Gas phase chemistry in gallium nitride CVD: Theoretical determination of the Arrhenius parameters for the first Ga-C bond homolysis of trimethylgallium.

Experimental evidence suggests that the energy of activation for the first homolytic Ga-C bond fission of GaMe3 of Ea = 249 kJ/mol, measured by Jacko and Price in a hot-wall tube reactor, is affected by surface catalytic effects. In this contribution, the rate constant for this crucial step in the gas-phase pyrolysis of GaMe3 has been calculated by variational transition state theory. By a basis set extrapolation on the MP2/cc-pVXZ level and a correlation correction from CCSD(T)/cc-pVDZ level, a theoretical "best estimate" for the bond energy of Delta H(289K) = 327.2 kJ/mol was derived. For the VTST calculation on the B3LYP/cc-pVDZ level, the energies were corrected to reproduce this bond energy. Partition functions of the transitional modes were approximated by a hindered rotor approximation to be valid along the whole reaction coordinate defined by the Ga-C bond length. On the basis of the canonical transition state theory, reaction rates were determined using the maxima of the free energy Delta G++. An Arrhenius-type rate law was fitted to these rate constants, yielding an apparent energy of activation of Ea = 316.7 kJ/mol. The preexponential factor A = 3.13 x 10(16) 1/s is an order of magnitude larger than the experimental results because of a larger release of entropy at the transition state as compared to that of the unknown surface catalyzed mechanism.