Institut für Angewandte Physik

Festkörperanalytik

Vorlesung „ Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik

FestkörperanalytikDünne Schichten

Organisatorisches

Johannes Heitmann Institut für Angewandte Physik Gellert-Bau, EG.17Tel.: 39 2590E-Mail: johannes.heitmann@physik.tu-freiberg.de

2

Vorlesungsfolien finden Sie unter:http://tu-freiberg.de/fakult2/angph/studium/

Nutzer: iapuserPasswort: iap0107

Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik

Dünnschichtanalytik1. Einleitung

- Übersicht über unterschiedliche Techniken

- Vergleich von Auflösung, Empfindlichkeit und Eindringtiefe

- Grundlagen der Vakuumtechnik

2. Ionenbasierte Anregung

- IonenstrahlanalytikRBS, PIXE, NRA, ERDA; Microprobes

- Ionenbasierte Sputter prozesseSIMS, SNMS

3. Elektronenbasierte Anregung

3

- Scanning Electron MicroscopyElektronenangeregt Abbildung, EDX, STEM

- Transmissionselektronmikroskopie (TEM)Abbildung, EELS, EDX, HRTEM, Dunkelfeld

4. Röntgenanregung

- Detektion von Röntgenstrahlung (XRF), Photoelektronen (XPS)

5. Photonen

- Ellipsometrie

- Infrarotspektroskopie

- Raman

- Photolumineszenz

Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik

Dünnschichtanalytik1. Einleitung

- Übersicht über unterschiedliche Techniken

- Vergleich von Auflösung, Empfindlichkeit und Eindringtiefe

- Grundlagen der Vakuumtechnik

2. Ionenbasierte Anregung

- IonenstrahlanalytikRBS, PIXE, NRA, ERDA; Microprobes

- Ionenbasierte Sputter prozesseSIMS, SNMS

3. Elektronenbasierte Anregung

4

- Scanning Electron MicroscopyElektronenangeregt Abbildung, EDX, STEM, Auger

- Transmissionselektronmikroskopie (TEM)Abbildung, EELS, EDX, HRTEM, Dunkelfeld

4. Röntgenanregung

- Detektion von Röntgenstrahlung (XRF), Photoelektronen (XPS)

5. Photonen

- Ellipsometrie

- Infrarotspektroskopie

- Raman

- Photolumineszenz

Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik

Scanning probe microscopy

SPM (Scanning probe micrioscopy)

- Atomic force microscopy(AFM)

- Conductive atomic force

5

- Conductive atomic forcemicroscopy (c-AFM)

- Scanning tunnelingmircoscopy (STM

Vorlesung „Festkörperanalytik″Johannes Heitmann, Institut für Angewandte Physik

Atomic force microscopy (AFM)

V

photo diodemirror

laser

piezo

feedback loop

6Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

tip

sample

Beispiele

7Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

Technical realization

Basic Components

measurement tip

movement of tip in all 3 directions

feedback loop

signal processing

8Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

many components same for STM, AFM, c-AFM, therefore often combined microscopes

...

signal processing

sample environment and sample holder

Cantilever & Tip

material often isolating (e.g. Si3N4)tip radius ≈ 10 .. 30 nm

spring constant of the cantilever: k = 0,005 - 50 N/m(for comparison spring in a ball point pen - 1000 N/mspring in a car - 10.000 N/m)

9Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

spring in a car - 10.000 N/m)

Technical Realization

The tips are sensitive ....

Measurement Tips

10page 10

... and therefore consumeables.

L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization

Cantilever & Tip control

11Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

PrinzipDistance-Force-Relation

repulsion

tip

distance

typical length ~ 1 nm

The atomic force microscopy is based on the measurement of the force between the tip and the sample.

12Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

Van-der-Waals bonding forcesdominate

forc

eo

n t

he

tip

distancetip - sample

SurfaceHow a surface looks like under ambient conditions ?

water layer

The surface of materials under ambient conditions has a thin water film on top, particularly, if the surface is hydrophilic.

This is often the case for oxides. In this case OH groups form the outer surface which favors the bonding of water molecules.

13Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

favors the bonding of water molecules.

The thickness of the water film depends on the properties of the surface and the humidity. Typical thicknesses on hydrophilic surfaces are 1.5 ... 3 nm.

