Hartmut S. Leipner: Structure of imperfect materialshsl/Realstruktur/Realstruktur_2_IV...

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2. Defects and semiconductor technology Hartmut S. Leipner: Structure of imperfect materials 2.1 Device effects of defects 2.2 Degradation

Transcript of Hartmut S. Leipner: Structure of imperfect materialshsl/Realstruktur/Realstruktur_2_IV...

Page 1: Hartmut S. Leipner: Structure of imperfect materialshsl/Realstruktur/Realstruktur_2_IV Devices.pdf · hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

2. Defects and semiconductor technology

Hartmut S. Leipner: Structure of imperfect materials

2.1 Device effects of defects

2.2 Degradation

Page 2: Hartmut S. Leipner: Structure of imperfect materialshsl/Realstruktur/Realstruktur_2_IV Devices.pdf · hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

„Über Halbleiter sollte man nicht arbeiten, das ist eine Schweinerei, wer weiß ob es überhaupt Halbleiter gibt.“

[Pauli 1931]

2.1 Device effects of defects

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

The first transistor and IC

Transistor as invented by Bardeen and Brattain at Bell labs, 1947

Integrated circuit of J. Kilby, 1958

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Cost reduction

“In 1958, the year of the invention of the integrated circuit, a single transistor sold for about $10. Today, it is possible to buy more than 50 million transistors for that price.”

[Kilby: 2000]

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Moore’s law

In 1965 Gordon E. Moore postulated his law: The doubling of transistors per circuit every couple of years.Intel expects that it will continue at least through the end of this decade.

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Moore’s laws

log

(num

ber)

Time

Functional form of key semiconductor industry business trends (Tx – transistors)

The exponential expansion of the total number of transistors per chip or area against the time (in years) over the last half-century is known as (the original) Moore’s law.

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

The field-effect transistor (FET)

SourceGate

Drain

n p n

Structure of a MOSFET (metal–oxide–semiconductor FET). The electrical contact to the gate is separated from the semiconductor by a thin layer of insulator, typically silicon dioxide.

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Mobility of electrons in n-type semiconductors

Mobility of electrons μn in Si[Yu, Cardona 2001]

Mobility related to acoustic phonon interaction

Mobility related to ionized donors

Conductivity

μ n (cm

2 V− 1

s−1 )

T (K)

1

µn=1

µv+1

µD

µv ! (m!)"5/3T"3/2

µD ! (m!)"1/2NDT 3/2

! = neµn

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Scattering at dislocations

Mobility of electrons in n-type Ge before and after plastic bending. Curve (a) was calculated with the space-charge cylinder model.

[Seeger 1997]

Type and direction of dislocations are

important:

I–U characteristics measured parallel and

perpendicular to the dislocations are differentT

a

μn

Two contributions: Scattering due to the deformation potentialLinear arrangement of charged scattering centers

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Electron mobility and dislocation density

Si Substrate

Graded Si1 − yGey, y = 0…0.3

Relaxed Si1 − xGex, x = 0.3

Effect of the grading rate in the buffer on the density of threading dislocations and the electron mobility measured at 0.4 K. The dotted line is an extrapolation showing that the mobility is not limited by the dislocations once their density is below 107 cm−2 (thickness of graded layer > 0.5 µm).

[Ismail 1996]

Thickness of graded layer (µm)

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Diode characteristics and dislocations

Reverse bias I–U characteristics for InP photodiodes containing different dislocation densities (given in cm−2)

[Beam et al. 1992]

Rev

erse

cur

rent

(A)

Bias (V)

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Dislocations at the p–n junction

♦ Dislocations as electrically active defects cause a shortening of the junction

♦ Transport of minority carriers under reverse bias affected

♦ Energy levels of dislocations (recombination centers): source of electrical noise

♦ Dopant precipitation at dislocations: local field enhancement under reverse

bias, drop in reverse breakdown voltage

♦ Microplasma (high electron–hole density) formation

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Charged-dislocation model

Model of Read (1957): The line charge of the dislocation is screened by a cylindrical space charge region, i. e. in an n-type semiconductor a negative dislocation is surrounded by positively charged donors.

