-ture morphology is dependent on the seed layer design. In both cases, honeycomb-likearranged seed patterns result in columnar structures with triangular cross-sections andhcp-arranged seed patterns result in columns that have close-to-circular cross-sections.Both, experiments and simulations result in the same trends regarding the morphologicaldevelopment of glancing angle sputter deposited rod-like nanostructures on patterned sub-strates, indicating a sufficiently good compliance between the physical deposition mech-anism and the model adapted for the simulations that involves an angular distributionfunction of the sputtered particle flux, diffusion by means of particle hopping, oblique

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5 GLAD of Si on Patterned Substrates

1 µm1 µm

(a)

(c) (d)

(b)

Figure 5.11: Top-view micrographs of columnar structures obtained with GLAD under fast substraterotation on honeycomb-like (a,c) and hcp (b,d) patterned substrates in both, experiment (a,b) and MCsimulation (c,d). The insets of (c) and (d) show the respective height matrix, resembling either honeycomb(c) or hcp (d) geometry. The experiments were done with the following parameters: nanosphere diameterD = 419 nm, height of the Au seed hs ≈ 50 nm, height of the Si column h ≈ 700 nm, ratio ofdeposition rate to substrate rotational speed ρ = r/ω ≈ 26 nm/rev, substrate temperature TS = RT.The simulations were run under the following parameter set: sphere diameter D = 70 lu, seed heighth = 15 lu, number of particles per rotation NPR = 1×106, number of particles deposited NOP = 1.5×107, and DS = 300 diffusion steps.

angle deposition and substrate rotation by changing the azimuthal angle of incomingparticles sent to the substrate one by one. Nevertheless, there exist differences betweensimulations on bare planar substrates and on patterned substrates. As mentioned in sec-tion 4.2.5, it is not possible to scale the number of surface diffusion steps DS with thesubstrate temperature TS. On the planar substrates, DS = 300 diffusion steps wasin good accordance with the experimental results at TS ≈ 300C. Different from thatfinding, for the case of the patterned substrate as studied in this section, DS = 300diffusion steps resembled experimental results at TS = RT.

5.2.2 Influence of deposition time

In the previous section, it was shown that glancing angle deposition with fast substraterotation on pre-patterned substrates does not automatically entail the growth of rota-tionally symmetric columns that adopt the template period. Rather, the geometry of theunderlying seed pattern influences the morphology of the vertical, columnar structures de-

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5.2 Honeycomb templates: experiments and MC simulations

posited thereon. Especially for the case of a honeycomb-like template pattern, the specialseed arrangement fosters the growth of columnar structures with triangular cross-section.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

200 nm

Figure 5.12: Top-view micrographs of columnar structures obtained with GLAD under fast substraterotation on honeycomb-like patterned substrates at different stages of growth. The experiments were donewith the following parameters: nanosphere diameter D = 287 nm, height of the Au seed hs ≈ 45 nm,ratio of deposition rate to substrate rotational speed ρ = r/ω ≈ 7 nm/rev, substrate temperatureTS = RT. The deposition time tdep and therefore the structure height h was varied as follows: (a)reference sample (patterned substrate before GLAD), (b) tdep = 10 min, h ≈ 13 nm, (c) tdep = 20 min,h ≈ 26 nm, (d) tdep = 30 min, h ≈ 39 nm, (e) tdep = 40 min, h ≈ 52 nm, (f) tdep = 50 min,h ≈ 66 nm, (g) tdep = 70 min, h ≈ 92 nm, (h) tdep = 140 min, h ≈ 184 nm, and (i) tdep = 200 min,h ≈ 262 nm. Highlighted in (g) are a two adjacent columns with big and small diameters to emphasizethe growth competition. The scale in (a) is valid for all micrographs.

To get a more detailed impression of this growth behavior, the morphological evolution ofthe columnar structures as function of deposition time tdep and therefore structure heighth on the honeycomb seed pattern is shown in Fig. 5.12. As template, honeycomb-likearranged Au dots were deposited on Si substrates with the NSL approach as describedin section 3.2, with a PS nanosphere size of D ≈ 287 nm. With an Au seed heighthS ≈ 45 nm, the deposition angle θ = 85 results in a nominal shadowing lengthl ≈ 515 nm, thus sufficiently shadowing the inter-seed-regions.

Although the Au dots that serve as artificial seeds are of triangular shape (Fig. 5.12(a)),this is clearly of no influence on the evolution of the triangular cross-section of the columnsdeposited thereon. On the one hand, when comparing the alignment of the triangularseeds with the alignment of the triangular shaped columns in the late stages of growth(Figs. 5.12(a) and (i)), it can be seen that the respective triangular shapes of seed and

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5 GLAD of Si on Patterned Substrates

column evolving on the seed are rotated against each other by 6015. On the otherhand, in the early stages of growth, a circular cross-section of the evolving structureson the quasi-triangular seeds can be observed (Figs. 5.12(b-d)), indicating that there isno impact of the seed cross-section on that of the structure growing thereon. A clearshift of the columnar shape from circular to triangular can only be observed after thestructures reach a height h of approximately (50-60) nm (Fig. 5.12(e)). Obviously, in thebeginning of growth, the growth competition of columns that evolve on the artificial seedsis insignificant. Although inter-seed-condensation can be observed16, the largest portionof the Si particles that reach the substrate attaches to the artificial seeds that effectivelyshadow the inter-seed region. Due to the fast substrate rotation used in the experiments,with a ratio of deposition rate to substrate rotational speed ρ = r/ω ≈ 7 nm/rev,in the beginning of growth the structures on the seeds encounter no efficient shadowingby adjacent structures and consequently evolve multidirectional, thereby increasing theirradius according to R ∝ hp with increasing structure height [25] in all directions.However, as growth continues, this rapid multidirectional diameter increase eventuallystops. Whereas columns growing on nearest neighbored seeds influence their respectivediameter increase, resulting in an early saturation of growth in this direction, the columnsare still free to laterally expand versus the “inner area” of each honeycomb pattern. Ineven later growth stages the lateral expansion in this direction is stopped as well, oncethe saturation radius is reached, that is a function of the inter-seed distances according toRsat,2 ∝ dq

2 [68]. As a result, columns with a triangular shape evolve that, according tothe mentioned scaling rule, should eventually only increase in height when the depositiongoes on, after a steady state of the lateral diameter increase is reached.

On a planar substrate, when GLAD is done with sufficiently fast substrate rotation thatresults in the growth of vertical, columnar structures, the growth competition betweenadjacent structures sets in right in the beginning of growth ([124], section 4.1). Once thegrowth continues, column extinction, broadening and merging effects lead to an increasein the column-column separation λ [69, 125], which might eventually stabilize to a steady-state regime [62].

Regarding the growth of columnar structures on honeycomb-like patterned substrates,however, the situation is different. In the initial stages of deposition, on every seed pointthe growing columns encounter no growth competition effects. On the one hand, theybenefit from the initial height advantage of the artificial seed to the surrounding planarsubstrates, enabling the columns to gather the uppermost portion of the incoming particleflux. On the other hand, depending on the inter-seed distances, in the early growth stagesthe mutual influence of columns evolving on adjacent seeds can be considered negligible,permitting the columns to expand in every direction. Thus, in contrast to the case ofglancing angle deposition on bare, planar substrates, on patterned substrates the growthcompetition is delayed to later stages of growth. The onset of growth competition can beseen in Fig. 5.12(g), where two columns that grow off nearest neighbored Au seeds arehighlighted, both having significantly different diameters.

In Fig. 5.13, both the delay of growth competition and the saturation of the lateralexpansion with increasing structure height h are quantified by means of the developmentof the average structure diameter 〈w〉 and the normalized diameter deviation ∆〈w〉/〈w〉for different structure heights h. The term “diameter” used here describes the diameter

15Whereas in the case of the triangular seeds, the pikes of the triangles of every seed point to each nearestneighbored seed, for the case of the columns, the flat sides of the triangles face each nearest neighbor.

16This is mainly an artifact of the angular flux deviation of the Si particles caused by the sputteringdeposition method.

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5.2 Honeycomb templates: experiments and MC simulations

w a cylindrical column would have at a given height h, when its circular cross-sectionhas the same area A as the non-cylindrical columnar structures under observation, i.e.w = 2

A/π. The average diameter 〈w〉 and its normalized deviation are derived withSPIP [115], taking into consideration between 40 and 60 different columns for every sampledeposited with a different deposition time tdep (and, therefore, column height h).

0 50 100 150 200 250 30050

75

100

125

150

175

200

<w>

D<

w>

/<w

>[n

m]

h [nm]<

w>

[nm

]

0.0

0.1

0.2

0.3

0.4

D<w>/<w>

Figure 5.13: Development of the average columndiameter 〈w〉 and the normalized diameter deviation∆〈w〉/w with column height h for columnar Si struc-tures deposited on honeycomb-like arranged seed pat-terns (PS nanosphere diameter D = 287 nm).

As Fig. 5.13 shows, after a rapid in-crease in 〈w〉 with h, once the film heightreaches h ≈ 90 nm, the average di-ameter saturates at 〈w〉 ≈ 150 nm.This is in accordance with the abovementioned scaling rule that predicts asaturation of the structure radius at avalue Rsat that depends on the inter-seeddistances d [68]. Additionally, besidesthe development of a steady state cross-sectional area, the normalized diameterdeviation ∆〈w〉/〈w〉 increases severelywith increasing column height: whereasthe Au seeds are uniformly spaced with∆〈w〉/〈w〉 ≈ 0.04, after h ≈ 92 nm,∆〈w〉/〈w〉 ≈ 0.11, and at even laterstages of growth, at h ≈ 260 nm, thenormalized diameter deviation increasesto ∆〈w〉/〈w〉 ≈ 0.26, thus exceeding therespective value of the seed pattern by afactor of 6.5. This increase of the diameter distribution with increasing column height canbe attributed to a gradual enhancement of inter-column competition. On the one hand,until the final column diameter (which in the concrete case is 〈w〉 ≈ 150 nm) is reached,the structures increase laterally, thus gradually approaching each other. The influence ofany growth irregularity17 on the inter-column competition will therefore be of more andmore importance when the deposition goes on. On the other hand, even when a steadystate is reached and an average column diameter 〈w〉 has evolved, competitive growthof neighbored structures that started while the average column diameters increased canstill go on and might result in column extinction and, likewise, irregular column expan-sion or structure merging. An increase of ∆〈w〉/〈w〉 with ongoing deposition time tdep

can therefore be expected. With increasing the inter-seed distances (by increasing thenanosphere diameter D during substrate patterning with NSL), for otherwise constantdeposition parameters the normalized diameter deviation is likely to decrease, as the in-creased distances between adjacent growing columns lead to less dominant inter-columncompetition [42].

If the saturation value of the average structure diameter, 〈w〉 ≈ 150 nm, is normalizedwith respect to the nanosphere diameter D = 287 nm, a value of 〈w〉/D ≈ 0.52 is gained.Similar experiments18 of Si glancing angle deposition with IBSD using fast substraterotation, but other D values resulted in saturation values of the average column diameters〈w〉 ≈ 233 nm at D = 419 nm (〈w〉/D ≈ 0.56) and 〈w〉 ≈ 267 nm at D = 508 nm(〈w〉/D ≈ 0.54). Thus, the normalized structure diameters, after reaching a steady

17For example a randomly formed protrusion on a column, or local and temporal inhomogeneities in thedeposition flux that might lead to a growth advantage for single columns.

18In those experiments, the Au dot seed height was hS ≈ 35 nm, and the total column height wash ≈ 500 nm for D = 419 nm and h ≈ 695 nm for D = 508 nm, respectively [144].

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5 GLAD of Si on Patterned Substrates

state, are approximately constant. It follows that the (saturated) average diameter 〈w〉(after the initial growth stages that are characterized by lateral structure expansion) ofcolumnar Si nanostructures on honeycomb seed patterns is predictable and scales with thediameter of the nanospheres that were used for the NSL patterning with approximately〈w〉/D ≈ (0.54 ± 0.02). This is in good agreement with similar results on Ta columnssputter deposited on honeycomb seed patterns, where the normalized structure diametersare found to be 〈w〉/D ≈ (0.39± 0.02) [42]. The differences between Si and Ta columnsconcerning the 〈w〉/D values lead to the assumption that 〈w〉/D is constant if the samegrowth conditions (deposited material, deposition angle θ, deposition rate r, substraterotational speed ω) are applied, but might vary due to different geometrical and intrinsicgrowth conditions (e.g. different sticking coefficients for Si and Ta) [144].

5.2.3 Influence of deposition angle

Fig. 5.14 shows the morphological evolution of Si films grown on rapidly rotating sub-strates patterned with a honeycomb-like seed arrangement with different deposition anglesθ.

The variation of θ was done in 4 steps: besides the cases of on-axis deposition (θ = 0)and glancing angle deposition (θ = 85), θ = 35 and θ = 70 were chosen to span therange between the two extreme cases. The films with heights h between 430 nm and 490nm show remarkable morphological differences: for θ = 0, a continuous, dense film isobtained, with hillocks on top that resemble the honeycomb pattern of the substrate (Fig.5.14(a)). At θ = 35, the shadowing effect of the Au dots is still insufficient to remarkablyshadow the inter-seed-regions19, as can be seen in Fig. 5.14(b). Thus, a continuous filmtopped with honeycomb-like arranged hillocks can still be found. At θ = 70, however,the effects of shadowing dominate the growth of the films: instead of a closed film, an arrayof vertical columnar Si structures, growing on the artificial, honeycomb-like arranged seedsis deposited (Fig. 5.14(c)). Unlike as in the case of glancing angle deposition ( θ = 85),those columns are partially merged to each other, depending on the growth directionon the honeycomb pattern, and are intersected by a pyramidal-shaped layer that fills thespace between the seeds and terminates growing at some stage. A better impression of theinter-seed-grown pyramids is given in Fig. 5.1520. Finally, at θ = 85 , the glancing anglecase is fulfilled, and separated, columnar nanostructures with triangular cross-section inhoneycomb arrangement are deposited.

In order to compare the experimental observations concerning the development of nanos-tructures on rapidly rotating honeycomb-like patterned substrates with appropriate sim-ulations, the deposition on honeycomb-like height matrices was simulated with varyingthe deposition angle θ in 4 steps between 0 and 85 and applying a rapid azimuthal sub-strate rotation with a number of particles per rotation that results in separated, columnarstructures in the glancing angle deposition case (NPR = 1×106 ). The results as shownin the right column of Fig. 5.14 indicate a good consilience of experimental and simulatedresults.

Just as in the experimental case, at θ = 0 a continuous film that is topped with hillocksreplicating the honeycomb arrangement of the underlying seed pattern is developing.

19At θ = 35, the shadowing length for a seed of height hS ≈ 50 nm equals approximately l ≈ 30 nm.20There, the cutting of a sample for SEM preparation incidentally resulted in the stripping off of the

columnar structures grown on the Au seeds, whereas the pyramidal-shaped inter-seed-grown layerremained on the substrate.

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5.2 Honeycomb templates: experiments and MC simulations

500 nm

(a)

(b)

(c)

(d)

Figure 5.14: Cross-sectional micrographs of columnar structures obtained with GLAD under fastsubstrate rotation on honeycomb-like patterned substrates at different deposition angles θ in experiment(left column) and MC simulation (right column). The insets show the respective top view micrographs.The experiments were done with the following parameters: nanosphere diameter D = 419 nm, heightof the Au seeds hS ≈ 50 nm, ratio of deposition rate to substrate rotational speed ρ ≤ 105 nm/rev,substrate temperature TS = RT. Due to different deposition rates r at different deposition angles θ, thedeposition time tdep was adjusted to gain comparable structure heights 430 nm . h . 490 nm. TheMC simulations were run with the following parameters: D = 70 lu, hS = 15 lu, number of particlesper rotation NPR = 1 × 106, total number of particles NOP = 1.5 × 107, number of diffusion stepsDS = 300. the deposition angle was varied as follows: (a) θ = 0, (b) θ = 35, (c) θ = 70 and (d)θ = 85. In (c), one of the pyramidal-shaped inter-seed grown structures is highlighted. The scale in(a) is valid for all SEM micrographs.

