Strong affinity of hydrogen for the GaN(000-1) surface: implications for MBE and MOCVD growth
The stability of clean and hydrogen covered GaN(000-1) surfaces is determined by density functional theory total energy calculations together with a finite temperature thermodynamics approach. The results reveal that H has an extremely high affinity for the N-face surface: Even under ultra-high vacuum conditions as realized in molecular beam epitaxial growth, with a temperature of 1000 K and assuming a residual hydrogen pressure of only 10-12 atmospheres, the hydrogen terminated surface is, for very N-rich conditions, more stable than any clean surface. A transition to a surface covered by a Ga adlayer is predicted to occur as the Ga chemical potential increases. In metal-organic chemical vapor deposition conditions the (000-1) surface is always covered by hydrogen, except possibly in extremely Ga-rich conditions. The slower growth rate on the (000-1) surface in comparison to the (0001) surface is attributed to low adsorption of N on the H-covered (000-1) surface.
Northrup, J. E. ; Neugebauer, J. Strong affinity of hydrogen for the GaN(000-1) surface: implications for MBE and MOCVD growth. Applied Physics Letters. 2004 October 18; 85 (16): 3429-3431.