KINETICS OF THE VERY LOW-PRESSURE PYROLYSIS OF TRIMETHYLGALLIUM AND ARSINE

We have studied the mechanism and kinetics of the decomposition reactions of trimethylgallium (TMG) and arsine in a very low pressure pyrolysis (VLPP) reactor. We have observed in situ the surface-catalyzed decomposition reactions of TMG and arsine on a growing GaAs surface. The primary step in the decomposition of TMG involves the adsorption and the successive release of the methyl radicals from the surface. The decomposition reactions of TMG in our Knudsen cell flow reactor exhibit a different type of behavior in the higher (p(TMG) > 10(-3) Torr) and lower pressure (p(TMG) < 10(-4) Torr) regime. The decomposition of TMG on a growing GaAs surface in the lower pressure regime shows an activation energy of about 26 +/- 3 kcal/mol and an A-factor of about 10(13.3) s-1. In the higher pressure regime the initially growing GaAs surface shows an activation energy of about 26 kcal/mol. The surface is deactivated slowly before the TMG signal reaches a steady state and the activation energy is reduced to 10 +/- 2 kcal/mol. The pre-exponential factor decreases even faster to about 10(10.3) s-1. We have suggested an overall reaction scheme to explain our observations. The SEM/EDS analysis of the growing film in the high pressure regime shows a carbon signal unlike the films grown in the low pressure regime. We attribute the significant carbon build-up on the surface to the secondary bimolecular reactions of the methyl radical producing carbon species (CH(x)) covering the surface. The introduction of arsine increases the decomposition of TMG and the methyl radical production. However, the activation energy for the production of the methyl radicals is not affected by the presence of arsine staying constant at about 20 +/- 3 kcal/mol and there is no evidence for the recombination of methyl and H species on the surface. This apparent activation energy is much lower than the average Ga-C bond energy probably due to the heat of adsorption and the concerted nature of the surface reactions.