bulk crystal growth

8/10/2019 Bulk Crystal Growth

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Sebastian Lourdudoss 1

BULK CRYSTAL GROWTH and LIQUID PHASE EPITAXYBULK CRYSTAL GROWTH and LIQUID PHASE EPITAXY

Lecture-3, 2B 1700, 2B1823 - Advanced Semiconductor Materials

Bulk crystal growth techniques

Need for bulk crytals

Horizontal/Vertical Bridgman technique

Liquid Encapsulated Czochralski technique

Dopant distribution

Wafer specification

Liquid Phase Epitaxy

Various epitaxial techniques

Liquid phase epitaxy Growth procedure and reactors

LPE phase diagrams


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Practical Difficulties with certain

III-V semiconductors

1) In general, high melting points=> Crucibles normally silica

(silica becomes soft at 1100 -

1200 oC) graphite or

pyrolytic boron nitride (PBN)

2) Vapour pressures high at m.pt.for InP, GaP and GaAs ( low for

InSb, GaSb and InAs)

3) Decomposition near the

melting point

=> loss of one of the elements=> defects

(Remedy = Evacuated and

closed systems)

Compound M.Pt.(oC)

Vap. Pr.at M.pt.(atm)

InSb 525 4x10

-8

GaSb 712 1x10-6

InAs 943 0.33

GaAs 1238 1.0

InP 1062 27.5

GaP 1465 32

HgSe 799

HgTe 670 12.5

CdSe 1239 0.3

CdTe 1092 0.65

ZnSe 1526 0.5

ZnTe 1300 0.6

Ge 960

Si 1420From Compound Semiconductor Devices,

Structures and Processing, Ed. K.A.Jackson,

Willey-VCH, Weinheim, 1998.


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Phase diagram for the Ga-As

system


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LIQUID ENCAPSULATED CZOCHRALSKI (LEC)

METHOD

Cold wall system High pressure with inert gas / active gas

Encapsulant (B2O3) hinders vapour escape from

the melt + wets the growing surface

Normally higher dislocation density than in

Bridgman technique (because of thermal

non-uniformity)

Contamination from the surrounding material

(e.g. carbon from graphite parts)

Low pressure LEC ( Dissociation pressure < 2 atm)

High pressure LEC (Dissociation pressure > 2 atm)

=> inert gas or active gas used


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CzochralskiCzochralski Growth MethodGrowth Method


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Dopants

k0, eqm. Distribution coefficient = Cs/Cli

Cli = concentration in the melt at the interface (weight/g melt)

Cs = concentration in the solid (weight/g solid)

ke, Effective distribution

coefficient = Cs/Cl where

Cl = concentration in the

melt far from the interface

(weight/1g melt)

v = crystal growth rate

= diffusion barrier width

D = diff. coeff. of dopant inthe melt

Dv

l

s

e

ekk

k

C

Ck

+==

)1(00

0

From S.M.Sze,Semiconductor

devices, Physics and

Technology, John

Wiley, NY, 2nd ed.,

2001


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Equilibrium segregation coefficients for dopants

in silicon and GaAs

From S.M.Sze,

Semiconductordevices, Physics and

Technology, John

Wiley, NY, 2nd ed.,

2001


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Dopant concentration in the solid Cs :

where k0, eqm. distribution coefficient

= Cs/Cl (Cl is the concn. in the melt),

C0 = Initial concentration in the melt and

M/M0 = Fraction of the melt solidified

10

0

100

=

k

M

MCk

sC

As solidification progress, i.e.As solidification progress, i.e.

when M/Mwhen M/M00 increases,increases,

CCss/C/C00 increases if kincreases if k00 < 1 and< 1 and

CCss/C/C00 decreases if kdecreases if k00 > 1> 1

(M/M0)

Seed endSeed end Tail endTail end

From S.M.Sze, Semiconductor

devices, Physics and Technology,John Wiley, NY, 2nd ed., 2001


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Orientation flat, index flat, G-type, J-type


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Dovetail groove and V-groove


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SEVERAL EPITAXIAL TECHNIQUES

Liquid Phase Epitaxy (LPE)Liquid Phase Epitaxy (LPE)-- Semiconductor solid from a liquid solutionSemiconductor solid from a liquid solution

-- An equilibrium process usingAn equilibrium process using liquidusliquidus -- solidussolidus

equilibriumequilibrium

Vapour Phase Epitaxy (VPE)Vapour Phase Epitaxy (VPE)-- Semiconductor solid from gas sourcesSemiconductor solid from gas sources

-- A special case of Chemical Vapour Deposition (CVD)A special case of Chemical Vapour Deposition (CVD)

Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy (MBE)-- Semiconductor solid from atomic or molecular beamsSemiconductor solid from atomic or molecular beams

-- Beams arrive directly on the growth surface withoutBeams arrive directly on the growth surface without

any priorany prior interferanceinterferance or interaction (feasible in anor interaction (feasible in an

ultra high vacuum environment)ultra high vacuum environment)


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Liquid Phase Epitaxy

Observations:

1) III-V comounds decompose before reaching their melting points (melting

points are very high)

This means normally Hfusion

/H0formation

> 1

AlSb 0.848 GaAs 1.26

GaSb 1.48 InAs 1.35

InSb 1.43

NaCl 0.07 KF 0.05

2) High vapour pressure of V species at the congruent melting point

Remedy:

Dissolve V species (solutes) in III species (solvents) Use solidus liquidus equilibrium to carry out epitaxy

THIS IS LPE!

Implication:

Growth predicted by thermodynamics almost accurately


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Liquid Phase Epitaxy reactors


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LPE PHASE DIAGRAMSLPE PHASE DIAGRAMS


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Doping ofDoping of InGaAsPInGaAsP latticelattice

matched to InP with LPEmatched to InP with LPE


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p-quaternary

contact layer

p-InP

cladding layer

n-InP

p-InP

n-InP

substrate

Regrowth by LPERegrowth by LPE

Active layer


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ADVANTAGES OF LPE

Simple

Inexpensive

Rather non-hazardous Suitable for selective growth

Al and Sb compounds possible

=> Highly suitable for simple structures

DISADVATAGES OF LPE

Too simple to grow quantum structures

Thickness control and composition control difficult

Redissolution of the grown material High growth temperatures for certain compounds

(e.g. GaAs at ~ 800-900 oC but InP at ~ 600 oC)

Fe doping (for semi-insulation) difficult because of low

distribution coefficient

bulk crystal growth

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