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NNSE 618 Lecture #24
1
Lecture contents
• Heteroepitaxy
• Growth technologies
• Strain
• Misfit dislocations
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
3 Bulk semiconductor crystal growth methods
From khup.com
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
5 Commercial Thomas Swan® MOCVD
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
7 Commercial Veeco GEN2000® MBE system
NNSE 618 Lecture #24
8
Strain in heterostructures
Strain:
Ref: Singh
Strain relaxed:
Lattice mismatch:
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
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)
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
15
Atomic structure of a 900 dislocation
NNSE 618 Lecture #24
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]
NNSE 618 Lecture #24
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
NNSE 618 Lecture #24
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)