Increasing θ to 35, the substrate-near zone of the simulated film is still continuous, andthe film is covered with features in honeycomb arrangement. At θ = 70, in goodcomparison to the experimental results, partially merged, broad, columnar structuresdevelop on the Au seeds. Those columns are intersected by pyramidal-shaped structures

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5 GLAD of Si on Patterned Substrates

growing on the substrate surface between the artificial seeding spaces. Finally, for glancingangle conditions ( θ = 85), clearly separated, vertical columnar structures that adopt theperiodicity of the template pattern evolve in the simulations, with only little condensationbetween the seeds.

500 nm

Figure 5.15: Pyramidal-shaped inter-seed grown layer at θ = 70 on honey-comb template pattern.

Evidently, three different cases of the depositionangle θ result in different structure (or, respectively,thin film) morphologies on the honeycomb-like pat-terned substrates: a low incidence case (realizedwith θ = 0 and θ = 35), an intermediate inci-dence case (realized with θ = 70), and a glanc-ing angle incidence case (realized with θ = 85).

At θ = 0 and θ = 35, both experiments andsimulations show the development of a continuousfilm that is capped with dots, arranged in a hon-eycomb pattern and replicating the Au dot matrixunderlying the deposited film. In contrast to sim-ilar experiments with parallel ballistic particle flux

on pre-patterned substrates at θ = 0 [78, 145], no fan-like aggregation that is reportedthere can be found here. This remarkable difference between thin films grown with parallelballistic flux (i.e. evaporation with a large source-substrate distance, thereby minimizingthe angular deviation ∆θ of the deposition angle) and the sputtered particle flux used inthe present experiments and MC simulations indicates that the angular distribution ofthe incoming particles plays an important role in thin film depositions on patterned sub-strates. Evidently, the non-ballistic particle flux of the ion beam sputter deposition setupfosters the formation of a continuous layer, topped with hillocks over the initial patternseeds. Although at θ = 35, both simulation and experiment still result in dense filmscapped with periodically arranged hillocks, the enhanced agglomeration of the Si particlesat the Au seeds (as compared to the case of θ = 0) is reflected in the increased diameterb of the circular dots that are arranged atop the closed film. At θ = 0, the averagemound diameter (measured with SPIP [115]) is approximately b ≈ (48± 6) nm, in con-trast to b(θ = 35) ≈ (126± 8) nm. In the simulations, the film development at θ = 0

and θ = 35 is qualitatively the same as in the experimental observations, although aquantitative comparison is difficult, as for example the ratios hS/D of seed height hS tonanosphere diameter D are not the same in experiment in simulation. Nevertheless, thedevelopment of continuous films that are capped with surface mounds arranged just asthe underlying honeycomb pattern for sputter deposition at low deposition angles θ is alsoevident in the simulations. The diameter b of those mounds in the MC simulation case atθ = 35 is approximately twice the diameter of the respective mound-like features thatblanket the film simulated at θ = 0.

At θ = 70, the effect of shadowing becomes more dominant, favoring the agglomerationat the seeds of the patterned substrate and leading to the development of columnarnanostructures that are intersected by pyramidal shaped structures in-between the seeds.

Unlike as in the glancing angle case ( θ = 85), however, those columns are partiallymerged to each other, depending on the growth direction on the honeycomb pattern.This can be understood as follows: on a honeycomb-like template pattern, the inter-seeddistances are different in different lateral growth directions. The dominating inter-seeddistances that govern the lateral column expansion are the distance from one seed toeach of its three nearest neighbors, d1 = D/

√3, and the distance from one seed to

its three third nearest neighbors, d2 = 2d1 (section 5.2.1). In the case of θ = 70,

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5.2 Honeycomb templates: experiments and MC simulations

(a) (b) (c)

(d) (e)

(f)

500 nm

500 nm

h1

h2

Figure 5.16: Top-view SEM micrographs of Si nanostructures deposited with a deposition angleθ = 70 on honeycomb-like patterned substrates (nanosphere diameter D = 419 nm, Au seed heighthS ≈ 50 nm) at different total film heights h ≈ 450 nm (a), h ≈ 670 nm (b) and h ≈ 1200 nm (c).Highlighted are examples of the remaining pores. The scale in (a) is valid for (a-c). A cross-sectionalSEM micrograph of the same film as in (c) is shown in (d). The height h1 ≈ 585 nm is the approximateheight of merging of nearest neighbors, and h2 ≈ 1060 nm indicates the height of merging of structuresthat are third nearest neighbors. In (e) and (f), top view micrographs of MC simulated films depositedon a honeycomb-like arranged height matrix (respective sphere diameter D = 70 lu and seed heighthS = 15 lu) are shown. The simulations were run under a deposition angle θ = 70, with a number ofparticles per rotation NPR = 1 × 106 at different film heights, i.e. total number of deposited particlesNOP = 1.5 × 107 (e) and NOP = 3.0 × 107 (f).

the self-shadowing effect that would lead to the growth of separated structures in theglancing angle case (θ & 80) is less pronounced, and consequently the radius R of thegrowing columnar structures is not saturating. The structures increase their diameter withincreasing height, until they eventually touch and merge together. Due to the differentinter-seed distances d1 and d2 = 2d1 between nearest and third nearest neighbors, themerging of one particular structure to its nearest neighbors is starting at an earlier stage ofgrowth (h1) as the merging between the observed structure to the third nearest neighborsdoes, that will not set in before a structure height h2 > h1 is reached. As a consequence,when the deposition takes place on honeycomb-like patterned substrates, there exists arange of structure heights h1 < h < h2 in which the columns that evolve on the seedsare already grown together in direction of the nearest neighbors, but not in the directiontowards the center of every honeycomb pattern of the templated substrate. Thus, in thisheight range, the developing film resembles a matrix of hcp arranged nanopores with porediameters wpore that gradually decrease with increasing structure height, until finally a

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5 GLAD of Si on Patterned Substrates

totally closed layer is lying atop the STF. This can be seen in Figs. 5.16(a-c) for thecase of a honeycomb-like patterned substrate (nanosphere diameter D = 419 nm, Auseed height hS ≈ 50 nm). With increasing the structure height h from h ≈ 450 nm(a) to h ≈ 670 nm (b) to finally h ≈ 1200 nm (c), the pore diameter wpore decreasesfrom wpore ≈ 85 nm to wpore ≈ 30 nm, until finally the pores are closed whenh & h2. As can be seen in Fig. 5.16(d), h1 ≈ 585 nm and h2 ≈ 1050 nm21. Figs.5.16(e) and (f) show respective MC simulations (in top view) at different total numbersof deposited particles NOP (i.e. different structure heights). A qualitative conformity ofexperiments and simulations is obvious: when NOP is doubled from NOP = 1.5 × 107

to NOP = 3 × 107, the diameter of the hcp arranged pores decreases.

The pyramidal-shaped inter-seed grown film at θ = 70 (Figs. 5.14(c) and 5.15) is adirect consequence of the reduced shadowing effect of the Au seeds as compared to the caseof θ = 85. At θ = 70, the shadowing length of the seeds is l ≈ 137 nm. Therefore, theradial space between six honeycomb-like arranged Au dots is not completely shadowed,with the center of this circle gaining the most flux. As growth continues, the columnsthat grow on the seeds increase their diameter and gather most of the flux, which in turnis then unable to reach the regions between the seeds any longer, until finally the growthof those inter-seed pyramids terminates.

At θ = 85, the shadowing length of the Au seeds is sufficient enough to shadowthe whole region between the seeds (l ≈ 570 nm). In this case, vertical columns with atriangular cross-section grow on the honeycomb-like seed pattern (Fig. 5.14(a)). However,unlike as in the case of θ = 70, the diameter of the columns saturates at a certain point,without merging of the separated structures22. The inter-seed growth can be attributedto non-directional particle flux. Again, the same trends as in the experiments can be seenin the MC simulated deposition.

5.2.4 Series of different deposition angles

In the previous section it was shown that deposition on honeycomb-like patterned sub-strates with a deposition angle of θ = 70 and sufficiently fast substrate rotation resultsin partially merged, columnar structures that evolve on the seeds and join together withtheir nearest neighbors. In a certain structure height region that basically depends on theinter-seed distances (which are determined by the diameter D of the nanospheres usedfor the NSL process), the columns will only merge to their nearest neighbors, but notyet in the direction of the third nearest neighbors, leaving open hcp arranged pores withgradually decreasing pore diameters as the deposition continues. Such a pore matrix withpore diameters dpore ≈ 30 nm could, for example, be used as mask aperture for ion im-plantation at specific sites, if the ions would penetrate the substrate only underneath theopen pores and would get absorbed in the layer of partially merged columns otherwise.

However, such an application would require the substrate area in-between the seedsto be as bare as possible. If the deposition is started at a deposition angle θ = 70

right from the beginning of growth, the shadowing effect of the seeds is insufficient toeffectively shadow the substrate region between the artificial seeds. This leads to a layerof pyramidally shaped, inter-seed grown structures as shown in Fig. 5.15. To circumventthis effect, a combination of different deposition angles can be used.

21With different nanosphere diameters, the values h1 and h2 are likely to alter, as the increase of thestructure radius R that influences h1 and h2 is a function of the inter-seed distances d1 and d2 [68].

22The saturation radius is found to be Rsat ≈ 117 nm for the nanosphere diameter D = 419 nm [144].

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5.2 Honeycomb templates: experiments and MC simulations

q1

q1

q2

hS

dS

Rsat

H

(a) (b) (c)

substrate

Figure 5.17: Deposition principle with variation of the deposition angle θ on honeycomb-like pat-terned substrates with inter-seed spacing dS and seed heights hS . When GLAD is started with θ1 thatfulfills dS < hS · tan(θ1), no inter-seed condensation will occur (a), and columnar structures will evolvethat quickly develop a saturation radius Rsat (b). When the deposition is continued with a depositionangle θ2 < θ1 after a height H of the columns is reached, the column radius will broaden due to theabsence of glancing angle conditions, but still no inter-seed growth will occur if dS −2Rsat < H ·tan(θ2).

Fig. 5.17 shows the principle of this deposition approach. In a first step, the depositionis done with a combination of (glancing) angle θ1, seed height hS and maximum inter-seed spacing dS (i.e. dS = D, where D is the nanosphere diameter) that allows for atotal shadowing of the substrate area between the seeds according to a shadowing lengthl = dS < hS · tan(θ1).

If chosen to be θ1 ≈ 85, in this glancing angle case with fast substrate rotation columnswill grow off the seeds that increase their radii R with increasing structure height haccording to a scaling law [25]. However, as the deposition takes place on a pre-patternedsubstrate, this increase in R with h quickly saturates and a saturation radius RSat evolvesthat is a function of the inter-seed distances d [68]. Thus, no column broadening effects willbe observed, and columnar structures with constant diameters (after reaching a saturationvalue thereof) will grow when the deposition time is increased. Decreasing the depositionangle at a later stage of growth to θ2 = 70 (after the already grown columns withsaturated radii Rsat have reached a column height H), changes the growth mode. Columnbroadening will lead to a subsequent merging of nearest neighbored structures, and afilm consisting of hcp arranged pores will develop, if the deposition is stopped before acolumn merging of third nearest neighbors sets in as well. In contrast to the case asdescribed in the previous section, were at θ = 70 the growth of such a porous layer isaccompanied by the development of a pyramidal-shaped film that fills the space betweenthe artificial seeds, in the case described here, no inter-seed condensation will set in whenthe deposition angle is decreased from θ1 = 85 to θ2 = 70 if the following equationis fullfilled: dS − 2Rsat < H · tan(θ2). In this case, the shadowing effect of the alreadyexisting columns on the seeds is sufficient enough to shadow the whole inter-seed areawhen the deposition angle θ is decreased.

Fig. 5.18(a) shows an example of such a porous layer with only sparse inter-seedcondensation (which is probably due to non-directional Si flux) in cross-section. Withan initial seed height hS ≈ 100 nm, the shadowing length at θ1 = 85 isl(θ1) ≈ 1145 nm > D = 419 nm, where D is the size of the nanosphere used for NSL.In contrast to that, if the deposition would have started with θ2 = 70, the shadowinglength would then have been l(θ2) ≈ 275 nm < D, and pyramidally shaped inter-seedstructures would have evolved. Here, θ1 = 85 is not changed to θ2 = 70 only until astructure height H ≈ 730 nm is reached. As the saturation radius is Rsat ≈ 130 nm in

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5 GLAD of Si on Patterned Substrates

H

Rsat

q1 = 85°

q2 = 70°

(a) (b) (c)

(d)

500 nm

2 µm

Figure 5.18: Sequence of two deposition angles, θ1 = 85 and θ2 = 70 on honeycomb-likepatterned substrates in experiment (nanosphere diameter D = 419 nm, seed height hS ≈ 100 nm,ρ ≈ 10 nm/rev, TS = RT, cross-sectional (a) and top-view (c) SEM micrographs) and MC simulation(nanosphere diameter D = 70 lu, seed height hS = 15 lu, number of diffusion steps DS = 300, numberof particles per rotation NPR = 1 × 106, in cross-section (b) and top view (d)). No pyramidal-shapedinter-seed condensation can be observed.

this case, dS − 2Rsat < H · tan(θ2) (154 nm < 2005 nm) is fulfilled, and no pyramidal-shaped inter-seed condensation occurs. Nevertheless, due to the decrease in θ from 85

to 70, after reaching the height H the column diameter broadens and merging sets in,which in turn leads to the evolution of a porous layer with hcp arranged pores, as can beseen in Fig. 5.18(c). The pore diameter decreases with increasing deposition time untileventually, a dense layer is capping the STF underneath.

A comparison of the experiment with a MC simulation of the respective deposition withchanging the deposition angle θ during deposition on honeycomb-like arranged substratesgains results that are qualitatively comparable, as Fig. 5.18 shows.

5.2.5 Influence of rotational speed

In section 4.1 the influence of the ratio ρ = r/ω of vertical deposition rate r to substraterotational speed ω on unpatterned substrates was discussed. In accordance with litera-ture, GLAD with continuous substrate rotation enables the deposition of either vertical,columnar structures (fast substrate rotation, low ρ values) or helical structures (screw-like for intermediate ρ values and spiral-like for large ρ values, respectively) [8]. For thespecial case of Si ion beam sputter glancing angle deposition, on bare, planar substratesthe ρ values at which a gradual shift of structure morphology appears are ρ ≈ 20 nm/rev(columnar → screw-like) and ρ ≈ 120 nm/rev (screw-like → spiral-like). Apart fromthis, the structure shape can gradually change during deposition from helical to columnar,once the structure diameter w that increases with increasing deposition time tdep [25] getslarger than the pitch c of the (formerly) helical structure. As discussed in section 5.6, onpatterned substrates the structure diameter w is ruled by the inter-seed distances as wellas by the size of the artificial seeds. Even in the very early stages of growth, w will inmost cases be at least one order of magnitude higher as compared to the case of the planarsubstrate [146]. Thus, the pitch-to-diameter ratio is different for the GLAD growth ofnanostructures on patterned substrates in comparison to unpatterned ones, and thereforethe ρ ranges that lead to either columnar or helical structures will be different for bothcases. It follows that the deposition conditions needed to deposit helical nanostructureswith GLAD and continuous substrate rotation are different for the cases of planar orpatterned substrates.

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5.2 Honeycomb templates: experiments and MC simulations

500 nm 500 nm500 nm

(a)

(b)

(c) (e)

(f)(d)

Figure 5.19: Cross-sectional micrographs of columnar and helical structures obtained with GLADunder continuous substrate rotation on honeycomb-like patterned substrates in experiment (a-c) andsimulation (d-f). The experiments were done with the following parameters: nanosphere diameter D =419 nm, height of the Au seeds hS ≈ 50 nm, substrate temperature TS = RT. The ratio of depositionrate to substrate rotational speed ρ = r/ω was varied as follows: (a) ρ = 100 nm/rev (fast rotation),(b) ρ = 180 nm/rev (intermediate rotation), (c) ρ = 270 nm/rev (slow rotation). The MC simulationswere run under the following parameter sets: sphere diameter D = 70 lu, seed height hS = 15 lu,DS = 300 diffusion steps. Different ρ values were simulated by changing the number of particles perrotation (NPR) as follows: (a) NPR = 1 × 106, (b) NPR = 3 × 106, (c) NPR = 5 × 106.

