Adatom diffusion on stepped square lattices was studied by means of Monte Carlo modeling. Steps consisting of two adjacent rows running in the x direction were separated by two terrace rows and it was assumed that one of the step rows was 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 was added or subtracted for adatoms on down-side or up-side step rows. Pair-wise interaction energies between nearest neighbors were also taken into account. It was assumed that these couplings were not effective between adatoms on up-side and down-side step sites. At low surface coverages and temperatures, most of the adatoms were trapped by down-side step sites. Both parallel and perpendicular diffusion require detachment from such step sites, and therefore diffusion was almost isotropic. As coverage increased the down-side step sites became highly populated and then acted as barriers for perpendicular diffusion. The up-side step sites constitute channels for fast parallel diffusion so that anisotropy increased. In the case of nearest-neighbor repulsion, the activation energies for surface diffusion were generally smaller. At low temperatures, (2x2) ordered lattice gas domains were observed for 0.5 with disordered regions nucleating at step rows in between. The effect of ordering on surface diffusion was described.

Diffusion on Stepped Square Surfaces: a Monte Carlo Approach. C.Uebing: Physical Review B, 1994, 49[19], 13913-20