bulk crystal growth
TRANSCRIPT
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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