As Fig. 5.19 shows, just as in the case of the unpatterned substrates, an increase of theratio ρ = r/ω induces a morphological change of the grown structures on the honeycomb-like patterned substrates from a vertical, columnar morphology over a screw-like one tofinally a spiral-like structure shape. As discussed above, on the patterned substrate thetotal values of ρ that result in the different morphologies are differing from the respectivevalues that are valid for bare, planar substrates.

Columnar structures evolve at ρ = 100 nm/rev, screw-like structures atρ = 180 nm/rev, and spirals are deposited with ρ = 270 nm/rev on top of theAu seeds.

Obviously, increased ρ values as compared to the GLAD growth with continuous sub-strate rotation on planar substrates are needed on the honeycomb-like patterned sub-strates in order to gain the respective structure shape. This is a result of the increaseddiameter w of the structures grown on a patterned substrate in comparison to thosegrown on a planar substrate. Without template pattern, the GLAD-grown structurestypically evolve out of fibers that have diameters w ≈ 30 nm, and eventually mergetogether to form broader structures. On patterned substrates, however, the diametersof the structures that grow on the artificially provided seeds are larger as in the case ofthe unpatterned substrate. In the present case, honeycomb patterned substrates withnanosphere diameters D = 419 nm are studied. The saturated diameter of the struc-tures growing on the Au dots is approximately 〈w〉 ≈ 233 nm (section 5.2.2, [144]).Under such circumstances, in order to keep the ratio of the pitch c (that is, the heightof the nanostructure after one full substrate revolution) to the structure diameter w highenough to still be able to recognize the screw- or spiral-like morphology, ρ = r/ω has tobe increased in comparison to the case of the unpatterned substrate.

Comparing the experimental results with MC simulated glancing angle deposition onhoneycomb templates with different values of the number of particles per rotation NPR

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5 GLAD of Si on Patterned Substrates

shows that comparable results are gained in both, experiments and simulations. As thelower row in Fig. 5.19 shows, just as in the experiment a change of ρ induces a morpho-logical shift from columnar over screw-like to spiral-like structures, changing NPR in thesimulations (NPR = 1 × 106, 3 × 106, 5 × 106) shifts the nanostructure morphology inan equal manner. A fast rotation is resembled by low NPR values, thus leading to up-right, columnar structures. Increasing NPR leads to a particle flux that circularly rotatesaround the seeds (as seen from the substrate). Therefore, screw- or spiral-like structureswhose ends “follow” the direction of the incoming flux will evolve.

The ratios of the experimental ρ and simulated NPR values that result in differentstructure morphologies are of the same order of magnitude (ρ(spiral)/ρ(column) ≈ 3,NPR(spiral)/NPR(column) = 5). This indicates that the used simulation parametersare well suited to reflect the experimental results.

For the case of glancing angle deposition on patterned substrates with continuous sub-strate rotation at different values of deposition rate to substrate rotational speed ρ = r/ω,it can be concluded that as in the case of an unpatterned substrate, the deposition of both,columnar and helically shaped structures is possible. However, due to the increased struc-ture diameters w on patterned substrates with respect to planar substrates, the absolutevalues of ρ needed to deposit a specific structure morphology are shifted to larger values.

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5.3 Nanospheres as hcp arranged seeds for GLAD

5.3 Nanospheres as hcp arranged seeds for GLAD

As described in chapter 3, self-assembled nanospheres on the substrate surface can be usedas periodically arranged, specific nucleation sites for GLAD directly, then resembling ahcp arranged pattern with a seed period P that equals the nanosphere diameter D.

The deposition experiments on such templates presented in this section were done witha slit aperture between target and substrate, with an opening of (20±3) mm and situated60 mm away from the center of the substrate. Therefore, the minimum angle of incidencefor the incoming particle flux with respect to the substrate normal was restricted toθmin ≈ 90 − arctan(20/60) = (72 ± 3). With the nominal deposition angle θ = 85,∆θ ≈ (13± 3). At RT, these deposition conditions resulted in vertical deposition ratesr ≈ (4.3± 0.3) nm/min. The experiments were done with ω = 0.2 nm/rev to resemblea fast substrate rotation with ρ ≈ 21 rev/min.

5.3.1 Influence of sphere diameter

To evaluate the influence of the sphere size D (and therefore of the area of the seed)on the growth of vertical, columnar Si nanostructures, hcp arranged polystyrene (PS)nanospheres with diameters D = 260 nm, 500 nm, 760 nm, 2550 nm and 3550 nm wereused as templates. The experiments were done at TS = RT for tdep = 150 min, yieldinga Si column height h0 ≈ 650 nm on unpatterned substrates.

1 µm 1 µm

1 µm1 µm

(b)

(d)

(a)

(c)

Figure 5.20: Example of a substrate patterned with hcp arranged PS nanospheres (sphere diameterD = 260 nm) before deposition (a). Top view micrographs of Si nanocolumns grown on (b) D = 260 nm,(c) D = 760 nm and (d) D = 3550 nm spheres. Columns with both, small and large diameters arehighlighted in (b), and some of the almost separated, column-like structures that contribute to the wholeSi column on spheres with large diameters D = 3550 nm are highlighted in (d).

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5 GLAD of Si on Patterned Substrates

Fig. 5.20 shows top view SEM micrographs of Si nanocolumns grown on hcp arrangednanospheres with different D values. Obviously, the hcp arrangement of the underlyingself-assembled nanosphere layer is adopted by the Si columns grown on it, indicating thepossibility to grow lateral periodically arranged columnar Si structures with this method.As is also observable, for instance in Figs. 5.21 and 5.22, the inter-column competitionthat is prominent on unpatterned substrates, leading to effects such as structure extinctionand broadening of the structure diameter w with increasing h [8, 25], is initially absenthere, as the artificial seeds have equal size and spacing [29, 40, 47]. The column diameter istherefore primarily defined by the size of the PS sphere and stays approximately constantwhen the structure height h is increased, in contrast to the common finding that on flatsubstrates, glancing angle deposited columnar structures show column broadening withincreasing h [25].

1 µm

whtot

h

hcap

D

a

1 µm 1 µm

(a) (b)

(c) (d)

F

Figure 5.21: (a) Illustration of the basic parameters to describe GLAD on nanospheres. (b)-(d)Cross-sectional SEM micrographs of Si nanocolumns grown on nanospheres with (b) D = 260 nm, (c)D = 760 nm (backscatter SEM micrograph to estimate the fraction F of the PS sphere (dark) thatis covered by the Si columns (bright)), and (d) D = 3550 nm. The sketched sphere in (c) shows thenanosphere and the fraction F of the sphere actually covered with Si.

However, there exist differences concerning the morphology of the columnar structuresgrown on the spheres, depending on the PS sphere diameter D. Fig. 5.20 shows that themorphology of the columns changes with increasing sphere diameter. For D = 260 nm,singular columns evolve out of one sphere that serves as nucleation site. Those columnshave average diameters23 〈w〉 ≈ 235 nm. The distribution of the w values is rather broad,

23The average diameters were calculated using SPIP [115] with the grain detection module on top viewSEM micrographs showing a sufficiently large number of columns to analyze their diameter values.For each nanosphere size D, a Gaussian distribution plot was performed according to the diametervalues to gather information on the average diameters 〈w〉.

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5.3 Nanospheres as hcp arranged seeds for GLAD

and columns with diameters ranging from w ≈ 130 nm to w ≈ 300 nm can be found. Tounderline this, two columns with small and big diameters are highlighted in Fig. 5.20(b).With increasing D, the growth on one sphere that acts as seed changes from single-columnar to multi-columnar: For D = 3550 nm, Fig. 5.20(d) shows that several singlecolumnar structures, each one having a diameter between 150 nm and 900 nm, grow on topof the PS sphere. If D = 3550 nm, the ”total” column consists of approximately (30-50)of those subcolumns and has an average diameter of 〈w〉 ≈ 3310 nm. With increasingD, the distribution in w sharpens. In Tab. 5.2, the values for 〈w〉, the standard deviationσ of the Gaussian distribution of w and the normalized standard deviation σ/ 〈w〉 arelisted. Obviously, an increase in D is accompanied by a decrease in σ/ 〈w〉.

500 nm

Figure 5.22: SEM mi-crograph of a Si columnevolving out of a single PSsphere with D = 500 nm.The sidewall of the columnis not smooth, but exhibitsrugged features.

Figs. 5.21(c)-(d) show cross-sectional micrographs of nano-columns grown on PS spheres with different D values. It canbe seen that the nanostructures growing on each nanosphereresemble cylindrically shaped columns that are terminated witha hemispherical cap. Fig. 5.21(a) shows a sketch of the columnsto underline the parameters used: htot is the total height of thePS sphere plus Si column grown on it, h is the height of a Sinanocolumn, measured with backscatter SEM micrographs likein Fig. 5.21(c), and hcap is the height of the hemispherical capthat terminates the column.

Two trends are observable with increasing nanosphere diam-eter D. The first trend can be seen as a change of the growthmode from single-column growth on spheres having small Dvalues to multi-column growth on spheres having larger diam-eters D. This trend can be attributed to the fact that on PSspheres with small D values as compared to the lateral size anddistance of self-arranging nuclei on flat substrates (that wouldserve as seeds for the subsequent growth of Si structures byglancing angle deposition in the absence of artificially providedseed points), only a few nuclei will form on each nanosphere.Considering a sphere with diameter D, its active surface areafor nucleation would be Aeff = 4π · (D/2)2 · F , where F isthe fraction of the sphere that will actually be exposed to theSi flux. As described earlier, at TS = RT the growth of Sistructures with oblique or glancing angle deposition will startwith the nucleation of seeds with lateral sizes and inter-seeddistances within the range of (20-30) nm. Assuming that thegrowth on the hemispherical surfaces of the PS nanospheres willstart with the nucleation of self-forming seeds as well, the num-ber of nuclei per PS sphere nn can be estimated by calculatingnn = Aeff/(πr2

n), where rn is the radius of one nuclei. Settingrn ≈ 15 nm, one gets nn ≈ 90 for D = 260 nm. Thus, theprobability for the survival of more than one column on one sphere will be small, and acollective growth of nanorods that form on the nuclei and merge together to form a singlenanocolumn on each sphere can be expected. In section 4.2.3, it is shown that for glancingangle deposited vertical Si columns on unpatterned flat substrates with h ≈ 490 nm,the structure density is n ≈ 45 µm−2. Here, if D = 260 nm, the effective surface areafor nucleation is Aeff ≈ 0.06 µm2. For this area in the case of a flat substrate, onewould therefore assume to find 2-3 columnar structures. As the nominal height of the Sicolumns grown here is h0 = 650 nm and therefore larger as for the columns grown as

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5 GLAD of Si on Patterned Substrates

described in section 4.2.3, but the structure density decreases with increasing h (section4.1), the evolution of only one columnar structure on one seed seems likely.

As can be seen in Fig. 5.22, even for small D values (500 nm in this case), the columnsgrowing on the spheres at TS = RT do not have a smooth surface, but exhibit ruggedfeatures, cusps and mounds. Following the discussion of Zhou et al., this can be attributedto limited adatom mobility, leading to kinetic roughening, as ”[...]once two or multiplesurface mounds have developed on a single pillar, atomic shadowing favors the growth ofthe mounds versus the valley that separates them, leading to deep cusps and ultimatelyvoids[...]” [44].

On PS spheres with large D values, however, more nuclei will form in the beginningof the growth process on the sphere surface, increasing the probability for inter-nucleicompetition on one sphere that finally results in the establishment of more than onesingular column per sphere. At D = 3550 nm, nn is on the range of approximately18500. Some of those nuclei on the surface of one sphere will capture more flux than theothers (for example because of local inhomogeneities in the deposition flux), thus quicklyevolving into nanorods that shadow the surrounding nuclei and increase their diameterwith increasing deposition time. Thus, as the growth continues, several nanocolumns willevolve on one PS sphere with sufficiently large D and will finally contribute to the ”big”nanocolumn that grows on the PS sphere.

The second trend concerning the growth of vertical Si columns on hcp arranged PSspheres is the increased homogeneity of the average column diameter 〈w〉 with increasingD. Tab. 5.2 shows that the normalized standard deviation of the column diameter dis-tribution gets almost bisected from D = 260 nm (σ/ 〈w〉 = 0.052) to D = 3550 nm(σ/ 〈w〉 = 0.027). This trend is attributed to less inter-column competition of columnsgrowing on adjacent PS spheres with increased D and therefore increased inter-columndistances a and is consistent with previous findings that indicate a decrease of the in-tercolumnar growth competition with increasing the pattern seed separation [29]. Thegrowth of a Si column will be more affected by irregularities in the growth of an adjacentcolumn nearby if the seeds of the columns (i.e. the tops of the PS spheres) are closer toeach other.

On substrates patterned with smaller nanospheres, column competition and flux cap-turing by one column on expense of a neighbored column will be more severe then in thecase of large nanosphere diameters. For large D values, the distance between the growing”big” columnar Si structures can be considered to be large enough to suppress stronginter-column-competition, hence the narrow distribution of 〈w〉 in this case. The aver-

D [nm] htot [nm] h [nm] F [%] 〈w〉 [nm] σ [nm] σ/ 〈w〉 a [nm] hcap [nm] h − hcap [nm] nn

0 650 650260 860 675 0.29 230 12 0.052 30 81 594 90500 1055 730 0.29 424 19 0.045 76 130 600 320760 1400 835 0.25 678 29 0.043 82 215 620 6402550 3080 1390 0.33 2360 70 0.029 190 600 790 95303550 4200 1840 0.33 3210 96 0.027 340 790 1050 18475

Table 5.2: Relations between sphere diameter D, total structure height htot, column height h, fractionof the nanosphere covered with Si F , average column diameter 〈w〉, standard deviation σ, normalizedstandard deviation of the column diameter distribution σ/ 〈w〉, inter-column-distance a, height of hemi-spherical cap hcap, h − hcap and estimated maximum number of nuclei per sphere nn for deposition atTS = RT.

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5.3 Nanospheres as hcp arranged seeds for GLAD

age column diameter 〈w〉 and the inter-column-distance a as function of the PS spherediameter D are shown in Figs 5.23(b) and (c). Both, 〈w〉 and a scale linear with D,with ratios 〈w〉 /D ≈ 0.92 and, respectively, a/D ≈ 0.09. Obviously, it is possible togrow hcp arranged Si nanocolums with predictable diameter and inter-column-distanceson self-assembled monolayers of PS nanospheres.

0 1000 2000 3000 40000

50

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350

d[n

m]

D[nm]

dlinear fit

slope: 0.09

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500

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3500<

w>

[nm

]

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<w>linear fit

slope: 0.92

100 200 300 400 500 600

0.00

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)

w [nm]

2s

0 750 1500 2250 3000 3750

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slope: 0.33

h[n

m]

D[nm]

htot

hlinear fitlinear fit slope: 1.00

(a)

(b)

(c)

a

a

Figure 5.23: Plots of htot and h (a), theaverage nanocolumn diameter 〈w〉 (b) and theintercolumnar distance a as function of D. Insetof (b): example of the Gaussian distribution ofcolumn diameters w for one value of D (here:D = 360 nm).

On bare substrates, the nominal heightof the growing Si columnar structures ish0 ≈ 650 nm for the chosen depositionconditions. On substrates patterned with PSspheres, the total height of the structures in-creases with increasing sphere diameter. Fig.5.23(a) shows the relation between D andhtot. The relationship is linear, with a ratioof htot/D ≈ 1. From the linear function, itfollows that a seed point (i.e. an island thecolumn starts to evolve from) with a diame-ter of approximately 32 nm would result in thegrowth of a column with h0 = 650 nm. Thisseed point diameter fits well with the exper-imental finding that on plain substrates, thegrowth of Si structures under oblique or glanc-ing angle deposition conditions starts withfibers having diameters within the range of (20-30) nm that merge together to form the actualnanostructures as the growth continues.

