The dissociative adsorption of silane and disilane on Si(100)-(2 x 1)

We investigate the dissociative adsorption of silane and disilane on Si(100)-(2 x 1) using pseudopotential planewave density functional theory calculations. These are important steps in the growth of silicon films. Although silane has been studied computationally in some detail previously, we find physisorbed precursor states for the intradimer and interdimer channels. The silane energetics calculated here are in good agreement with experimental data and previous theoretical estimates and provide us with a useful reference point for our disilane calculations. Disilane has not been studied as intensively as silane. We investigate both silicon-silicon bond cleavage and silicon-hydrogen bond cleavage mechanisms, and for each we investigate intradimer, interdimer, and inter-row channels. As in the case of silane, we also find precursor states in the adsorption path in agreement with molecular beam experiments. The qualitative picture that emerges is that adsorption takes place through a weakly bound precursor state with a transition state to chemisorption that is low lying in energy relative to the gas phase. This is in good agreement with experimental data. However, the calculated energetics are only in fair agreement with experiments, with our transition state to chemisorption being about 0.02 eV above the gas phase while experimentally it is estimated to be approximately 0.28 eV below the gas phase. This suggests that accurate theoretical characterization of these weakly bound precursor states and the adsorption barriers requires further computational work.