Density Functional Theory Study on Gas Reactions of Trimethylaluminum and Hydrogen for Aluminum Layer Growth

The gas-phase reaction mechanism between trimethylaluminum and hydrogen for aluminum layer growth via metal–organic chemical vapor deposition was investigated using quantum chemical calculations based on density functional theory. A total of 14 elementary reactions, comprising 10 pyrolysis and 4 hydrogenolysis reactions, were proposed. From these, 10 reaction pathways were constructed. Intrinsic reaction coordinate calculations were employed to identify transition states and determine the electronic free energy changes associated with each reaction. Furthermore, vibrational frequency analyses of reactants, products, transition states, and intermediates were performed to explore the temperature dependence of competing reaction pathways originating from different reactants. To determine the most plausible reaction pathway at various temperatures, the Gibbs free energy difference between the reactant and transition state (ΔG‡) was calculated for each reaction and normalized by the product of the molar gas constant (R) and temperature (T), yielding ΔG‡/(RT). Based on transition state theory, the Arrhenius equation for each elementary reaction was derived. The results indicated that at the reaction temperatures, the process predominantly followed the pathway: Al(CH3)3 → Al(CH3)2 → AlCH3 → AlH. By further microkinetic simulations and comparing the reaction rate and diffusion rate of reactant in each elementary step along the reaction pathway, the results indicated that that the major gas-phase reaction product at the growth temperature is AlCH3. And the structural design conditions of MOCVD reactor for the Al layer growth are proposed.