Comparing the h values for differentnanosphere diameters D (listed in Tab. 5.2), itcan be seen that although on unpatterned sub-strates the Si column height is h0 ≈ 650 nm,and the amount of deposited Si is the samefor all experiments, h increases with increasingD, from h = 675 nm (D = 260 nm) toh = 1840 nm (D = 3550 nm). This increasecan be understood if each Si column that growson one PS sphere is seen as a cylinder, termi-nated with a hemispherical cap of height hcap

on the side of the growth front and a hemi-spherical trough on the other side, were the Sicovers the top of the PS nanosphere.

In this simple model, the volume of the col-umn equals the volume of a cylinder withheight h−hcap , if one assumes the hemispher-ical cap to fit into the trough. As Tab. 5.2shows, up to D = 760 nm, the values forh − hcap are approximately constant and withh − hcap = (605 ± 15) nm close to the valueh0 ≈ 650 nm for unpatterned substrates. ForD = 2550 nm and D = 3550 nm, however,the values for h − hcap increase to 790 nm and

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5 GLAD of Si on Patterned Substrates

1050 nm, respectively. This indicates that for larger sphere diameters, the assumptionthat the Si nanocolumn consists of a cap-terminated, dense cylinder with a hemisphericaltrough on one side does not hold any more. As discussed above, the growth changes froma single-column growth mode to a multi-columnar growth mode on one sphere when D isincreased. Hence, for the multi-columnar case, the final Si column that consists of many,rod-like sub-columns that grow on one nanosphere is less dense than the single columnsgrowing on PS spheres with smaller diameters, due to the intra-columnar voids betweenthe rods. As the same amount of material is deposited in each case, it follows that h andh − hcap increase with increasing D due to a decrease in the density of the total, ”big”column on a single sphere that consists of plenty of sub-columns for large D values.

The relationship between D and h is found to be linear as well. In Fig. 5.23(a), theslope of the linear function is 0.33 in this case. This value is in good accordance with thefraction F of the PS sphere that is actually covered with Si. In Tab. 5.2, the F values,ranging from F ≈ 0.25 to F ≈ 0.33 are listed.

5.3.2 Influence of substrate temperature

Substrates patterned with SiO2 nanospheres24 (D = 360 nm) were used to evaluatethe effect of TS on the growth of vertical columnar Si structures with glancing angledeposition. GLAD with fast substrate rotation (ρ = 21 nm/rev) was performed onboth, patterned and unpatterned substrates for a deposition time tdep = 162 min atthree different substrate temperatures TS = RT, 150C and 350C. The results areshown in the micrographs of Fig. 5.24 and in Tab. 5.3.

At TS = RT, the Si columns on unpatterned substrates25 have an average diameter〈w〉0(RT) ≈ 99 nm and a height of h0(RT) ≈ 700 nm. With increasing TS, the columnarSi structures on bare substrates increase their diameter w, while their height h decreases,until at TS = 350C, some of the columns are broad enough to touch each other andeventually merge together, with 〈w〉0(350C) ≈ 167 nm and h0(350C) ≈ 570 nm. Thisdensification of the growing film with increasing TS can be attributed to surface diffusioneffects. At increased TS, diffusion-driven mass transport attributes to column broadeningas well as to an intra-columnar-densification, as voids in a single nanocolumn get filledmore easily by Si particles when the distance an adatom is able to diffuse away from itsimpact point is enhanced. As a result, broad, densely packed Si columns grow at elevatedsubstrate temperatures on bare substrates, in contrast to the highly underdense films ofwell-separated structures that grow at RT.

In contrast to the growth on unpatterned substrates, the Si columns on SiO2 spheresdo not show this extreme increase of 〈w〉 at elevated TS. At TS = RT, on the sphere-patterned substrates the average column diameter is 〈wRT〉 ≈ 320 nm. As D = 360 nm,〈wRT〉 /D = 0.89. This is in good accordance with the constant 〈w〉 /D ≈ 0.92 whichhas been found in the previous section for the growth of Si columns on PS spheres withdifferent D values.

An increase of TS induces an increase of 〈w〉 just as in the case of the unpatternedsubstrate, as can be seen in Tab. 5.3. However, this increase of 〈w〉 with TS on patternedsubstrates is only slight when compared to the temperature-induced increase of 〈w〉0 onflat substrates. While on sphere-patterned substrates with 〈w〉(350C) ≈ 340 nm as

24SiO2 spheres were taken for those experiments as PS is a low-melting organic material.25In the following, the parameters describing structures grown on unpatterned substrates are marked

with an index “0”, e.g. h0 decribing the structure height on an unpatterned substrate.

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5.3 Nanospheres as hcp arranged seeds for GLAD

500nm500nm

500nm500nm

500nm500nm

500nm500nm

500nm500nm

500nm500nm 500nm500nm

500nm500nm

500nm500nm

RT

350°C

150°C

cross-sectionpatterned

top viewpatterned

top viewbare substrate

Figure 5.24: Cross-sectional and top view SEM micrographs of columnar Si structures deposited onSiO2 nanospheres with D = 360 nm and and on unpatterned substrates at different TS .

compared to 〈w〉(RT) ≈ 320 nm, the ratio 〈w〉(350C) / 〈w〉(RT) ≈ 1.06, on unpatterned

substrates this ratio amounts to 〈w〉0(350C) / 〈w〉0(RT) ≈ 1.69.

As in the case of the unpatterned substrate, an increase of TS results in a decrease ofthe column height h. If one assumes the column to have the volume V of a cylinder withheight h − hcap and diameter 〈w〉, a decrease of h at higher TS should be compensatedwith an increase of 〈w〉 in order to keep V constant, as the same amount of material is

TS htot h 〈w〉 σ/ 〈w〉 a hcap h − hcap h0 〈w〉0

(σ/ 〈w〉)0

[nm] [nm] [nm] [nm] [nm] [nm] [nm] [nm]

RT 1060 760 320 0.072 40 80 680 700 99 0.25150 C 1000 715 336 0.065 24 95 620 610 117 0.23350 C 890 635 340 0.082 20 75 560 570 167 0.31

Table 5.3: Relations between substrate temperature TS , total structure height htot, column heighth, average column diameter 〈w〉, normalized standard deviation σ/ 〈w〉, intercolumnar distance a, heightof hemispherical cap hcap, h − hcap, height of Si column on unpatterned substrate h0, average columndiameter on unpatterned substrate 〈w〉

0, and normalized standard deviation on unpatterned substrate

(σ/ 〈w〉)0.

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5 GLAD of Si on Patterned Substrates

deposited for every TS value and if the sticking coefficients are assumed to be constant inthe applied range of TS. Thus, if V(RT) = V(T ), where T is the (elevated) temperature, itshould follow that

(h − hcap)(T ) =(

〈w〉(RT) / 〈w〉(T )

)2

· (h − hcap)(RT)

.

Therefore, theoretically one should obtain values of (h − hcap)(150C) ≈ 617 nm and

(h − hcap)(350C) ≈ 602 nm. At TS = 150C, the measured value h− hcap ≈ 620 nm is

close to the value as predicted above. However, if TS is increased to 350C, the measuredvalue h − hcap ≈ 560 nm is less than the predicted value of approximately 600 nm.

It follows that at TS = 350C, the height decrease of the Si column as compared to theSi columns deposited at lower substrate temperatures can not be explained by a strongcolumn broadening effect, as would be in the case of an unpatterned substrate. Therefore,it is likely that other effects play a role here, for example intra-columnar densification. Atelevated TS, the length scale for surface diffusion increases, and intra-columnar voids fillwith diffusing adatoms more easily, which can lead to a denser Si column with decreasedheight as compared to columns grown at TS = RT [47]. On the other hand, as theseed points of the Si nanocolums (i.e. the top surfaces of the SiO2 spheres) are well-separated, it is unlikely that a sufficient amount of Si adatoms will be able to overcomethe distance between two SiO2 spheres by surface diffusion at TS = 350C. Therefore, nodistinct column broadening or even merging of adjacent nanocolumns will appear betweenTS = RT and TS = 350C on substrates patterned with SiO2 spheres of sphere diameterD = 360 nm due to negligible inter-column growth competition. This is in contrast tothe case of an unpatterned substrate, where an increased surface diffusion length increasesthe average column width, which in turn leads to a strongly competitive growth modethat favors the survival of the broadest columns on expense of smaller ones.

It is likely, though, that when TS will be increased to values that by far exceed 350C,the diameters w of the columnar Si structures will severely increase and even mergingof neighbored structures to conglomerates might set in, once the adatom mobility willbe high enough to allow the diffusing atoms to overcome the inter-seed distances. Forinstance, in Ref. [47], it is reported that for the growth of Ta columns on self-assembledSiO2 spheres with diameters of D ≈ 250 nm, merging of adjacent columns sets in onceTS & 700C. As the melting point of Ta, TM(Ta) = 3017C exceeds that of Si,TM(Si) = 1415C by far, it is likely that similar merging effects for Si columns depositedon hcp arranged SiO2 spheres might set in even at TS . 700C. However, the applicableTS range for the experiments was restricted by the experimental setup to Tmax

S ≈ 400C.

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5.4 Periodically arranged structures with arbitrary periods

5.4 Periodically arranged structures with arbitrary

periods

1 µm

P d

l < d l >> d

Figure 5.25: Transition zone between patterned and un-patterned part of substrate. Left part: tetragonally pat-terned, right part: planar substrate.

Different seed design considerationshave to be taken into account, ifno unwanted inter-seed condensa-tion or structure broadening is tooccur (section 2.2, [5, 28]). Thus,the obtainable periodicities are re-stricted and usually span the rangebetween 100 nm . P . 1 µm[27]. The problem is displayed inthe SEM micrograph of Fig. 5.25.The GLAD film grown there con-sists of chevrons, grown with asubstrate rotation scheme as ex-plained in section 5.1. The micro-graph shows the boundary zone be-tween a part of the substrate thatis tetragonally patterned with seedsof height hS ≈ 100 nm, seed di-ameter wS ≈ 150 nm and periodP ≈ 400 nm, thus leading to aninter-seed spacing dS = P − wS = 250 nm. With the Si flux incident under θ ≈ 85,the shadowing length is l ≈ 1150 nm.

100 1000100

1000

p = 0.31

h [nm]

P = 1100 nmP = infinite

w[n

m] w ~ h

p

p = 0.62

2 µm

R

R ~ hp

Figure 5.26: Transition zone between tetragonally pat-terned (P = 1100 nm) and unpatterned part of substrateand growth front evolution of Si columnar structure grownwith ρ ≈ 100 nm/rev on both sides of the patterns bound-ary row.

For the patterned part of the sub-strate, both, Eqs. 2.9 and 2.10 are ful-filled, leading to only very little con-densation between the seeds (prob-ably due to parts of the sputteredflux being non-directional). No struc-ture broadening effects are observable,the zig-zag morphology of the struc-tures (as intended by the substraterotational scheme) is clearly visible.A clear shift of the structure mor-phology is observable for the struc-tures growing on the boundary seedrow of the patterned part of the sub-strate. To the left, facing the pat-tern, the zig-zag morphology is stillinherited by the growing structures onthe boundary row. To the right, how-ever, facing the unpatterned part ofthe substrate, equivalent to a inter-seed spacing dS = ∞, neither Eq.2.9 nor Eq. 2.10 are fulfilled anymore.This leads, on the one hand, to non-

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5 GLAD of Si on Patterned Substrates

periodic growth on the right-hand side of the structures growing on the boundary row.On the other hand, in an attempt to keep the equilibrium volume density constant, theright-hand side of the (intended) chevronic structure broadens disproportionately. As itstarts growing on the seed with hS = 100 nm above the level of the planar substrate, itcaptures more flux then the substrate region to its right, thereby getting a growth advan-tage over structures that start growing adjacent to it. By this, the self-shadowing effect isexacerbated, and the structure at the template boundary, filling the ”empty space” to itsright, widens in a form that it not distinguishable as chevron any more. Quite similarly,the same effect is shown in Fig. 5.26 for rotationally symmetric structures grown withρ ≈ 100 nm/rev. The figure shows that the growth exponent p of the scaling law (Eq.2.7) R ∝ hp changes26 if the pattern period drastically changes (from P = 1100 nm toP = ∞ in this case). Facing the patterned part of the substrate, on the boundary rowof the pattern the structure radius increases according to R ∝ hp with p ≈ 0.31, butquickly saturates after h ≈ 400 nm to a saturation value RSat ≈ 440 nm. On the otherside, facing the planar part of the substrate, however, R extends quicker to larger values,with p ≈ 0.62. The maximum value of R ≈ 1200 nm at h ≈ 1100 nm found here islikely to increase further when the deposition time is increased.

Both examples illustrate that conventional lithography approaches fail if non-broadenedstructures with periods larger than P & 1 µm or even non-periodically arranged, free-standing isolated structures with distinct morphology are to be deposited with GLAD.In this section, a new, two-step patterning principle will be introduced that allows toovercome those restrictions.

5.4.1 Principle of two-step lithography patterning process

The process as depicted in Fig. 5.27 comprises two main (electron beam) lithographysteps, followed by the actual GLAD process and resist removal.

• Step 1: initial patterning:

– The substrate is covered with negative photoresist and exposed to the electronbeam at the positions where the structures are supposed to grow (Fig. 5.27(a)).The distances between those positions can be chosen freely. As the exposedarea at each position defines the seed diameter wS, it should be restricted to afew hundred nanometers in order to prevent bifurcation and growth of multiplestructures on one seed [27].

– After development of the photoresist, the remaining resist pattern as shownin Fig. 5.27(b) is transferred into the substrate, for example by (reactive) ionbeam etching [144] or other transfer techniques. The substrate, now initiallypatterned with seeds for GLAD, is shown in Fig. 5.27(c).

• Step 2: second patterning:

– Now patterned with seeds having seed heights hS and inter-seed distances dwhich can be chosen freely, the substrate is covered with positive photoresist.

26As on the boundary row of the pattern the growing structure evolves into a non-rotational symmetricform, the scaling law is used in terms of R rather than w here, as this allows to compare the growthsexponents of both ”radii” in direction of the patterned and, respectively, unpatterned parts of thesubstrate.

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5.4 Periodically arranged structures with arbitrary periods

(a) (b) (d)

(e) (f)

positive resist

e -beam-

b

(c)

(g)

negative resistpatterning

patterntransfer

positive resistpatterning

GLAD resistremoval

d Rgap

substrate

negative resist

substratepositive resist

Figure 5.27: Principle of two-step lithography process to deposit free-standing nanostructures.Electron beam exposal of negative resist at the positions of the seeds with inter-seed-distances d (a). Seedpattern in resist after development (b). Seed pattern in substrate after pattern transfer (c). Electronbeam exposal of positive resist in circular area around the initial seeds (d). After resist development,a circular opening (radius Rgap) surrounding the seeds remains (e). Glancing angle deposition step (f).After resist removal, only the structures grown on the seeds remain (g).The lower row shows a cross-sectional sketch of the process.

The resist thickness should equal the seed height hS, as depicted in Fig. 5.27(d).The positive resist is then to be exposed in a circular area surrounding the seedsof the initial patterning step. The radius Rgap of the circular area has to bechosen to fulfill both, Eq. 2.8

Rgap ≤ hS · tan(θ)

and Eq. 2.9

wS ≥ Rgap − hS · tan(θ)

in order to prevent inter-seed condensation on the substrate and structurebroadening effects in the GLAD process.

– Fig. 5.27(e) shows that after resist development, the patterned substrate con-sists of artificial seeds with inter-seed-distances d that do not necessarily haveto fulfill the design considerations expressed in Eqs. 2.8 and 2.9. Additionally,the whole substrate is covered with photoresist whose thickness equals the seedheight hS, leaving open only a circular gap with radius Rgap surrounding eachseed.

• Deposition:

– When GLAD is performed on substrates patterned in the described way (thesimplified sketch of Fig. 5.27(f) shows the OAD case), structures will bothstart to grow on the defined seeds as well as, statistically distributed, on theplanar surface of the photoresist. The photoresist shadows the circular gap

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5 GLAD of Si on Patterned Substrates

that surrounds the artificial seeds, preventing nucleation and structure growththere. Likewise, the structures that grow on the resist close to the edge of theresist at the circular gaps prevent structure broadening on the seeds (in orderto keep Fig. 5.27(f) simple, structures growing on the resist are only sketchedin the lower left part of the figure).