Radius of the Read cylinder:(Q charge per unit length)

Smearing out of the screening cloud for T > 0

------------

++

+

+

++

++

+

+

+

+

Captured majority carriers

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Influence of dislocations on the p–n junction

Scheme according to Holt (1996) illustrating the absorption of the diffusing impurity (e. g. phosphorus) by dislocations and the retardation of the diffusion front. In this way, cusps in the p–n junction are produced. The magnitude of the effect depends on the depth of the dislocation; shallower ones produce the larger effects. A and B are strongly decorated dislocations. The impurity atmosphere at dislocation C is weaker.

Top surface

High concentration zone of phosphorus donors

p–n+ junction

n+ laye

rA

B

C

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Defects in GaAs field-effect transistors

SI GaAs substrate

n channel

DrainGateSource

BB

Structure of a FET. Si implantation is used for the production of the n-conductive channel. Lateral isolation is provided by amorphization in B. The symmetrical n+ contacts are made of Ni/Au–Ge, the gate by Ti–Pt–Au. The drain/source–gate separation is 2 µm.

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Precipitates in FET structures

Laser scattering tomography (LST) of precipitates (a) decorating dislocations in the substrate and (b) in the channel between drain and gate.

[Castagné et al. 1992]

(a) (b)

LST LST

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

FET characteristic curvesP

reci

pita

te d

ensi

ty (c

m−

3 )

Breakdown voltage (V)Th

resh

old

volta

ge (m

V)

Precipitate density (cm–3)

2 µm20 µm50 µm

Theory

Influence of the precipitates on the breakdown voltage (a) and the threshold voltage (b) of the GaAs FET. The symbols in (b) represent different lengths of the channels. [Castagné et al. 1992]

(b)(a)

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

2.2 Degradation

♦ Limitation of the lifetime of semiconductor devices, especially optoelectronic devices

♦ Defect formation during operation

♦ Classification of degradation modes, recombination enhanced defect

generation and motion

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Scheme of a laser diode

Basic structure of a laser diode

Optical resonator realized with

the Fabry–Perot condition

mλ = 2nL(m integer number, λ wavelength, n refraction index, L resonator length)

Current

Mirror face

Active zone

Radiation

Opticallyrough

p doped

n doped

d

w

L

Population inversion at the p–n junction as the lasing condition (i. e. stimulated emission larger than optical absorption)

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

GaN laser diode

Nichia blue laser diode consisting of a stack of AlGaN, GaN, and InGaN

layers. The SiO2 islands are stoppers for threading dislocations.

Top surfaceDouble heterostructure

GaN

Substrate–film interfaceAl2O3

TEM cross-section image of the layer containing threading dislocations [Lester et al. 1995]

1 µm

TEM20

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Influence of dislocations on device efficiency

Dislocation density (cm–2)

Qua

ntum

effi

cienc

y

Light output as a function of the dislocation density in the active region for various laser diodes according to Lester et al. (1995) and Sugahara et al. (1998)

104 105 106 107 108 109

0.20.40.60.81.0

GaAs

GaAlAsGaP GaAsP

GaN

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Degradation of light-emitting devices

A representative degradation curve of light output versus time. The scale along the ordinate can vary over hours to ten on thousands of hours.

Ligh

t out

put

Time of operation

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Formation of dark-line defects

Micro-electroluminescence distribution of -oriented dark-line defects in a high-power GaAs light-emitting diode of the 1980ies.

