MOCVD growth of GaAS

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MOCVD growth of GaAS

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Contents

• VPE
• MOCVD
• Grow mechanism
• MOCVD process
• Surface process
• Conclusion

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VPE (vapor phase epitaxy)

• All reactants in vapor phase, deposited on heated
  substrate

halide AsCl3, H2, dopants hydride AsH3, H2,
dopants
As4
?,?
substrate
?
HCl
halide AsCl3, H2 hydride HCl, H2
Ga metal
GaAs
Halide or hydride process ? GaAs(s) HCl(g) ?
GaCl(g)1/4 As4(g)1/2H2(g) ? 3GaCl(g)1/2As4(g)
? 2GaAs(s)GaCl3(g) ? GaCl(g)1/4As4(g) 1/2H2
? GaAs(s)HCl(g) Advantage fast
rate(0.10.5um/min), easy, safe(w/o arsine
process) Disadvantage Al compounds difficult,
thickness resolution
www.hait.ac.il/staff/reuvend/micro/ee418_c6.pdf
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MOCVD (Metalorganic chemical vapor deposition)

• History
• - growth thin single crystals of GaAs by
  Manasevit, in 1968
• - Source trimethylgallium(TMGa) - arsine(AsH3)
  - H2
• - Pyrolized temp. 600700?
• - Substrate GaAs, sapphire or others
• - Basic reaction Ga(CH3)3 AsH3 ? GaAs2CH4
• advantages compared to other VPE
• - Irreversible reaction
• - Relatively low temperatures (minimized effect
  of interdiffusion)
• - Phosphorus poses no problems compared to MBE
• disadvantages
• - in-situ etching is not possible
• - AsH3 are very toxic

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CVD Reactor configuration
Pancake reactor
Horizontal reactor
Vertical reactor
Barrel reactor
Rotating disk reactor
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Growth mechanism
Main gas flow region
Gas phase reactions
Desorption of volatile surface reaction products
Redesorption of Film precusor
Transport to surface
Surface diffusion
Adsorption of film precursor
Nucleation And island growth
Step growth
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MOCVD process

• Mass transport
• Carry reactants to reaction cell
• Laminary boundary layer above growth surface
• (determined by Ptot and Vgas)
• Diffusion of reactants througth laminary boundary
  layer
• Chemical reactions
• about 200 chemical reactions involved
• Net reaction for GaAs for TMG and arsine
• Thermodynamics
• Rate constants
• Dictate the deviation from equilibrium
• Maximum growth rate
• Physical surface processes
• Surface diffusion

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MOCVD growth kinetics
http//www.teknisknanovetenskap.lth.se/programmet/
arskurs3/FFF110/PDF/lecture3.pdf
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Surface process

• Deposition
• Surface diffusion
• Competition deposition versus surface diffusion
• 2D growth step flow growth or 2D island
  nucleation
• 3D growth 3D island nucleation, roughening

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Deposition

• Deposition of atoms ( or precusor molecules) from
  the gas phase
• Deposition rate R sometimes measured in nr of
  atoms/s-cm2

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Surface diffusion

• Surface diffusion random movement
• (Brownian motion),
• hopping between sites
• Difussion constant, D
• DDo exp(-Ea/kT) cm2/s
• Do attempt frequency
• Ea activation energy

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Surface diffusion length

• Competition between diffusion and deposition

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Layer-by-layer growth

• Growth by successive completion of monolayers
• Step flow growth or 2D island nucleation

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Surface roughness

• Very low ?(low T or high R) leads to 3D islad
  nucleation surface roughening

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Growth rate of GaAs
Partial pressure of TMGa
Different TEGa flow rate

• Materials aspaets of GaAs and InP based
  structures, V Swanminathan, p136

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Conclusion

• Growth parameter of MOCVD
• Temperature of growth
• Flows of the precursors
• Reactor pressure
• Surface processes
• Growth at too low T or too high R leads to rough
  surfaces 3D island nucleation
• Growth at higher T and lower R(optimun) gives
  smooth surfaces-step flow growth, 2D)

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MOCVD model
S.Yu. Karpov, Journal of Crystal Growth 248
(2003)1-7