– After removal of the photoresist (and the unperiodically arranged, glancingangle deposited structures on it), only the structures grown on the seeds remainon the substrate, at positions defined by the seed layer arrangement in step 1of the process.

5.4.2 Results and drawbacks

The described two-step lithography process with subsequent Si GLAD was performed ona set of sample templates with tetragonally arranged seeds, with different seed periods10 µm ≤ P ≤ 50 µm, different radii Rgap of the circular gap around the seeds anddifferent seed diameters 200 nm ≤ wS ≤ 400 nm. Dependent on wS, the radius Rgap

of the circular gap surrounding the seed was adjusted to keep the distance between seedand resist (i.e. the length to be shadowed) at 1200 nm and 800 nm, respectively. The SiGLAD deposition was done with a slit aperture between target and substrate, setting theangular deviation from the average deposition angle θ = 85 to ∆θ ≈ (9 ± 3). Theexperiments were done at TS = RT, with ρ ≈ 11 nm/rev (fast substrate rotation) togrow vertical, columnar structures. The nominal film height amounted to h ≈ 640 nmin this case. With a seed height hS ≈ 200 nm, the shadowing length was l ≈ 2300 nm,exceeding the radius of the circular gap Rgap by far.

(a) (b) (c)

(d) (e) (f)

1 µm 1 µm 1 µm

1 µm 1 µm 1 µm

Figure 5.28: Top-view SEM micrographs of vertical columns grown on substrates patterned with thedescribed two-step lithography process. The seed period was P = 20 µm in each case. (b-d): beforeremoval of resist, (e,f): after resist removal. The following values of seed diameter wS and gap radiusRgap were used: (a) wS = 300 nm, without second lithography step (Rgap = ∞); (b) wS = 200 nm,Rgap = 1300 nm; (c) wS = 400 nm, Rgap = 1400 nm; (d) wS = 400 nm, Rgap = 1000 nm; (e)wS = 400 nm, Rgap = 1400 nm; (f) wS = 400 nm, Rgap = 1000 nm.

Fig. 5.28 shows examples of columnar Si structures, deposited with the described pro-cess. It is observable that both, the area of the circular opening that surrounds the seed

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5.4 Periodically arranged structures with arbitrary periods

and the seed diameter influence the diameter27 of the column. Without the second stepof the process, the lack of resist that would otherwise shadow the region around the dotleads to a large broadening of the growing structure on the seed, as can be seen in Fig.5.28(a). With a seed diameter wS = 400 nm, the structure diameter at h ≈ 640 nm isw ≈ 1680 nm, which is approximately 13 times the average diameter of the surroundingcolumns on the planar substrate, which is 〈w〉 ≈ (130 ± 40) nm in this case (measuredwith SPIP [115]). As the period is P = 20 µm here, Eqs. 2.9 and 2.10 are not fullfilled.Therefore, the column growing on the seed broadens significantly28 as it is insufficientlyshadowed by the columns that evolve on the planar substrate surrounding the seed. Addi-tionally, it can be seen that not a single column develops on the seed, but due to branchingand bifurcation, the growth mode is multicolumnar. Apart from that, as the shadowinglength l ≈ 2300 nm is by far insufficient to shadow the whole inter-seed distance, thewhole area next to the seed is covered with randomly distributed Si columns.

In contrast to that, if there exists a second layer of resist, there is only marginal growthin the circular gap around the seed, as can be seen in Figs. 5.28 (b) - (f). The existence ofstructures in the gap is most likely due to deviations in the angular flux of the incomingSi particles on the one hand, and to adjustment problems of the substrate between thetwo lithography steps on the other hand: As the figure shows, the seed is not situatedin the center of the circular gap, but rather placed closer to the edge of the resist. Thisenhances the distance between seed and resist (and vice versa) in the opposite direction,leading to an insufficient shadowing effect.

200 250 300 350 400750

800

850

900

950

1000

1050

1100

1150

1200

colu

mn

dia

met

erw

[nm

]

seed diameter wS

[nm]

wlinear fit

Figure 5.29: Dependence of w on wS at constantRgap = 1300 nm.

If, however, the radius Rgap ofthe gap is reduced, the inter-seed-growth is reduced as well: com-paring Figs. 5.28(b) and (d), it isobservable that reducing Rgap from1300 nm to 1000 nm lowers thenumber of perturbing structures inclose vicinity of the seed from ap-proximately 100 to approximately30.

Concerning the influence of Rgap

and wS on the growth of thecolumns on the seeds, it is foundthat reducing Rgap at constantwS reduces the structure diam-eter as well. From infiniteRgap (no second lithography step)over Rgap = 1300 nm toRgap = 1000 nm, w is reduced from1680 nm over 1150 nm to 1000 nm,if the seed diameter is kept constantat wS = 400 nm. Likewise, if Rgap is kept constant at Rgap = 1300 nm, an increaseof wS is followed by an increase of w, as is shown in Fig. 5.29. This increase seems tobe linear with w ≈ 1.75 · wS, but as the seed is not placed properly in the center of thecircular gap, a verification of this relation is difficult. However, the anticipated monotonic

27The diameter measured at the top of the column, in that case.28In fact, the term ”pillar” rather than ”column” describes the structure morphology better, as the

structure diameter wS at the top of the structure is nearly three times the structure height h.

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5 GLAD of Si on Patterned Substrates

increase of w with wS is clearly observable. Additionally, as can be seen when compar-ing Figs. 5.28 (b) and (c), increasing wS also increases the probability of branching intosubcolumns and bifurcation on the seed [27, 28]. Whereas a clear division of the growingcolumn into subcolumnar entities is existent for wS = 400 nm (Fig. 5.28(c)), this is notthe case for wS = 200 nm (Fig. 5.28(b)).

The main drawback of the proposed process is visualized in Fig. 5.28(e): The removal ofthe photoresist after the deposition process29 proved to be problematic. The figure showsthat at the edge of the resist, the same broadening effects as described in section 5.4 atthe boundary row of the patterned part of the substrate takes place. Thus, a ring-shapedSi layer grows at the edge of the resist, which tends to remain on the substrate as tubearound the column that grows on the seed. To overcome this problem, the radius ofthe circular gap has to be as small as possible in order to enhance the effectivity of theshadowing process and to circumvent the detrimental condensation of Si on the barrierof the resist. On the other hand, decreasing the radius of the circular opening aroundthe seed effectively decreases the free distance between the structure growing on the seedand the inter-seed deposited structures on the resist, thereby enhancing the possibilitythat during the process of resist removal, the resist and Si structures thereon touch thestructures on the seeds and sweep them away as well. Fig. 5.30(a) shows all the possibleresults after resist removal: free-standing, shaped nanostructures (two-armed chevrons inthis case) as well as empty spaces where the structure was ruptured from the seed duringresist removal and also structures that are (partly) surrounded by a ring-like Si structurethat condensates at the edge of the resist during deposition.

10 µm 500 nm500 nm

(a) (b) (c)

Figure 5.30: (a) Top-view SEM micrographs of two-armed chevrons, P = 10 µm, Rgap = 1000 nm,dS = 200 nm, after resist removal. Top and bottom left: isolated structures. Top right: structure sweptaway during resist removal. Bottom right: parts of the resist near the structure not gone in solutionduring resist removal. (b) Isolated Si column, surrounded by ring-like resist leftovers covered with Si(Rgap = 1000 nm, micrograph taken under 45). (c) Isolated two-armed Si chevron (Rgap = 1000 nm,micrograph taken under 45).

Experimentally, the following pattern parameters lead to the best results, concerningthe amount of remaining, free-standing columnar Si structures without being surroundedby a perturbing Si tube, which then was approximately 35% of the initial seed number:P & 20 µm, wS & 300 nm, Rgap . 1000 nm.

Clearly, further work has to be applied here to improve the process. It is likely that adifferent deposition principle than ion beam sputtering as used in the experiments here, forexample thermal or electron beam evaporation, will help to enhance the shadowing effectand reduce detrimental condensation both at the edge of the resist and in the circulargap, as the the deposition itself is more ballistic then, with the particles strongly centered

29The resist used here, PMMA (Polymethyl methacrylate), is dissolved in either acetone or a specialresist removal solution, AR300 − 70, with the help of ultrasonification.

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5.4 Periodically arranged structures with arbitrary periods

around the direction of the source normal [8]. Additionally, the alignment of the substratefor the second lithography step has to be improved, in order to gain a patterned substratewith the seeds placed centrally in the circular gaps. The misalignment in the present caseis most likely a result of thermal creep in the EBL system. Nevertheless, in principle,the introduced approach is suited to deposit isolated, nearly arbitrarily shaped structuresof multiplex elements, alloys and heterostructures. To illustrate this, Figs. 5.30(b) and(c) show free-standing Si structures (columnar and chevronic), deposited on substratespatterned with the explained process.

The patterning approach presented here could be of particular importance for the growthof square spiral 3D photonic band gap crystals of Si as predicted by Toader and John[6]. The band structure of such photonic band gap crystals can be tailored by adjustingthe lateral expansion W , the arm diameter w and the height of a single four-fold spiralafter one full turn (pitch c)30 with relation to the period P of the underlying square seedtemplate [142]. Whereas the lateral expansion W and the pitch c are not independentof each other as they are linked by the inclination angle β of the arms of the four-foldspirals (c = 4W · tan(β)), at least the structure arm diameter w will be independent ofthe period P and rather be governed by the radius of the circular gap Rgap and the seeddiameter wS, as can be seen in Fig. 5.29, when making use of the two-step lithographypatterning process. Therefore, in principal w could be specifically tailored with respectto P , whereas in traditional approaches of performing GLAD on patterned substrates,“[...] Control of the square side length in the fabrication process is a function of both,incidence angle and structure-to-structure or seed spacing.[...]”, as Kennedy et al. statein Ref. [34].

30W and w are shown in Fig. 5.2.

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5 GLAD of Si on Patterned Substrates

5.5 Summary of results: GLAD on patterned

substrates

GLAD on tetragonally arranged templates

Concerning the growth of non-columnar Si nanostructures deposited on substratespatterned with tetragonally arranged seeds with certain seed periods P , seed heightshS and seed diameters wS it has been found that

• both, structure shape (as intented by the applied substrate rotation algorithm) andstructure arrangement (as intented by the use of a pre-patterned substrate with seedsat pre-defined places) are not only influenced by the seed design considerations asproposed in Ref. [28] (Eqs. 2.8 and 2.9),

• but also by the ratio P/W of the template period P to the lateral structure expansionW . If P/W . 1, inter-structure competition (comparable to the case of a planarsubstrate) sets in that prevents the adaptation of the periodicity of the underlyingseed pattern by the glancing angle deposited structures.

• Compared to the growth on planar substrates, the diameter w of the structuresgrown on template seeds stays approximately uniform, if P/W & 1.

• For the growth of four-fold spirals and chevrons, the structure diameter w is con-trolled by adjusting the template period P with P/w ≈ (1.25 ± 0.4).

• The seed diameter wS defines the lower limit for the structure diameter w, butthe structures growing on the seeds are likely to have diameters larger than wS,depending on the template period P .

• The smaller wS, the less bifurcation and branching of the growing structures can beexpected.

Concerning the growth of columnar Si nanostructures deposited on continuouslyrotating substrates patterned with tetragonally arranged seeds with seed periods P ,seed heights hS and seed diameters wS it has been found that

• The template period P has to be chosen to be in the order of or larger than theaverage column-column separation λ0 for the case of GLAD on a planar substrate,if the periodicity is to be maintained after deposition, as

• if P < λ0, inter-column competition leads to structure extinction and structurebroadening effects, resulting in a structure growth mode where the periodic order ofthe underlying patterned substrate vanishes, which is in good accordance to similarfindings of Dick et al. (Ref. [140]).

GLAD on honeycomb-like or hcp arranged templates

Concerning the glancing angle (θ = 85) deposition of columnar Si nanostructuresdeposited with fast substrate rotation on NSL patterned substrates, it has been foundthat

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5.5 Summary of results: GLAD on patterned substrates

• the columns evolve on the artificial seeds, leading to a replication of the underlyingtemplate pattern which is either honeycomb-like (single layer NSL) or hcp (doublelayer NSL) arranged.

• The cross-section of the columns is different for both patterns. In the hcp case, itis close to circular just as in the case of the tetragonal pattern. In the honeycombcase, however, the columns evolve a triangular cross-section.

• The differences concerning the cross-sectional shape of the columns are linked tothe symmetry of the respective template pattern:

– In the symmetric hcp case, six equidistant neighbors induce a homogenousshadowing effect and thereby the evolution of a saturated column radius Rsat

that depends on the distances d between nearest neighbors with respect to amodel developed for GLAD on tetragonally patterned substrates [68].

– In contrast to that, in the less symmetric honeycomb case, only three nearestneighbors in distance d1 result in a quick saturation of the lateral structureexpansion in the respective direction, whereas in the direction of the thirdnearest neighbors, the inter-seed distance d2 = 2d1 induces a saturation ofthe lateral structure broadening in that direction at a later stage of growth.Thus, 2 different saturation radii Rsat,1 and Rsat,2 develop in those differentdirections, finally resulting in a structure with a triangular cross-section.

– It is proposed that the more nearest neighbors a seed in a specific templatepattern has, the more circular the resulting cross-section of the GLAD-growncolumn will be.

For the special case of GLAD with fast substrate rotation on the honeycomb-liketemplate pattern, it was found that

• With increasing deposition time, the morphology of the columns evolving on theseeds changes:

– In the early growth stages, the height advantage of the artificial seeds withheight hS in comparison to the surrounding substrate dominates the struc-ture evolution, whereas the inter-column competition of columns that grow onthe seeds is negligible then. Thus, the columns increase their diameter withincreasing height uniformly in each direction [25].

– With increased deposition time, the columns develop a triangular cross-sectiondue to different inter-seed distances in different growth directions. Addition-ally, growth competition sets in at later stages of growth, once the columnarstructures are large enough to influence the growth of each other.

– For the special case of GLAD on honeycomb templates patterned with ananosphere diameter D = 287 nm and a seed height hS ≈ 45 nm, this wasquantified with an increase of the normalized diameter distribution ∆〈w〉/〈w〉with increasing structure height h. From h ≈ 13 nm to h ≈ 262 nm,∆〈w〉/〈w〉 increases from approximately 7% to 26%.

– This finding is in contrast to GLAD on unpatterned substrates, where theinter-column competition is strongest in the early stages of growth and thengradually reaches a steady state.

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5 GLAD of Si on Patterned Substrates

• The saturation diameter 〈w〉 of the columnar structures scales with the inter-seeddistances that are governed by the nanosphere size D. For Si GLAD with ion beamsputter deposition and fast substrate rotation, it is found that 〈w〉 can be adjustedwith D according to 〈w〉/D ≈ (0.54 ± 0.02).

A variation of the deposition angle θ at fast substrate rotation on honeycomb-likepatterned substrates results in

• three growth regimes depending on θ that lead to different film morphologies:

– The low incidence case (θ . 35) results in dense films, capped with hillock-like features that replicate the underlying honeycomb template pattern.

– The intermediate incidence case (θ ≈ 70) results in columnar structuresevolving on the seeds and a pyramidal-shaped inter-seed grown film. Due tothe non-glancing angle conditions, no saturation radius of the columns evolves.The columnar structures touch each other and merge after a certain stage ofgrowth. The onset of merging is different in different growth directions, thusleaving a height range in which a porous layer with hcp arranged pores exists.

– The glancing angle case (θ ≈ 85) results in separated columnar structuresthat are typical for GLAD.

• A series of intermediate and glancing angle case during one deposition cycle canlead to the growth of a porous layer without inter-seed condensation.

Concerning GLAD on honeycomb templates at different rotational speeds itwas found that

• Just as in the case of an unpatterned substrate, slow substrate rotation results inspiral structures, intermediate substrate rotation in screw-like structures and fastsubstrate rotation in columnar structures.

