MOCVD growth of GaAS

1. MOCVD growth of GaAS 광공학 협동과정 박은주

2. Contents • VPE • MOCVD • Grow mechanism • MOCVD process • Surface process • Conclusion

3. VPE (vapor phase epitaxy) • All reactants in vapor phase, deposited on heated substrate halide: AsCl3, H2, dopants hydride: AsH3, H2, dopants As4 Ⅱ,Ⅲ substrate Ⅰ HCl halide: AsCl3, H2 hydride: HCl, H2 Ga metal GaAs Halide or hydride process Ⅰ: GaAs(s) +HCl(g) → GaCl(g)+1/4 As4(g)+1/2H2(g) Ⅱ: 3GaCl(g)+1/2As4(g) → 2GaAs(s)+GaCl3(g) Ⅲ: GaCl(g)+1/4As4(g) +1/2H2 → GaAs(s)+HCl(g) Advantage: fast rate(0.1~0.5um/min), easy, safe(w/o arsine process) Disadvantage: Al compounds difficult, thickness resolution www.hait.ac.il/staff/reuvend/micro/ee418_c6.pdf

4. MOCVD(Metalorganic chemical vapor deposition) • History - growth thin single crystals of GaAs by Manasevit, in 1968 - Source : trimethylgallium(TMGa) - arsine(AsH3) - H2 - Pyrolized temp.: 600~700℃ - Substrate: GaAs, sapphire or others - Basic reaction : Ga(CH3)3 + AsH3→ GaAs+2CH4 • 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

5. CVD Reactor configuration Pancake reactor Horizontal reactor Vertical reactor Barrel reactor Rotating disk reactor

6. 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

7. MOCVD process • Mass transport • Carry reactants to reaction cell • Laminary boundary layer above growth surface (determined by Ptotand 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

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

9. 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

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

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

12. Surface diffusion length • Competition between diffusion and deposition

13. Layer-by-layer growth • Growth by successive completion of monolayers • Step flow growth or 2D island nucleation

14. Surface roughness • Very low λ(low T or high R) leads to 3D islad nucleation – surface roughening

15. Growth rate of GaAs Materials aspaets of GaAs and InP based structures, V Swanminathan, p136 Different TEGa flow rate Partial pressure of TMGa

16. 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)

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