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 en ergy phi(st) has been added or subtracted for adatoms on down-side or up-side step rows. Pairwise interaction energies between nearest neigh bors phi(NN) have also been taken into account. It has been assumed th at these couplings are not effective between adatoms on up-side and do wn-side step sites. At low surface coverages theta and temperatures T, most of the adatoms are trapped by down-side step sites. Both paralle l and perpendicular diffusion require detachment from such step sites, and therefore diffusion is almost isotropic. As coverage increases th e down-side step sites become highly populated and then act as barrier s for perpendicular diffusion. The up-side step sites constitute chann els for fast parallel diffusion so that anisotropy increases. In the c ase of nearest-neighbor repulsion phi(NN) < 0 the activation energies for surface diffusion are generally smaller. At low temperatures T les s-than-or-equal-to T(c) c(2 x 2) ordered lattice gas domains are obser ved for theta almost-equal-to 0.5 with disordered regions nucleating a t step rows in between. The effect of ordering on surface diffusion is described.