• As the initial seed size and inter-seed distance is different compared to the caseof the bare substrate, different values ρ = r/ω of vertical deposition rate r tosubstrate rotational speed ω result in the respective structure morphology.

• For the case of the honeycomb template, it is found that columnar structures evolveat ρ = 100 nm/rev, screw-like structures at ρ = 180 nm/rev, and spirals aredeposited with ρ = 270 nm/rev on top of the Au seeds.

GLAD on hcp arranged nanospheres

Concerning the growth of columnar Si nanostructures deposited on substrates patternedwith hcp arranged PS spheres of different sphere diameter 260 nm < D < 3550 nmat TS = RT, it has been found that

• independent of D, the hcp arrangement of the nanosphere pattern is adopted.

• The column diameter w stays approximately constant and is determined by D withw ≈ 0.9 D.

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5.5 Summary of results: GLAD on patterned substrates

• With increasing D, more Si nuclei evolve on the surface of a single PS sphere,thus gradually increasing the probability for competitive growth of sub-columnarstructures that grow on one seed;

• which leads to a change of the growth mode from dense, single-columnar growth forsmall sphere diameters (D . 760 nm) to underdense, multi-columnar growth onspheres with larger diameters.

• With increased D, the inter-columnar competition decreases and therefore the ho-mogeneity of the average Si column diameter 〈w〉 increases with increasing D.

Evaluating the influence of the substrate temperature TS on the growth of colum-nar Si structures on hcp arranged SiO2 spheres (D = 360 nm), the following conclusionscan be drawn:

• In the temperature range RT . TS . 350C, the average column diameter 〈w〉is mainly influenced by D and not by mass transport through surface diffusion;

• whereas on a flat substrate, 〈w〉031 increases by 69% when TS is increased from RTto 350C, on the substrates patterned with SiO2 spheres, 〈w〉 only increases by 6%for the same increase in TS.

• With increasing TS, the height h of the Si column decreases. For TS & 150C, thisdecrease is not compensated any more by an increase of 〈w〉. Thus,

• if re-evaporation effects are neglected, increasing TS & 150C leads to an intra-columnar densification of the Si structures deposited on the spherical seeds.

GLAD on two-step lithography patterned substrates

• A novel approach to pattern substrates prior to GLAD with a two-step lithographyprocess has been introduced, allowing the possibility to deposit isolated nanostruc-tures at arbitrary positions on the substrate;

• without having to obey the design considerations (Eqs. 2.8 and 2.9) for GLADtemplates if structure broadening and inter-seed nucleation is unwanted.

• First results have shown that the process works in priciple, and that it is possibleto adjust the structure diameter w with disregard to the template period P ,

• but pattern design improvement is needed to overcome some more technical dif-ficulties, especially the existence of a tube-like Si remnant around the separatedstructures.

31This 〈w〉 value is measured at the structures top then, as on unpatterned substrates, w is a functionof h.

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5 GLAD of Si on Patterned Substrates

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6 Applications of Glancing AngleDeposited Si Nanostructures

In this chapter, some further characteristics of Si nanostructures grown with ion beamsputter glancing angle deposition will shortly be presented.

In the following section, it will be shown that the experimental setup can easily beused for the deposition of multilayered hetero-nanostructures, exemplary for the materialcombination Si-SiOx.

In section 6.2, mechanical measurements on helical Si nanostructures with nanoinden-tation will be presented, and the spring constants of the nanoscaled helical Si structureswill be derived.

Finally, in the last section of the chapter, it will be shown that GLAD-grown Si nanos-tructures can be used as templates for a subsequent atomic layer deposition (ALD) pro-cess. If using vertical and slanted columnar Si structures for ALD of Fe2O3 with a sub-sequent reduction step from Fe2O3 to Fe3O4, the generation of tube-like (ferrimagnetic)structures is possible. First results on direction-dependent magnetization measurementswill be presented, showing that the magnetic response in terms of the coercivity Hc isstrongly influenced by the orientation of the sample, and therefore the orientation of theALD-coated columns, with respect to the applied magnetic field H.

6.1 Si-SiOx-multilayers

Besides few reported investigations on GLAD with a combination of different materials(for example, in Refs. [4, 58, 111, 112, 147, 148]), most GLAD research so far hasbeen done on single elemental systems, without combining different materials in order todeposit heterostructures. Multilayered thin films and nanostructures, however, dependingon the material combinations, can be expected to enhance the films’ functionality, andcould find applications as smart and wear resistant coatings, or as highly effective anti-reflection layers in a broad spectral range [4]. Those findings therefore motivate the studyof the growth behavior of glancing angle deposited, multilayered nanostructures.

With the deposition setup used in the scope of this work, a simple realization of suchmultilayered glancing angle deposited STFs can be achieved in terms of the materialcombination Si-SiOx.

If the ion beam sputter glancing angle deposition is done with a Si target, a par-tial O2 pressure pO2

during deposition leads to the formation of SiOx structures, with0 . x . 1.8, depending on pO2

that can be adjusted with the oxygen gas flow fO2. If

an intermittent oxygen gas flow fO2is applied, the resulting STF will comprise structures

that consist of Si and SiOx stacks.

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6 Applications of Glancing Angle Deposited Si Nanostructures

500 nm10

310

4

0

200

40030

Si

O2

Intensity [a.u.]

h[n

m]

(b) (c)(a)

Figure 6.1: Screw-like, multilayered (Si-SiOx-Si) STF. (a): Cross-sectional SEM micrograph (sec-ondary electrons signal). (b): Cross-sectional backscatter SEM micrograph (the darker regions originatefrom the SiOx layer). (c): Corresponding TOF-SIMS depth profile (measurement done by Dr. J.W.Gerlach).

0 1x1018

2x1018

3x1018

4x1018

0

20

40

60

80

100

Si

O

C

N

elem

enta

lco

nce

ntr

atio

n[a

t.%

]

depth [at./cm²]

500 nm

(b)

(a)

Si

Si

SiOx

Figure 6.2: Six-armed glancing angledeposited chevron structure depositedwith intermittent oxygen gas flow. Thesubstrate-nearest two arms of the struc-ture consist of Si, the third and fourtharm of the structure consist of SiOx

(x ≈ 1.8 in this case), and the two up-permost arms consist of Si again. (a):ERDA depth profile (measurement doneby Dr. J.W. Gerlach). (b): Cross-sectional SEM micrograph.

Fig. 6.1 depicts the result of this variation in the de-position principle by means of a screw-like STF witha SiOx layer sandwiched in-between two Si layers. Forthe deposition, the following parameters were used:after tdep = 30 min at ρ ≈ 33 nm/rev, for thefollowing 10 min of deposition, an oxygen gas flowfO2

= 10 sccm, resulting in an oxygen partial pres-sure of pO2

≈ 8.6 × 10−5 mbar, was applied. Then,fO2

was set to zero again, and the first depositioncycle (30 min of Si deposition) was repeated1. TheTOF-SIMS depth profile in Fig. 6.1(c) reveals sharpdecreases and increases of the intensities of the 30Si-and the O2-signal at the interfaces between Si andSiOx, in accordance with the backscattered electronSEM micrograph in Fig. 6.1(b), where the darkercontrast in the “center” of the STF is due to thelarger fraction of scattered primary electrons at theSiOx layer in comparison to the pure Si layers.

With adjusting the oxygen gas flow fO2, the oxygen

content in the oxygen-rich layers can be adjusted toa certain degree. As ERDA measurements on non-glancing angle ion beam sputter deposited SiOx thinfilms (θ = 0) showed, pO2

≈ 9 × 10−5 mbar re-sults in x ≈ 1.3, whereas pO2

≈ 19 × 10−5 mbarresults in x ≈ 1.8. Fig. 6.2 shows an example of anelemental depth profile gained with ERDA and theSEM cross-sectional micrograph for the respectivesample, a six-armed, Si-SiO1.8-Si chevronic structure(the oxygen-rich part of the sample was depositedwith fO2

= 20 sccm, i.e. pO2≈ 19 × 10−5 mbar).

The fact that even without oxygen flow during depo-

1The experiment took place at TS = RT, with a deposition angle θ ≈ 85, without flux aperture.

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6.1 Si-SiOx-multilayers

sition, the ERDA depth profile shows up to approximately 10 atomic percent of oxygenin the Si layers, can be attributed to the porosity of the film2.

Not only the oxgyen content of SiOx, but also the growth morphology of the as-deposited,GLAD-grown structures is affected by adjusting the oxygen partial pressure pO2

duringdeposition.

0 5 10 15 20150

200

250

300

350

400

450

500

p(O2)*10

-5[mbar]

h[n

m]

40

44

48

52

56

60

b[°

]

b bhb

(a) (b) (c)

h

Figure 6.3: (a): Structure height h and column inclination angle β as function of the oxygen par-tial pressure pO2

for slanted, columnar glancing angle deposited SiOx structures. Cross-sectional SEMmicrographs of the respective STFs with pO2

≈ 0 mbar (b) and pO2≈ 19 × 10−5 mbar (c).

Fig. 6.3 shows the effect of an increased oxygen partial pressure on both structureheight h and column inclination angle β for otherwise constant deposition parameters(deposition angle θ ≈ 85, no substrate rotation, no flux aperture, deposition timetdep = 40 min). Obviously, as not only Si atoms, but more and more Si-O bonds areincorporated in the growing structures with increasing the partial pressure of oxygen,the overall structure height approximately doubles, from h(pO2

≈ 0 mbar) = 215 nm toh(pO2

≈ 19×10−5mbar) = 445 nm. Furthermore, a shift of the column inclination anglefrom β ≈ 59, a value in good accordance to the cosine rule (Eq. 2.3) without additionaloxygen to β ≈ 44 at pO2

≈ 19 × 10−5 mbar can be observed. Thus, an increaseof the oxygen gas flow whilst deposition shifts the orientation of the columnar obliqueangle deposited structures towards the substrate normal. This behavior is probably dueto the oxygen gas flow being non-directed, in comparison to the silicon atom flux that isdirected from the target surface towards the substrate surface. Whereas the (directed) Siparticle flux yields the columnar, inclined structures as a direct result of the self-shadowingprocess in the oblique angle deposition setup, the O2 molecules reach the substrate withoutpreferred direction, thus disturbing the self-shadowing effect when they get incorporatedin the growing STF. Additionally, increasing the oxygen gas flow significantly increasesthe total pressure during deposition pdep, and therefore induces a decrease in the meanfree path l of the sputtered Si atoms. With pO2

≈ 19 × 10−5 mbar at fO2= 20 sccm,

the total pressure during deposition increases to pdep ≈ 3 × 10−4 mbar. Estimating thediameter of the residual gas atoms (Ar) and molecules (O2) as dRG ≈ 0.3 nm, withEq. 3.4, the mean free path is l ≈ 0.35 m. This is approximately one half of the lvalue gained without additional oxygen flow in the deposition chamber. Consequently,the scattering probability for the sputtered Si atoms increases. As a result, with increasedO2 background pressure in the deposition chamber, the columnar inclination β shifts tosmaller values. This effect of lowering β with increasing pO2

is opposite to the effect of

2Therefore, non-glancing angle deposited, dense thin films were used to deduce the oxygen amount x inthe SiOx layers by means of ERDA.

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6 Applications of Glancing Angle Deposited Si Nanostructures

increasing β with increasing the substrate temperature TS, as described in section 4.2.1.Thus, it is likely that increasing TS while increasing pO2

at the same time might lead toSiOx columnar thin films with inclination angles β as high as predicted by the cosine rule(Eq. 2.3).

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6.2 Spring constants of Si nanospiral arrays

6.2 Spring constants of Si nanospiral arrays

When the ratio of vertical deposition rate to substrate rotational speed ρ = r/ω isadjusted to be larger than approximately 120 nm/rev, STFs comprising of spiral-like Sistructures (helices with open core) are deposited on the substrate, as already described insection 4.1. Such spiral-like STFs, if flexible enough to withstand defined loads withoutplastic deformation, could be promising candidates for a variety of applications in thesensing domain, for example as electromechanical actuators [149]. This motivates theexamination of their mechanical behavior, especially in comparison to macroscopic spirals,as will be presented here in terms of the spirals spring constant k [121].

(a) (b)

1 µm 2 µm

b

w

W

Figure 6.4: Cross-sectional SEM micrographs of a 4-turned Si spiral-like STF (sample A) (a) and a13-turned Si spiral-like STF (sample B) (b). The micrograph in the center shows one spiral of sampleA in enlargement to underline the parameters needed for the evaluation of the theoretical values of thespring constant ktheo. w: spiral arm diameter, W : lateral structure expansion (i.e. outer spiral diameter),β: inclination angle.

To analyze the elastic deformation of spring-like Si STFs, two samples comprising ofspirals with different structure parameters have been deposited with GLAD: one four-turned spiral-like sample of total height h = 2850±50 nm (sample A) and one 13-turnedspiral-like sample of total height h = 8400 ± 50 nm (sample B). Fig. 6.4 shows bothsamples in cross-section.

As described in section 3.3.4, load-displacement curves at 4 different maximum forcesFmax

i = 100 mN, 200 mN, 300 mN and 400 mN were taken from both samples by meansof nanoindentation with a flat punch indenter, at 20 different spots for each applied Fmax

i

on both samples. The spring constants kexp were deduced from the contact stiffnessS = dFi

dhdis(i.e. the slope of the unloading curve in the point of maximum displacement

hdis, Eq. 3.8) using Eq. 3.9: k = S/(Ac · n), where Ac is the contact area of the indenterwith respect to the sample surface, and n is the areal structure number density of thespring-like nanostructures. The parameters for both samples are shown in Tab. 6.1.

Experimentally, the four-turned Si spirals of sample A exhibit a spring constant ofkexp = (64 ± 12) N/m, whereas the 13-turned Si spirals of sample B show a value ofkexp = (107 ± 35) N/m.

For macroscopic springs, the following formula containing the material parameters(Young’s modulus E and Poisson ratio ν) as well as the morphological sample parameters(arm diameter w, lateral structure expansion (“coil diameter”) W , structure inclinationangle β, and areal structure density n)

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6 Applications of Glancing Angle Deposited Si Nanostructures

sample h [nm] w [nm] W/2 [nm] β [] γ n [µm−2] kexp [N/m] ktheo [N/m]

A 2850 ± 50 160 ± 50 260 ± 25 58 ± 3 4 16 ± 4 64 ± 12 1B 8400 ± 50 250 ± 70 300 ± 50 58 ± 3 13 10 ± 4 107 ± 35 9.6

Table 6.1: Dimensions of two helical Si STF samples used for the determination of their springconstants. h: spiral height, w: spiral arm diameter, W : lateral structure expansion (i.e. outer spiraldiameter), β: inclination angle, γ: number of turns, and n: areal structure density (number of spiralsper µm2). The resulting spring constants in experiment and theory are labeled kexp and ktheo.

ktheo =E

2(1 + ν)

w4

8W 3n·

(

1 − 3w2

16W 2+

3 + ν

2(1 + ν)· tan2(90 − β)

)−1

(6.1)

can be used to calculate the spring constant ktheo [123, 150, 151].

With the use of E = (160±5) GPa (a value experimentally obtained by nanoindentationon a dense, amorphous Si film deposited on a Si substrate under normal particle incidenceθ = 0), ν = 0.279 [152], and the morphological sample parameters as listed in Tab.6.1, the theoretical spring constants can be calculated.

Following Eq. 6.1, the Si spirals of the four-turned sample A should exhibit a springconstant of ktheo = 1.0 N/m, whereas the 13-turned Si spirals of sample B should havea value of ktheo = 9.6 N/m.

Comparing experiment and theory, it is obvious that although the trends concerning kexp

and ktheo are the same when comparing sample A with sample B (the 13-turned spirals ofsample B are stiffer than the 4-turned spirals of sample A, exhibiting higher k values inboth theory and experiment), the absolute values of kexp and ktheo are differing in aboutone order of magnitude for both samples. However, those discrepancies in the springconstant values can be understood as follows: on the one hand, the third and fourth orderpower terms in Eq. 6.1 demand a precise knowledge on the spiral dimensions, whichcan not be given, as, for example, neighbored spirals are not uniform in diameter onunpatterned substrates, and the diameter of the spiral arm w for a single spiral is notconstant, but increases with increasing structure height. Within the error bars of w, W ,β and n, the theoretical values thus vary between (0.2 . ktheo . 18.5) N/m (sampleA), or between (1.2 . ktheo . 21.1) N/m (sample B), respectively. On the other hand,it is likely that the spirals in the STFs do not stay separated, but touch each other undermechanical load. Thus, when experimentally determining k with nanoindentation, theresulting contact stiffness S can not be seen as sum of the kexp values of n independentlyreacting spirals, but should rather be seen as reaction of a whole “bed” of interconnectedsprings.

