DIFFUSION ON STEPPED SQUARE SURFACES - A MONTE-CARLO APPROACH

Adatom diffusion on stepped square lattices has been studied by means of Monte Carlo modeling. Steps consisting of two adjacent rows running in the x direction have been separated by two terrace rows and it has been assumed that one of the step rows is located on the up side and the other on the down side of a lattice step. In order to account for differences between up and down sides of steps, an extra adsorption energy phi(st) has been added or subtracted for adatoms on down-side or up-side step rows. Pairwise interaction energies between nearest neighbors phi(NN) have also been taken into account. It has been assumed that these couplings are not effective between adatoms on up-side and down-side step sites. At low surface coverages theta and temperatures T, most of the adatoms are trapped by down-side step sites. Both parallel and perpendicular diffusion require detachment from such step sites, and therefore diffusion is almost isotropic. As coverage increases the down-side step sites become highly populated and then act as barriers for perpendicular diffusion. The up-side step sites constitute channels for fast parallel diffusion so that anisotropy increases. In the case of nearest-neighbor repulsion phi(NN) < 0 the activation energies for surface diffusion are generally smaller. At low temperatures T less-than-or-equal-to T(c) c(2 x 2) ordered lattice gas domains are observed for theta almost-equal-to 0.5 with disordered regions nucleating at step rows in between. The effect of ordering on surface diffusion is described.