The diffusion of hydrogen atoms on a partially hydrogen-covered Si(111) surface was studied by using Monte Carlo techniques with a potential-energy surface based upon available ab initio results and experimental data. The potential described two kinds of binding sites: a covalent Si-H bond (top site) and an interstitial three-fold bonding site (open site). Classical jump frequencies between the top and open sites were calculated using Monte Carlo variation phase-space theory with importance sampling at 300, 600, 900 and 1200K. The approach used for treating tunneling assumed continuum-to-continuum WKB tunneling with classical Monte Carlo phase-space averaging. The diffusional barriers between the two binding sites on the equilibrium surface were 2.79 and 0.65eV for top-to-open site and open-to-top site jumps, respectively. The calculated classical jump frequencies gave Arrhenius parameters of 1.3 x 1014 and 9.9 x 1013/s and 2.72 and 0.59eV, for top-to-open and open-to-top site jumps, respectively. Monte Carlo techniques were used to compute the minimum energy path. The dynamic barrier was 2.64eV for top-to-open site jumps. Tunneling rates were calculated at 300K, and estimated at higher temperatures. Due partly to the small width of the barrier, the tunneling rate at 300K was 257 times larger than the classical value. Tunneling was important at room temperature, but its importance relative to the classical rate decreased with increasing temperature. The results indicated that surface phonons significantly enhanced the tunneling rate.

Diffusion of H Atoms on a Si(111) Surface with Partial Hydrogen Coverage: Monte Carlo Variational Phase-Space Theory with Tunneling Correction. B.M.Rice, L.M.Raff, D.L.Thompson: The Journal of Chemical Physics, 1988, 88[11], 7221-31