When trying to determine the spring constants of glancing angle deposited STFs con-sisting of spring-like SiO structures, Seto et al. found kexp values that are larger thanthe theoretically expected values ktheo (according to Eq. 6.1) in about one order of mag-nitude as well [123]. Therefore, the observed discrepancies in theoretically expected andexperimentally gained spring constant values are not surprising.

It follows that the prediction of the mechanical behavior of GLAD-grown helical STFs isnot possible in an easy way (e.g. applying Eq. 6.1, that is valid for macroscopic, separated

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6.2 Spring constants of Si nanospiral arrays

spirals, on the nanoscopic spiral-like STFs) and should rather be done experimentally toobtain reliable results.

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6 Applications of Glancing Angle Deposited Si Nanostructures

6.3 GLAD Si structures as templates for atomic layer

deposition of magnetic thin films

6.3.1 Fe3O4 tubes with glancing angle deposited Si STF as

substrate

Atomic layer deposition (ALD) is a special case of chemical vapor deposition (CVD),that is based on sequences of self-terminating gas-solid (surface) reactions. The techniqueoffers the possibility to cover complex surface shapes in a homogeneous way, with precise(sub-nm) thickness control [153]. A basic ALD cycle consists of four steps: first, a pre-cursor material A is brought to the substrate, where a self-terminating chemical reactiontakes place at the surface. In the next step, the deposition chamber is purged (e.g. byevacuation) to remove excessive precursor material A and reaction by-products. Then,the precursor B is introduced in the reaction chamber, leading to a self-terminating re-action of B with the intermediate prepared surface state. Finally, non-reacted remnantsof B (and reaction by-products) are evacuated, and the cycle can start again [153]. Ineach cycle, a well-defined amount of material is deposited. Thus, the film thickness canbe controlled with the number of cycles.

substrate substrate

Si column

Fe O

coating3 4

Si column

ALD (Fe O )

+ reduction2 3

Figure 6.5: Principle sketch of the tube-like coating of glancing angle deposited (inclined) Si columnswith Fe3O4 by atomic layer deposition of Fe2O3 and subsequent reduction step.

For the present study, ALD of Fe2O3 with a subsequent reduction to Fe3O4 was doneat the Universität Hamburg on Si STFs consisting of either vertical columnar structuresdeposited with glancing angle deposition and fast substrate rotation (structure heighth ≈ 520 nm) or inclined columnar structures deposited by oblique angle deposition(structure height h ≈ 600 nm). The coating of the Si STFs took place in an ALDreactor3 at TS = 200C, with ferrocene (Fe(C5H5)2, usually abbreviated Cp2Fe) as firstand ozone as second precursor [154]4. The first part of the ALD cycle consisted of agaseous5 ferrocene pulse of 2 seconds, followed by an exposure time of 30 seconds and apumping time of 40 seconds. The second part of the cycle started with a 0.2 second pulseof ozone, with a subsequent exposure time of 30 seconds, followed by a pumping time of30 seconds. The chamber and lines of the precursors were permanently flooded with N2

(fN2= 40 sccm). With a deposition rate of nominally 0.02 nm/cycle, 500 cycles resulted

in a Fe2O3 coating of the Si columns with a thickness of approximately 10 nm.

3Cambridge NanoTech Savannah 100.4The oxidation reaction is 2 Cp2Fe + O3 → Fe2O3 + 2 Cp2.5Ferrocene is heated up to 100C in order to gain a sufficient vapor pressure.

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6.3 GLAD Si structures as templates for atomic layer deposition of magnetic thin films

After ALD, the STFs (now coated with a tube-like layer of Fe2O3 around each column)were annealed for 12 hours in an Ar/H2 (95%/5%) atmosphere at 400C in an annealingoven6, resulting in a reduction of Fe2O3 to ferrimagnetic Fe3O4

7 [156].

The principle of the deposition is sketched in Fig. 6.5, and SEM micrographs of the usedsamples before and after ALD are shown in Figs. 6.6(a-c). To underline the coating effectof the GLAD-grown Si structures with ALD, a TEM bright field micrograph showing thetop of a Si chevron that was covered with approximately 15 nm of Fe3O4 with the samecombination of ALD and following reduction step (in a previous experiment) can be seenin Fig. 6.6(d).

500 nm 100 nm

Fe O3 4

Si(a) (b) (c) (d)

Figure 6.6: Cross-sectional SEM micrographs of (a) vertical columnar Si structures and (b) inclinedcolumnar Si structures before ALD. Cross-sectional SEM micrograph of the inclined columnar Si struc-tures after coverage with Fe3O4 by means of ALD and reduction (c). TEM bright field micrograph of thetop of a chevronic Si structure covered with a Fe3O4 layer of approximately 15 nm thickness (measurementdone by Prof. Dr. T. Höche) (d).

As Fig. 6.6 shows, obviously it is possible to deposit tube-like Fe3O4 nanostructureswith ALD on a substrate pre-patterned with GLAD-grown Si structures.

6.3.2 Magnetization measurements

In order to evaluate the magnetic response of the two different Fe3O4-coated samples of SiSTFs, the changes in the coercive field HC (i.e. the field strength needed to demagnetizethe material after magnetization in a field of strength H) were measured and recordedangular-dependent (with respect to the substrate normal) by SQUID magnetometry8 atT = 300 K.

For the STF sample that consists of inclined columns (Figs. 6.6(b) and (c)), the mag-netization experiments were done as follows: first, the sample was aligned such that theprojection of the length axis of the columns (and therefore the long axis of the column-surrounding Fe3O4 tubes) was perpendicular with respect to the direction of the appliedfield H. Then, a hysteresis loop was recorded to extract the coercivity HC in this position(an example of such a curve is exemplarily shown in Fig. 6.7(a)). In the following steps,the sample was rotated around the substrate normal in angular steps of ∆δ = 10, and themeasurement of HC was repeated at every angle δ in the angular range 0 ≤ δ ≤ 180.

6MILA-3000 Mini Lamp Annealer (ULVAC Technologies, Inc.).7To prevent the Fe3O4 layer from re-oxidation, before annealing a covering layer of SiO2 (thickness

approximately 5 nm) was deposited with ALD (as described in Ref. [155]).8A SQUID is a Superconducting QUantum Interference Device, here implemented in the Magnetic

Properties Measurement System MPMS-XL (Quantum Design).

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6 Applications of Glancing Angle Deposited Si Nanostructures

-6000-4000-2000 0 2000 4000 6000-4

-3

-2

-1

0

1

2

3m

[10

-4em

u]

H [Oe]

HC

0 30 60 90 120 150 180-50

0

50

100

150

200 inclined columsvertical columns

DH

C[O

e]

d [°]

(a) (b)

Figure 6.7: Example of a hysteresis loop to extract HC of a STF consisting of tube-like Fe3O4

structures (H is the magnetic field, m the magnetic moment, and HC the coercivity) (a). Depen-dency of HC on the angle δ between magnetic field H and direction of column growth in terms of∆HC = HC(δ) − HC(0) (b). In SI units, [Oe] equals [103/(4π) A/m], and [emu] equals [10−3 Am2].Measurements done by O. Albrecht (Universität Hamburg).

Similar experiments were done with a sample that contains vertical, upright columnarstructures (as shown in Fig. 6.6(a)). Due to the rotational symmetry of this sample withrespect to the substrate normal, the “starting point” δ = 0 was chosen arbitrarily in thiscase. A top-view sketch of the measuring setup, pointing out the difference between thetwo cases of either inclined or upright columns, is shown in Fig. 6.8.

H

dd

(a) (b) (c) (d)H

d = 0° d = 90°inclinedcolumns

verticalcolumns

HH inclinedcolumn

verticalcolumn

inclinedcolumn

verticalcolumn

Fe O

Si3 4

Figure 6.8: Sketch (top view) to underline the angle dependency of the coercivity HC on the alignmentof the column growth direction with the applied magnetic field H for inclined (a) and vertical, rotationalsymmetric columns (b) if the sample is rotated with the substrate normal as rotation axis. For the caseof the vertical columns, a cross section of the columns is drawn to illustrate the difference between theSi core and the Fe3O4 cladding. Whereas for the case of inclined columns, HC can be expected to showa minimum if the projection on the substrate surface of the long axis of the Fe3O4 tube is perpendicularto the direction of H (c) and vice versa (d), for the vertical columns, no dependency of HC on H shouldbe expected (c,d).

When ∆HC = HC(δ) − HC(0) is drawn as function of δ for both samples, remarkabledifferences are observable, as can be seen in Fig. 6.7(b). Obviously, whereas HC staysapproximately constant in the case of the vertical columnar structures, in the case of theinclined columns, ∆HC increases monotonically from δ = 0 (perpendicular alignmentof the projection of the long axis of the Fe3O4 tubes on the substrate surface with respectto the direction of H) to δ ≈ 90 (parallel alignment of the directions of H and the

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6.3 GLAD Si structures as templates for atomic layer deposition of magnetic thin films

projection on the substrate surface of the long tube axis)9. After surpassing δ ≈ 90,∆HC decreases again, until it reaches ∆HC ≈ 0 at δ = 180 (direction of H againperpendicular to the long axis of the Fe3O4 tubes).

Those results lead to the assumption that the tube-like ferrimagnetic Fe3O4 structuresexhibit the magnetic easy axis in the direction of the long axis of the tube. Whereas in thecase of vertical columns, the coercivity HC is angle-independent and stays approximatelyconstant for all values of δ, once the Si columns (and therefore the ferrimagnetic tubessurrounding them as well) are not rotational symmetric (with respect to the substratenormal) anymore, the magnetic response of the sample shows an angular dependency,having the largest HC values (i.e. the “strongest” magnetization) when the projection ofthe columnar growth direction on the substrate is aligned parallel with the direction ofthe external magnetic field H. Figs. 6.8(c) and (d) depict the influence of the alignmentof the tube axis with the direction of H on HC for the two extreme cases δ = 0 andδ = 90.

As this example shows, the combination of GLAD and ALD offers the possibility todeposit tube-like magnetic nanostructures, whose magnetic behavior could be tailored toa certain extent, as GLAD allows the deposition of structures with a very complex shape.

9The small deviation of the maximum in the ∆HC curve from δ = 90 is probably due to uncertaintiesin the initial alignment of the sample in the measuring setup.

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6 Applications of Glancing Angle Deposited Si Nanostructures

6.4 Summary of results: applications of glancing

angle deposited Si nanostructures

Si-SiOx multilayers with GLAD

Concerning the growth of Si-SiOx multilayers with ion beam sputter glancing angledeposition the following was found:

• The growth of Si-SiOx multilayered STFs with the experimental setup is possibleby applying an intermittent oxygen flow fO2

in the deposition chamber.

• TOF-SIMS measurements of the multilayered STFs reveal remarkably sharp edgesin the depth profiles at the Si-SiOx interfaces, the dynamic range of the signalscorresponding to Si and O being about one order of magnitude.

• ERDA measurements showed that the oxygen content x in the SiOx layers can bevaried with fO2

between 1.3 . x . 1.8.

• For the growth of slanted, columnar SiOx structures in the oblique angle depositionsetup (no substrate rotation), it was found that the deposition rate r increases withincreasing oxygen partial pressure pO2

as more and more Si-O bonds are incorporatedin the evolving columns, whereas the column inclination angle β decreases. Thelatter effect can be explained by increased Si particle scattering at residual gasatoms with increasing pO2

(and, consequently, decreasing mean free path l) and theundirected nature of the oxygen gas flow in the chamber, both effects counteractingthe atomic self shadowing.

Spring constants of helical Si GLAD structures

Performing nanoindentation experiments with a flat punch indenter on glancingangle deposited Si STFs that comprise spring-like structures, the findings were as follows:

• Up to a maximum load Fmax . 400 mN, the STFs react quasi-elastically, with onlylittle plastic deformation.

• Spring constants k of the spiral-like structures could experimentally be obtainedfrom the contact stiffness S of the films.

• Comparing the experimental spring constant values kexp with theoretical values ktheo,it was found that

– both experiment and theory show the same trends concerning the developmentof k with changing the structure morphology, but

– the experimental values are about one order of magnitude larger than thetheoretical predictions.

– This discrepancy is possibly due to interactions of adjacent nanospirals in theSTF and uncertainties in the determination of the spirals’ morphological vari-ables (e.g. structure diameter and inclination angle).

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6.4 Summary of results: applications of glancing angle deposited Si nanostructures

• It follows that the mechanical response of STFs in terms of the spring constant kcan not easily be predicted using formulas valid for macroscopic springs, but shouldrather be determined experimentally for the respective samples.

GLAD Si structures as template for ALD of magnetic thin films

First experiments of atomic layer deposition on glancing angle deposited SiSTFs show that

• the formation of a tube-like, ferrimagnetic Fe3O4 cladding that surroundsthe columnar Si structures is possible with ALD of Fe2O3 and a subsequentreduction step.

• Magnetization experiments show that inclined Si columns with a tubular Fe3O4 shellexhibit an angular-dependent coercivity HC , depending on the orientation ofthe growth direction of the column (i.e. the magnetic easy axis of the Fe3O4 tube).

Thus, the combination of GLAD (with its possibility to deposit nearly arbitrarily shapednanostructures) with ALD (giving the possibility to uniformly coat surfaces with verycomplex shapes) appears to be a very promising approach for the synthesis of tubularmagnetic structures. It seems likely that the magnetic behavior of such structures can beinfluenced and even tuned by control of the pre-ALD structure morphology with GLADand precise control of the tube thickness with the number of ALD cycles.

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6 Applications of Glancing Angle Deposited Si Nanostructures

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7 Summary and Conclusions

In this work, several aspects concerning the morphological evolution of Si nanostructuresthat were grown using glancing angle ion beam sputter deposition were examined. Asystematic experimental study on the effect of several parameters that influence the mor-phological evolution of the Si nanostructures that grow in form of sculptured thin films waspresented. Additionally, the experimental results were compared with three-dimensionalMonte Carlo computer simulations.

In the first part of the work, the influence of different deposition parameters on theglancing angle deposition of Si nanostructures on unpatterned substrates was pre-sented. Besides the study of the influence of parameters like deposition angle, ratio ofvertical deposition rate to substrate rotational speed and deposition time (i.e. structureheight) on the morphological evolution of the as-deposited Si structures, in particular theinfluence of the substrate temperature TS on the structure development of helical andcolumnar nanostructures was examined. For spiral-like Si STFs, it was found that a “tem-perature window” exists in which increasing TS leads to the growth of fibrous, denselypacked, separated Si spirals with diameters of approximately 30 nm and less. Those STFsexhibit no structure diameter broadening with increasing structure height (as would beexpected from the power law scaling model by Karabacak et al. [25] that is expressed inEq. 2.7). This behavior was qualitatively explained with a simple growth model and isbacked by MC simulations of glancing angle sputter deposition with rotating substrateand different surface diffusion steps. Therefore, it could be shown that increasing TS inGLAD does not automatically result in severe structure broadening and merging due tothe adatom diffusion length overcoming the inter-structure distance.

