Bulk and LPE-Lecture 3-2005

Sebastian Lourdudoss, KTH

BULK CRYSTAL GROWTH and LIQUID PHASE EPITAXYBULK CRYSTAL GROWTH and LIQUID PHASE EPITAXYLecture-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

Sebastian Lourdudoss, KTH

NEED• Device structure:

Several epitaxial layers often with different compositions and/or doping on a substrate

• Epitaxy (from Greek: epi = upon; taxis= ordered):Growth of a crystal on a substrate with the same crystallographic structure as the substrate

=> Monocrystalline substrate needed to grow epitaxial layers• Homoepitaxy: e.g., InP/InP

Heteroepitaxy: e.g., InGaAs/InP

Layer 3Layer 3Layer 2Layer 2Layer 1Layer 1

SubstrateSubstrate

Epitaxial growth

Bulk growth

Sebastian Lourdudoss, KTH

Requirements:1) Wafers with least defects and dislocations

(if present can propagate into the epitaxial layers)

2) Wafers of ultimate purity(electrically active impurity atoms < 10 parts per billion atomic)

3) Wafers with controlled electrically active dopant atoms

Sebastian Lourdudoss, KTH

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 0.33GaAs 1238 1.0InP 1062 27.5GaP 1465 32

HgSe 799HgTe 670 12.5CdSe 1239 0.3CdTe 1092 0.65ZnSe 1526 0.5ZnTe 1300 0.6

Ge 960Si 1420From “Compound Semiconductor Devices,Structures and Processing, Ed. K.A.Jackson,Willey-VCH, Weinheim, 1998.

Sebastian Lourdudoss, KTH

Phase diagram for the Ga-As system

Sebastian Lourdudoss, KTH

BRIDGMAN TECHNIQUE

• Growth on the seed from a melt ( Tseed < T melt)

• Hot wall• Growth in evacuated and

sealed containers => purity higher

• Easy to control the vapour pressure• Low vapour pressure system only

Sebastian Lourdudoss, KTH

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

Sebastian Lourdudoss, KTH

Czochralski Czochralski Growth MethodGrowth Method

Sebastian Lourdudoss, KTH

Sebastian Lourdudoss, KTH

Dopants

k0, eqm. Distribution coefficient = Cs/Cli

Cli = concentration in the melt at the interface (weight/1g melt)

Cs = concentration in the solid (weight/1g solid)

ke, Effective distribution coefficient = Cs /Cl where Cl = concentration in the melt far from the interface (weight/1g melt)

v = crystal growth rate

δ = diffusion barrier width

D = diff. coeff. of dopant in the melt

Dv

l

s

e ekkk

CC

k δ−

−+==

)1(00

0

Sebastian Lourdudoss, KTH

Equilibrium segregation coefficients for dopants in silicon and GaAs

Sebastian Lourdudoss, KTH

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

01

00

−

−=

k

MM

Cks

C

Sebastian Lourdudoss, KTH

Sebastian Lourdudoss, KTH

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

Sebastian Lourdudoss, KTH

Dovetail groove and V-groove

Sebastian Lourdudoss, KTH

Sebastian Lourdudoss, KTH

Sebastian Lourdudoss, KTH

SEVERAL EPITAXIAL TECHNIQUES

•• Liquid Phase Epitaxy (LPE)Liquid Phase Epitaxy (LPE)-- Semiconductor solid from a liquid solution Semiconductor solid from a liquid solution -- An equilibrium process using An equilibrium process using liquidusliquidus -- solidussolidusequilibriumequilibrium

•• 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 without Beams arrive directly on the growth surface without

any prior any prior interferanceinterferance or interaction or interaction (feasible(feasible in an in an ultraultra high high vacuum environment)vacuum environment)

Sebastian Lourdudoss, KTH

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 1.26GaSb 1.48 InAs 1.35InSb 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

Sebastian Lourdudoss, KTH

Liquid Phase Epitaxy reactors

Sebastian Lourdudoss, KTH

LPE PHASE DIAGRAMSLPE PHASE DIAGRAMS

Sebastian Lourdudoss, KTH

Doping of Doping of InGaAsP InGaAsP lattice lattice matched to InP with LPEmatched to InP with LPE

Sebastian Lourdudoss, KTH

p-quaternary contact layer

p-InP cladding layer

n-InP

p-InP

n-InP substrate

Regrowth by LPERegrowth by LPE

Active layer

Sebastian Lourdudoss, KTH

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