The diffusion of hydrogen atoms on a reconstructed Si(111)-(7x7) surface was investigated using variational phase-space theory. The dimer-adatom stacking fault model of the reconstructed Si(111)-(7x7) surface, proposed by Takayanagi et al., was used to describe a four-layer lattice structure containing 292 atoms. The lattice potential was that developed by Bolding and Andersen and the gas-lattice interaction potential was described by a sum of Morse functions and bending terms between the hydrogen adatom and the Si atoms in the first and second layers. Canonical Markov walks were used to evaluate the flux across a set of dividing surfaces separating different adsorption sites. The minimum jump frequencies were used as input to a set of coupled phenomenological kinetics equations which described the diffusion rates of adatoms between adjacent adsorption sites. The diffusion coefficients at various temperatures were deduced from the slope of plots of the time variation of the root mean-square displacements obtained from the solution of the rate equations. The results for 300, 500 and 800K yielded the Arrhenius equation, D(cm2/s) = 0.023 exp[-1.54(eV)/kT]. The latter activation energy was in excellent agreement with experimental results obtained using an optical second-harmonic diffraction technique. The coordinates corresponding to the minimum energy diffusion path suggested that hydrogen-atom diffusion between atop sites occurred along paths that involved lattice penetration. Calculated upper limits on the tunneling rates at 300, 500 and 800K showed that tunneling processes made only a small contribution to the total diffusion rate.

Diffusion of Hydrogen Atoms on a Si(111)-(7x7) Reconstructed Surface: Monte Carlo Variational Phase-Space Theory. D.C.Sorescu, D.J.Thompson, L.M.Raff: The Journal of Chemical Physics, 1994, 101[2], 1638-47