In the second part of the work, the influence of the pattern geometry on the GLADgrowth of Si nanostructures on artificially seeded substrates was examined. As mostresearchers so far concentrated on highly symmetric, usually tetragonal pattern arrange-ments for the artificial seeds, here the focus of the analysis was lying on the honeycomb andhcp seed arrangements that result from the nanosphere lithography patterning principle.It could be shown that not only the aspect ratio of seed height to inter-seed distance, thedeposition time and the substrate rotational speed, but also the pattern geometry itselfhas a profound influence on the growth behavior of the Si structures deposited thereon.Due to different inter-seed distances in different growth directions, on rapidly rotatinghoneycomb-patterned substrates the resulting columnar structures were found to exhibita triangular cross-section, in contrast to those grown on more regular patterns (tetragonaland hcp), that exhibit cylindrical shapes. Again, those findings are in accordance withMC simulations run on height matrices that represent the underlying pattern geometry.Additionally, the influence of the deposition angle θ on the Si sputter deposition on hon-eycomb templated substrates was systematically examined, both in experiment and MCsimulation. Three θ-dependent growth regimes were identified that either lead to hillock-capped dense films (low θ values), columns growing on the seeds that eventually touch eachother and are intersected by pyramidal-shaped inter-seed grown structures (intermediateθ values), or to separated, columnar structures with only little inter-seed condensation(high θ values, glancing angle case). It could be shown that a series of intermediate and

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7 Summary and Conclusions

glancing angle θ values during one deposition cycle results in a porous layer of hexagonallyclose packed arranged pores, without disturbing inter-seed growth. The pore diameter de-creases with deposition time and can therefore be adjusted.Finally, a new two-step EBL lithography patterning approach was introduced, that inprinciple allows the deposition of isolated, free-standing nanostructures with a manifoldof different shapes (depending on the substrate rotation algorithm), and of basically anymaterial at arbitrary positions on the substrate, without having to obey certain pre-pattern design considerations (Eqs. 2.8 and 2.9, [28]). First results have shown that thepatterning technique with subsequent GLAD works in principle, although further workhas to be done to overcome certain technical defiances.

In the third and last part of the work, some further aspects and applications of the ionbeam sputter glancing angle deposited Si nanostructures were presented. In particular, itwas shown that Si-SiOx multilayered STFs can easily be deposited with the existingdeposition setup, by applying an intermittent oxygen gas flow during GLAD. Furthermore,nanoindentation experiments with Si STFs consisting of spiral-like nanostructurescould be used to determine the spring constants of those nano-springs. In the end,it was shown for the first time that glancing angle deposited Si nanostructures can beused as templates for subsequent atomic layer deposition. In the scope of this work,columnar Si STFs with different inclination angles of the columns with respect to thesubstrate normal were deposited with GLAD and used as backbone for the ALD of Fe3O4

tube-like magnetic films surrounding the Si columns. It could be demonstrated thatthe magnetic properties of the samples strongly depend on the column inclination angle,allowing the proposal that they can be tailored by adjusting the right glancing angledeposition parameters.

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List of Acronyms

The following list summarizes acronyms used throughout the text.

GLAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . glancing angle depositionOAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . oblique angle depositionIBSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ion beam sputter depositionALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . atomic layer depositionPVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .physical vapor depositionCVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chemical vapor depositionPLD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .pulsed laser depositionSEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . scanning electron microscopyTOF-SIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . time-of-flight secondary ion mass spectrometryERDA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .elastic recoil detection analysisNSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . nanosphere lithographyEBL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . electron beam lithographyPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .polystyrenehcp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . hexagonally closed packedSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . single layerDL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .double layerMC simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monte Carlo simulationlu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lattice unitRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . room temperatureSTF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sculptured thin filmFFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . fast Fourier transformationPSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . power spectral density function

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List of Variables

List of variables used throughout the text:

l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . shadowing length [nm]hS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . seed height [nm]θ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . deposition angle relative to substrate normal []β . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . column inclination angle relative to substrate normal []Λ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . surface diffusion length [nm]L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . surface diffusion coefficient [nm2/s]TS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . substrate temperature [C]φ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . azimuthal angle []ω . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . rotational speed [rev/min]r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vertical deposition rate [nm/min]ρ = r/ω . . . . . . . . . . . . . . . . . . . ratio of vertical deposition rate to rotational speed [nm/rev]c . . . . . . . . . . . . . . . . . . . . . . . . . . . pitch (height after one full turn for helical structures) [nm]w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . structure (arm) diameter [nm]h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . structure height [nm]p, q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . growth exponentsdS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . inter-seed-spacing [nm]wS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . seed diameter [nm]P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .period (of template pattern) [nm]TM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .melting point [C]NOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . number of particles depositedNPR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . number of particles per rotationDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . number of diffusion stepsD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .NSL sphere diameter [nm]pdep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . deposition pressure [mbar]E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ion energy [eV]MT /MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ratio of target atom mass to incident ion massαS−T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ion incidence angle on target []Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sputtering yieldξ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .atom emission angle from target []EA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . energy of sputtered atom [eV]〈ESi〉 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . average energy of sputtered Si particles [eV]UB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . beam voltage [V]UA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .acceleration voltage [V]fAr, fO2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .gas flow [sccm]l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .mean free path of sputtered atoms [m]dRG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . diameter of residual gas atom or molecule [nm]PHF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RF generator power [W]IB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .beam current [mA]IA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . acceleration current [mA]tdep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . deposition time [min]o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .opening of slit aperture [mm]

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d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . inter-seed distance (center-to-center) [nm]d1, d2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . inter-seed distances in NSL case [nm]f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . spatial frequency [nm−1]fmax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . maximum peak in spatial frequency [nm−1]PSD . . . . . . . . . . . . . . . . . . . . . . . . . . . angular averaged power spectral density function [nm4]λ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . column-column separation [nm]Fi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . loading force of indentation [mN]Ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . contact area (of indenter) [µm2]hdis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . displacement (of sample under load) [nm]S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . contact stiffness [N/m]k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .spring constant [N/m]n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . areal structure number density [µm−2]hcol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . column length [nm]hcrit(TS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . critical height of merging [nm]W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lateral structure expansion [nm]R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . structure radius [nm]Rsat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . structure saturation radius [nm]A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . area of the cross-section of a columnar structure [nm2]b . . . . . . . . dot diameter for non-glancing angle deposition on honeycomb substrates [nm]wpore . . . . . . . . . . . . . . . pore diameter of hcp arranged pores on honeycomb substrates [nm]H . . . . . . . . . . . . . . . . . . . . . . . . . . . structure height at which a shift from θ1 to θ1 occurs [nm]a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . inter-structure-distance (on nanospheres) [nm]hcap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .height of hemispherical cap [nm]F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . fraction of nanosphere diameter covered with Si [%]nn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . estimated number of nuclei on sphereAeff . . . . . . . . . . . . . . . . . . . . . . . . . effective surface area for nucleation on nanospheres [nm2]Rgap . . . . . . . . . . . . . . . . . . . . . . . . . radius of circular gap (two-step lithography process) [nm]pO2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . oxygen partial pressure [mbar]γ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .number of turns (spiral STFs)H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . magnetic field [Oe]HC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . coercive magnetic field [Oe]m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . magnetic moment [emu]δ . . . . . . . . . . . . . angle between magnetic field H and long axis of a columnar structure []

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Acknowledgements

Quite naturally, it is rather impossible for one person to perform all the work that underliesa thesis all alone. Consequently, I owe a lot of thanks to many co-workers, and beforeI start doing this, I would like to apologize to anyone I might have forgotten, which forsure was not on purpose.

I would like to thank Prof. Dr. Bernd Rauschenbach for giving me the possibility towork on this project at the IOM, the many helpful and stimulating discussions we had,and generally for his support during the last three years.

Dr. Jürgen Gerlach I would like to thank for quite a lot, basically for always helpingme when “technical” problems occurred, for the helpful discussions, hints and suggestionsand the scientific input therein, for performing TOF-SIMS and ERDA measurements, forhis critical comments concerning this thesis, and basically for being a good friend.

Prof. Dr. Thomas Höche I would like to thank for TEM measurements and for thelearning sessions at the SEM, which I would also like to thank Dietmar Hirsch for.

Dr. Frank Frost and Jens Völlner I would like to thank for their help with the PSDcalculations.

Asst. Prof. Dr. Tansel Karabacak I would like to thank for performing the MC sim-ulations and allowing me to use them in this thesis, for the fruitful discussions, and thegood co-operation.

For their enduring help concerning all questions on ion beam sources I am greatlythankful to Horst Neumann, Dr. Bernd Faust and Dr. Carsten Bundesmann.

There were many, sometimes frustrating, attempts to pattern substrates with EBL in theoften unconventional ways I needed it, which I would like to thank Dr. Joachim Zajadaczand Dr. Wilfried Erfurth for. Additionally, I would like to thank Renate Fechner andPetra Hertel for the pattern transfer, and Dr. Klaus Zimmer for the lot of stimulatinginput concerning substrate patterning. Furthermore, many thanks to Dr. Bodo Fuhrmannand his team for always providing me with NSL patterned substrates, and the stimulatingcollaboration.

For the always fast and well-performed technical solutions, I would like to thank thewhole team of the IOM workshop, and in particular Stefan Daum and Ann-Grit Birnbaum.

A big “Thank you!!!” also goes to my companions at the GLAD deposition chamber, He-lena Hilmer (with a special thank for helping me during my first days at the IOM), AndréMießler (whom I owe a special thank for performing the nanoindentation measurements)and Chinmay Khare.

I am thankful to Dr. Eva Schubert for establishing GLAD research at the IOM.

For always supplying me with wafer material and her help in the chemical clean room,I would like to thank Ingrid Herold.

Furthermore, I am grateful to Uta Gleisberg for the TEM sample preparation.

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Of course, the work on a thesis benefits from mutual help in many aspects. Therefore,I would like to thank all the other PhD candidates that stayed at the IOM during theperiod of my PhD work there.

For performing the ALD experiments as well as the magnetic measurements, and thegood co-operation I would like to thank Robert Zierold, Ole Albrecht and Prof. Dr.Kornelius Nielsch.

mens sana in corpore sano, as Juvenal is often cited incompletely. Consequently,I would like to thank the IOM football team for keeping me fit on the pitch, and all therunners at the IOM for keeping me fit on the jogging track.

Last, not least, I thank Antje and my whole family and friends for years of enduringencouragement, understanding and never really questioning what I am spending my timeon.

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Curriculum Vitae

Name Christian Konrad Patzig

Date of birth October 10th, 1980

Place of birth Weida (Germany)

Marital status engaged

Education and scientific activities

1987 - 1991 Primary school (Max-Greil-Schule, Weida)

1991 - 1999 Secondary school (Georg-Samuel-Dörffel-Gymnasium Weida)

1999 School-leaving certificate (Abitur)

1999 - 2000 Military service (1. PiBtl 701, Gera)

2000 - 2001 Study of Media- and Communication Sciences(Technische Universität Ilmenau)

2001 - 2006 Study of Physics(Friedrich-Schiller-Universität Jena andCork Institute of Technology (Cork, Ireland))

2006 Diploma in Physics(Friedrich-Schiller-Universität Jena)

Topic of Diploma thesis: Herstellung und Charakterisierungdünner Wismutschichten auf gitterunangepassten Substraten

since 07/2006 Doctorial candidateLeibniz-Institut für Oberflächenmodifizierung e.V., Leipzig

since 11/2007 Member of the Graduate School BuildMoNaUniversität Leipzig

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Publications

The following articles have been published in the course of this thesis, are submitted, orin preparation for future publication.

1. C.Patzig, B. Rauschenbach, W. Erfurth and A. Milenin. Ordered silicon nanostruc-tures by ion beam induced glancing angle deposition. Journal of Vacuum Scienceand Technology B, 25(3), 833, (2007)

2. C. Patzig, B. Rauschenbach, B. Fuhrmann and H.S. Leipner. Growth of Si nanorodsin honeycomb and hcp arrays using glancing angle deposition. Journal of AppliedPhysics 103, 024313, (2008)

3. C. Patzig and B. Rauschenbach. Temperature effect on the glancing angle depositionof Si sculptured thin films. Journal of Vacuum Science and Technology A, 26(4),881 (2008)

4. C. Patzig, T. Karabacak, B. Fuhrmann and B. Rauschenbach. Glancing angle sput-ter deposited nanostructures on rotating substrates: experiments and simulations.Journal of Applied Physics 104, 094318, (2008)

5. C. Patzig, B. Rauschenbach, B. Fuhrmann and H.S. Leipner. Silicon nanocolumnson nanosphere lithography templated substrates: effects of sphere size and substratetemperature. Journal of Nanoscience and Nanotechnology 9, 1985, (2009)

6. I. Abdulhalim, A. Karabchevsky, C. Patzig, B. Rauschenbach, B. Fuhrmann, E. Elt-zov, R.S. Marks, J. Xu, F. Zhang, and A. Lakthakia. Surface-enhanced fluorescencefrom metallic nano-sculptured thin films with application to biosensing in water.Applied Physics Letters 94, 063106 (2009)

7. R. Nagar, B.R. Mehta, J.P. Singh, C. Patzig, B. Rauschenbach, V. Sathe, and D.Kanjilal. Ion beam induced anisotropic deformation of Si nanosprings. Journal ofPhysics D 42, 145404 (2009)

8. A. Shalabaney, A. Lakhtakia, I. Abdulhalim, A. Lahav, Christian Patzig, I. Hazek,A. Karabchevky, Bernd Rauschenbach, F. Zhang, und J. Xu. Surface plasmonresonance from metallic columnar thin films. Photonics and Nanostructures, inpress, (2009)

9. R. Nagar, C. Patzig, B. Rauschenbach, B.R. Mehta, und J.P. Singh. Factors affect-ing the mechanical response of chevron nanostructures: A finite element analysisand force-deflection spectroscopy study. Journal of Nanoscience and Nanotechnol-ogy, accepted for publication (2009)

10. C. Patzig, J. Zajadacz, K. Zimmer, R. Fechner, C. Khare, und B. Rauschenbach.Deposition of nanostructures with arbitrary periods: new patterning concept forglancing angle deposition. Applied Physics Letters, submitted (2009)

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11. O. Abrecht, R. Zierold, D. Görlitz, K. Nielsch, C. Patzig, B. Rauschenbach, und S.Allende. Experimental evidence for an angular dependent transition of magnetiza-tion reversal modes in magnetic nanotubes. in preparation

conference proceedings

1. I. Abdulhalim, A. Karabchevsky, C. Patzig, B. Rauschenbach and B. Fuhrmann.Comparative study of enhanced fluorescence from nano sculptured thin films. Proc.SPIE, (nanostructured thin films), 7041, 70410G, (2008)

2. B. Rauschenbach and C. Patzig. Periodic nanoscale Si structures by ion beaminduced glancing angle deposition. Proc. 2nd IEEE International NanoelectronicsConference, Shanghai 1084-1088, (2008)

3. J. W. Gerlach, C. Patzig, W. Assmann, A. Bergmaier, Th. Höche, J. Zajadacz,R. Fechner, and B. Rauschenbach. Swift Heavy Ion Irradiation Induced Effects inSi/SiOx Multi-Layered Films and Nanostructures. Proc. 2009 MRS spring meeting,(in preparation)

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Selbstständigkeitserklärung

Hiermit versichere ich, dass die vorliegende Arbeit ohne unzulässige Hilfe und ohne Be-nutzung anderer als der angegebenen Hilfsmittel angefertigt und dass die aus fremdenQuellen direkt oder indirekt übernommenen Gedanken in der Arbeit als solche kenntlichgemacht wurden.

Ich versichere, dass alle Personen, von denen ich bei der Auswahl und Auswertung desMaterials sowie bei der Herstellung des Manuskripts Unterstützungsleistungen erhaltenhabe, in der Danksagung der vorliegenden Arbeit aufgeführt sind.

Ich versichere, dass außer den in der Danksagung genannten, weitere Personen bei dergeistigen Herstellung der vorliegenden Arbeit nicht beteiligt waren, und insbesonderevon mir oder in meinem Auftrag weder unmittelbar noch mittelbar geldwerte Leistungenfür Arbeiten erhalten haben, die im Zusammenhang mit dem Inhalt der vorliegendenDissertation stehen.

Ich versichere, dass die vorliegende Arbeit weder im Inland noch im Ausland in gleicheroder in ähnlicher Form einer anderen Prüfungsbehörde zum Zwecke einer Promotion odereines anderen Prüfungsverfahrens vorgelegt und in ihrer Gesamtheit noch nicht veröf-fentlicht wurde.

Ich versichere, dass keine früheren erfolglosen Promotionsversuche stattgefunden haben.

Leipzig, 08.05.2009 Christian Patzig

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