heteroepitaxy growth technologies strain misfit dislocationssoktyabr/nnse618/nnse618-l...•...

NNSE 618 Lecture #24

1

Lecture contents

• Heteroepitaxy

• Growth technologies

• Strain

• Misfit dislocations


2 Heteroepitaxy

• Single crystalline layer

• on Single crystalline substrate

• Strong layer-substrate interaction orientation

• Surface diffusion of adatoms

• “Clean” process semiconductors

Epitaxy

Heterostructures

Major challenge: Tailor electronic and/or

optical properties •Bandgap engineering

• Carrier confinement (DHS, 2DEG...)

•Wavefunction engineering

• Creating “wavefunction logic”

•Optical elements engineering

• Integrated waveguides


3 Bulk semiconductor crystal growth methods

From khup.com


4 Chemical vapor epitaxy

Si epitaxy: Chlorine process

Chemical reaction on a heated substrate

- SiCl4 process

- Metal-Organic Chemical Vapor

Deposition (MOCVD)

- Low vapor pressure

- Weak bond with metal (organic

byproducts are pumped away)

- Carbon and Hydrogen incorporation

are sometimes a problem

III-As MOCVD

Trimethylaluminum (actually it exists as a

dimer Al2(CH3)6


5 Commercial Thomas Swan® MOCVD


6

Atomic structure of a 900 dialocation

Physical vapor epitaxy: Molecular Beam Epitaxy (MBE)

Cryo-shrouds

Sample holder

Loadlock

Outgasing

stage

- UHV evaporator (~10-10 Torr)

- Ultra-high purity materials

(6N’s)

- “Molecular beams” due to

UHV – mean free path

~hundreds of meters)

- Adatom diffusion on the

substrate (hot substrate) for

epitaxy and low defect

density

- Flux is controlled by effusion

cell temperature and shutters

- Elaborated cells: valved

cracker fro As, Sb; e-beam

evaporation cells, chemical

cells.

Effusion Cells


7 Commercial Veeco GEN2000® MBE system


8

Strain in heterostructures

Strain:

Ref: Singh

Strain relaxed:

Lattice mismatch:


9 Strain in Heterostructures

Misfit strain (0 - 10%) • due to lattice mismatch • accommodates at high (growth)

temperatures

Thermal strain (~ 10-3) • due to difference in thermal expansion

coefficients • accommodates at lower temperatures

Tfs

s

fs

a

aa


10 How to Deal with Misfit Strain ?

• Decrease misfit and thermal strain

• Limit the thickness

Strained layers/superlattices

• Limit lateral dimensions

Quantum wires and quantum dots

Strain coherent energy :

t

cm

Energy 2

2 1

12

Prevent plastic deformation

Allow plastic deformation

• Increase the fraction of 900 dislocations

• Suppress threading dislocations

Dislocation filters (MQW)

Thick buffer layers


11

Heterostructures with Low Defect Density

Low mismatch

(<0.5-1%)

High mismatch

(>2%)

Prevention from

dislocation

nucleation

Reduction of

dislocation

density in the

active region

Reduction of

thickness:

strained layers

Reduction of

lateral size:

3D islands

Graded

buffer layers Usage of

dislocation

filters: SLS

Search for lattice-matched materials

(for Si):

“old” - GaP

“new” - SiGeC

Thick buffer

layers


12

Equilibrium separation:

Biaxial strain accommodation

Elastic: Roughness, Islanding

Plastic: Introducing defects

(Misfit Dislocations)

sf aa

ab

bd

int

int

Morphological instability (ratio of

bulk and surface energy):

2

22

4

Et

t


13 Growth morphology

2D (layer-by-layer) growth => smooth surface morphology s2 12 s2 < s1

3D (Volmer-Weber or Stranski-Krastanov) growth mode s2 12 s2 > s1

is thermodynamically equilibrium for most heterostructures,

in particular with high mismathch

Ge on Si (4% mismatch)

1s

2s

12

2S

2D 3D

thermodynamics kinetics

a poly

170 C o

350 C o

Islanding can be suppressed by kinetics =>

higher supersaturation (low temperature

and high growth rate)


14

Dislocations: Equilibrium misfit dislocation structure

(001) - diamond and/or zinc-blende

heterostructures (Si, Ge, III-V, II-VI

semiconductors)

b=1/2[110]

[110]

Rectangular array of 90o (Lomer) dislocations

Also the most desirable defect structure:

- most effective for MS relief

- glide plane (001) => sessile dislocation

- no dangling bonds => electrically inactive !?

Low-energy dislocations in diamond and

zinc-blende (001) structures


15

Atomic structure of a 900 dislocation


16

- critical thickness half-loops 60o

misfit segments glide of threading

segments

- Grown-in threading dislocations

critical thickness Elongating 60o

misfit segment glide of threading

segments

- 3D growth critical thickness 90o

misfit segment at the island edge

overgrowth and glide of threading

segments

Nucleation of misfit dislocations

b=1/2[011]

[110]


17

Critical thickness

From People and Bean, 1985

154 nm In0.07Ga0.93As on GaAs

From Maree, 1987

From energy balance

From force balance for dislocation bending

From energy balance for dislocation array


18 NNSE 618 Recap

Bandstructure:

E(k)

Statistics:

f(E)

Optics and

recombination:

(E), I(E)

Scattering:

t(E)

Defects:

ED, EA

Junctions:

f, J Band-

engineering:

, x, f

Scattering:

t(E)

heteroepitaxy growth technologies strain misfit dislocationssoktyabr/nnse618/nnse618-l...•...

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