Reaction dynamics of molecular hydrogen on silicon surfaces

Abstract

Experimental and theoretical results on the dynamics of dissociative adsorption and recombinative desorption of hydrogen on silicon are presented. Using optical second-harmonic generation, extremely small sticking probabilities in the range ${10}^{\mathrm{\ensuremath{-}}9}$-${10}^{\mathrm{\ensuremath{-}}5}$ could be measured for ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ on Si(111)7\ifmmode\times\else\texttimes\fi{}7 and Si(100)2\ifmmode\times\else\texttimes\fi{}1. Strong phonon-assisted sticking was observed for gases at 300 K and surface temperatures between 550 K and 1050 K. The absolute values as well as the temperature variation of the adsorption and desorption rates show surprisingly little isotope effect, and they differ only little between the two surfaces. These results indicate that tunneling, molecular vibrations, and the structural details of the surface play only a minor role for the adsorption dynamics. Instead, they appear to be governed by the localized H--Si bonding and Si--Si lattice vibrations. Theoretically, an effective five-dimensional model is presented taking lattice distortion, corrugation, and molecular vibrations into account within the framework of coupled-channel calculations. While the temperature dependence of the sticking is dominated by lattice distortion, the main effect of corrugation is a reduction of the preexponential factor by about one order of magnitude per lateral degree of freedom. Molecular vibrations have practically no effect on the adsorption/desorption dynamics itself, but lead to vibrational heating in desorption with a strong isotope effect. Ab initio calculations for the ${\mathrm{H}}_{2}$ interaction with the dimers of Si(100)2\ifmmode\times\else\texttimes\fi{}1 show properties of the potential surface in qualitative agreement with the model, but its dynamics differs quantitatively from the experimental results. \textcopyright{} 1996 The American Physical Society.