Monte Carlo random walk study of recombination and desorption of hydrogen on Si(111)

Abstract

The recombination/desorption of H2 and the desorption of hydrogen atoms from a Si(111) surface have been investigated using Monte Carlo transition-state theory methods with a biased random walk. Rate coefficients, activation energies, preexponential factors, and angular desorption distributions have been computed for both reaction channels. The distribution of polarization angles for the H2 rotational angular momentum vector is also reported. The potential-energy surface is expressed as the sum of a lattice potential, a lattice–adatom interaction term, and an adatom–adatom interaction. Keating’s formulation as given by Weber is used for the lattice potential. A pairwise sum of 60 Morse potentials represents the adatom–lattice term. The adatom–adatom interaction is a Morse function multiplied by a hyperbolic switching function. The potential parameters are adjusted to fit the theoretical data for the Si(111)–H interaction potential and the measured adsorption energy of H2 on Si(111). The surface predicts a barrier of 0.61 eV for H2 adsorption and the existence of an H*2 precursor state in the recombination/desorption process. Thermal desorption of hydrogen atoms is predicted to be too slow to be an observable process. The computed activation energies are in good agreement with the experimental data. The calculated preexponential factor for H2 recombination/desorption is a factor of 103 smaller than the measured results. A detailed treatment of a reaction mechanism involving an H*2 precursor intermediate indicates that this difference is due to differences in the surface coverage present in the calculations and in the experiments. In general, the theoretical methods are shown to be well suited for the study of this type of rare-event process.