NH3 dissociative adsorption on Si(100)-(2 x 1) surface: A B3LYP quantum chemical cluster model study

Dissociative adsorption of ammonia on the Si(100)-(2X1) surface has been in vestigated using the hybrid density functional B3LYP method and the Si9H12 one-dimer cluster model of the surface. The adsorption,geometries and the r eaction energetics from NH3(a) to NH2(a) and H(a) have been studied with Si basis sets varying from the standard all electron 6-31G(d) to the LanL2DZ with effective core potential, while keeping NH3 basis sets as 6-31G(d,p) i n all cases. It was found that B3LYP/LanL2DZ is a reasonable level of theor y for the calculations of not only the geometries, but also the reaction en ergetics. This provides a cost-effective way to extend the size of the Si c luster models and to study in details the energetics of the whole reaction path. Based on the experimental and theoretical results available, we recom mend an "optimal" bond geometry of NH2(a) and H(a) on the Si(100)-(2x1) sur face to be: Si-Si (symmetric dimer) 2.42 Angstrom, Si-N 1.75 Angstrom, Si-H 1.50 Angstrom, N-H 1.01 Angstrom, Si-Si-N 115 degrees, Si-N-H 118 degrees, Si-Si-H 111 degrees, H-N-H 109 degrees. The calculated results on the reac tion energetics are in general agreement with the experimental findings as well as the previous theoretical ones.