AVS 45th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA2
Combined Experimental and Quantum Chemistry Study of the Disproportionation of Dimethylalane on Aluminum Surfaces

Tuesday, November 3, 1998, 2:20 pm, Room 309

Session: Morton M. Traum Student Award Session
Presenter: B.G. Willis, Massachusetts Institute of Technology
Authors: B.G. Willis, Massachusetts Institute of Technology
K.F. Jensen, Massachusetts Institute of Technology
Correspondent: Click to Email
A combined experimental/theoretical approach has been employed to study the decomposition reactions of trimethylaluminum and dimethylalane on aluminum surfaces. Together with UHV surface science experiments, plane wave pseudopotential density functional theory calculations have been implemented to augment the experimentally derived reaction mechanism. The plane wave surface calculations provide additional details of the surface reactions not easily probed with experiments, and where the two approaches overlap, comparisons are made. Dimethylalane is found experimentally to decompose on aluminum via a disproportionation reaction to produce trimethylaluminum. Trimethylaluminum follows the same reaction pathways on aluminum, but due to the difference in stoichiometry there is no net growth, and a dynamic equilibrium exists between the gas phase and surface at low temperatures. Based on the experimentally observed mechanism, the calculations are employed to generate thermodynamic heats of reaction for each elementary reaction step, and a complete mechanism is presented for the surface reactions. Results suggest exothermic steps for breaking down the dimethylalane and trimethylaluminum monomers to surface monomethylaluminum fragments. Further decomposition of these fragments, the desorption of hydrogen, and additional recombination reactions are found to be endothermic. The overall heat of reaction is calculated to be approximately -16 kcal if written in terms of monomeric gas phase units. If it is considered that trimethylaluminum and dimethylalane may exist as dimer gas phase units, the reaction is endothermic by approximately 16 kcal. At high temperatures both dimethylalane and trimethylaluminum share a strongly activated pathway for methyl dehydrogenation which produces a carbon contaminated surface. The experimental barrier is found to lie near 40 kcal for the methyl decomposition reaction. Theoretical calculations of this impurity incorporation pathway predict a methyl decomposition activation barrier near 40 kcal, in good agreement with experiments. The full ab initio model includes elementary reactions leading to growth, surface diffusion of the active methyl groups, methyl decomposition reactions to produce surface carbon, and the alternative (not experimentally observed) surface reaction pathway to form methane. Comparisons are made where both theory and experiment overlap and agreement is found to be good (within approximately ±5 kcal).