!110"

EL EL

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Dislocations in a GaAs LED

Dislocations in a GaAs light-emitting diode in EBIC (a) and cathodoluminescence (b) images. Note the vanishing contrast in (b) for the dislocations 1 to 3. [Schreiber et al. 1984]

25 µm1

23

(a) (b)

1

23

EBIC – electron beam induced current

CLEBIC

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Dark-line defects and dislocation motion

Dark-line defects of a GaAlAs laser diode in the electroluminescence distribution and in a transmission electron microscope image. B denotes a threading dislocation and A the formation of a dislocation dipole in a direction. g is the diffraction vector. [Hutchinson, Dobson 1980]

!100"

10 µm

!100"

TEM

EL

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Growth of dark-line defects

Scheme of the growth of a dislocation network in the active layer of the laser device. (a) Initially, the dislocation PN with the Burgers vector b crosses the layer; (b) climb into the active zone; (c) further climb confined to the active zone causes elongation along the [100] direction.

[Hutchinson, Dobson 1975]P

N

bActive layer

TEM

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Dislocation climb in the active layer

Two models proposed for the extension of -oriented dark-line defects in degraded optoelectronic devices via dislocation climb. (a) Model related to absorption of interstitials, (b) emission of vacancies.

(a) (b)

!100"

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Influencing defects by carrier recombination

♦ Release of energy by recombination of excess carriers (photons, phonons)

♦ Recombination energy may be directly used for defect reactions

(generation, motion, transformation) – recombination enhanced defect reaction

♦ Energy transfer can be directly an special sites on the dislocation (kinks) –

recombination enhanced dislocation glide

♦ Stimulated activity of the kink migration: increase in the dislocation velocity

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Electron-beam-stimulated dislocation motion

Moving dislocations near a scratch on GaAs observed by cathodoluminescence imaging

[Höring et al. 2000]

CL

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Recombination enhanced dislocation glide

Temperature dependence of the thermal and recombination-enhanced dislocation velocity υ of polar 60° α and β dislocations in GaAs

[Maeda, Takeuchi 1983]

1000/T (K−1)

v (m

/s)

I

I

with/without irradiation

n-type GaAsτ = 26 MN m−2

αβ

I = 0.26 A m−2

I = 2.6 A m−2

(Q activation energy in the dark, I beam current density, ΔE reduction in the activation energy by the electron beam, τ stress, m stress exponent, υ0 and η prefactors)

! = !0"m exp!! Q

kBT

"+#I exp

!!Q!!E

kBT

"

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Defect structures

♦〈110〉-oriented dark-line defects (GaAlAs)

dislocation glide enhanced by nonradiative recombination of carriers

♦〈100〉-oriented dark-line defects (GaAlAs)

recombination-enhanced dislocation climb

♦ Dark-spot defects (InGaAsP)

reaction between metal contacts and the matrix, microdefects

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hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects

Degradation modes

♦ Catastrophic optical damage (< 1 h)

Local heating at the mirror surface related to high output-power density

♦ Rapid degradation (< 100 h)

Formation of dark-line defects

♦ Gradual degradation (several 1000 h)

Microdefects (loops, point defect clusters)

♦ Facet degradation (> 10000 h)

Oxidation or point defect formation at the mirror facets

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References

J. C. Kilby in: Handbook of semiconductor manufacturing technology, Marcel Dekker 2000.P. Y. Yu, M. Cardona: Fundamentals of Semiconductors. Berlin: Springer 2001.K. Seeger: Semiconductor Physics. Berlin: Springer 1997.K. Ismail: Sol. State Phen. 47–48, (1996) 409.E. A. Beam et al.: Semicond. Sci. Technol. A 7 (1992) 229.D. B. Holt: Scanning Microsc. 10 (1996) 1047.S. D. Lester et al.: Appl. Phys. Lett. 66 (1995) 1249.T. Sugahara et al.: Jpn. J. Appl. Phys. Pt. 2 37 (1998) L398.J. Schreiber et al. in: 6. Tagung Mikrosonde, Dresden 1984, p. 151.P. W. Hutchinson, P. S. Dobson: Phil. Mag. A 41 (1980) 601.K. Maeda, S. Takeuchi: J. Physique 44 (1983) C4/375.

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