A kinetic Study of the MOCVD of GaN

Quantum chemistry investigations have been performed to study the gas phase chemistry active during the MOVPE of GaN when Ga(CH3)3 and NH3, diluted in a H2 carrier gas, are used as precursors. Optimized molecular geometries, energies, and transition state structures of gas phase species have been determined with density functional theory at the B3LYP/6-311+G(d,p) level. We found that the GaN gas phase chemistry is dominated by a radical chain mechanism, started by the CH3· groups originated by the gas phase and surface decomposition of Ga(CH3)3, and resulting in the fast formation of linear covalently bonded R(GaN)x adducts. These molecules can give fast cyclization reactions that lead to the formation of six membered cyclic species, which, similarly to benzene for combustion can be considered as the first GaN nuclei. The determined mechanism was then reduced through sensitivity analysis to a set of 15 reactions involving 15 species, which can be easily introduced in a suitable reactor model to simulate the global growth process. As a results of these simulations we found that the presence of H2 as a carrier gas can greatly enhance the rate of formation of gas phase particles as it is a major source of atomic hydrogen, promoter of gas phase reactivity.