Hydrophilic materials (e.g. noble metals) the thickness is reduced to about 1 ... 4 monolayers of water.

atth

etip distance

tip - sample

Contact mode

14page 14L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes

forc

e

• constant deflection of the cantilever• sample contact through the water film• force range nN ... µN

Contact mode

advantages • high scan speed • especially suitable for ratherrough samples

• very good spatial resolution

15Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

Contact mode

disadvantages • lateral forces may distort the image• forces rather large due to the capillary forces of the water film• deterioration of the surface possible

16Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

1 µm scan 2 µm scan

forc

eat

the

tip distancetip - sample

Non-Contact mode

17page 17L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes

forc

e

• forced vibration of the cantilever close to the resonance frequency

• oscillation amplitude ≤ 10 nm• approach to the surface changes frequency, hence, the amplitude as well

Non-Contact mode

Advantages: • small deterioration of the sample• practically no lateral forces• high spatial resolution

disadvantages • often usable only in HV/UHV

18Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

NaCl surface

• often usable only in HV/UHV• therefore not useable forbiologic sample

forc

eat

the

tip distancetip - sample

Intermittent Mode

19page 19L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.2. Physical Processes

forc

e

• forced vibration of the cantilever withresonance frequency

• oscillation amplitude 20 .. 100 nm• forces in the range ≤ 200 pN• change of the amplitude by approachingthe sample surface

Intermittent Mode

advantages • small deterioration of samples• works in liquids as well • usable for biologic samples

disadvantages• rather strong load of the cantilever• reduced spatial resolution

20Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

• reduced spatial resolution

structure of chromosoms

Influence of the Tip Radius

The tip radius and the tip aspect determines the smallest resolvablesurface features.

21Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

Stiffness Measurements Contact Mode

Special Application

22page 22L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.2. AFM 2.3.2.4. Application Examples

topography force modulation

Special ApplicationFriction Force Microscopy

topography

23Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

topography

friction force

Self-assembly of alkanethiol on gold

STM - Scanning Tunneling Microscopy

nA

Itip

piezo actor

24page 24

L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.1. Basic Principle & Result

Icontrol feedback loop

R

∆I -> ∆V

examples

Copper surface

25Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

Fe on Cu (4K)

Ni (011) surface

7x7 reconstructed Si(111)

Basic principle

The function of the STM needs a description on the basis of quantum physics.

Tip Sample

STM - Scanning Tunneling Microscopy

26page 26

Quantenmechaniktunnel

currentClassical

Mechanics

L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes

Parameters Influencing the Tunneling Current

Sample Tip

Ene

rgy

voltage U appliedat the tip (positive)

WF,S WF,T

Basic principle

27page 27L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes

Ene

rgy

Distance

at the tip (positive)

Distance d

( )SFT

WdUI,

exp ⋅−⋅∝

Parameters Influencing the Tunneling Current

Sample Tip

Ene

rgy

voltage U appliedat the tip (positive)

Basic principle

28page 28L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes

Ene

rgy

Distance

at the tip (positive)

Distance d

Furthermore, the current density is influenced by the density of states (DOS) in the tip and in the sample

Basic principleParameters Influencing the Tunneling Current

29page 29L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.2. Physical Processes

NiAu

For example, the density of electronic states of Au is smaller in comparison to Ni. To keep the current constant the tip has to be approached closer to the surface at the position of the Au atom.

Basic principleSTM Tunneling Current

typical relation between tunneling current and distance sample - tip

Tun

nelin

g C

urre

nt /

nA

Half Atomic Radius

30Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

the exponential correlation results in an extreme sensitivity of the current vs. distance

Distance / nm

Tun

nelin

g C

urre

nt /

nA

Technical RealizationMeasurement Tips

conditions - conducting tip (and sample!)- extreme small tip radius (~ 10 nm) - chemically stable

most common tip material:tungsten tips, produced with

31page 31

tungsten tips, produced with an electrochemical etching process in NaOH

Technical RealizationMovement of the Tip and Feedback Loop

fixed: constant height z fixed: constant current i

32page 32

L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization

current i

position x position x

position z

Environmental Conditions

STM only for a small number of materials possible under ambient conditionsexamples: graphite, noble metals

the majority of applications requires UHV

Technical Realization

33page 33L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.3. Technical Realization

Example: surface reconstruction Si(7x7)

Example

34page 34L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM 2.3.1.4. Application Examples

ExampleExamples: details of thin film growth

Tb on W(110)(200 x 150 nm2)

blue - H adsorption sitesyellow/green - stacking faults in Tb

35Vorlesung „Festkörperanalytik″

Johannes Heitmann, Institut für Angewandte Physik

yellow/green - stacking faults in Tb

step height: 0.28 nm

AFM•Morpholgy sensetive in nm scale(2D) + Angström resoltution in heightc-AfM•use of conductive tips•local current measurementspossibleSTM

Summary

36page 36L-MA-ME 2. Microscopy Based 2.3. Scanning Probe Microscopy 2.3.1. STM

STM •atomic resolution in three dimension on surfaces

• limitation to conducting/metallic samples

• investigation of details of the electron distribution in thematerial

• possibility to manipulate atoms on the surface

48 Fe atoms on Cu (111)