Indirect mechanism of Au adatom diffusion on the Si(100) surface

Calculations of the diffusion of a Au adatom on the dimer reconstructed Si(100)-2×1 surface reveal an interesting mechanism that differs significantly from a direct path between optimal binding sites, which are located in between dimer rows. Instead, the active diffusion mechanism involves promotion of the adatom to higher-energy sites on top of a dimer row and then fast migration along the row, visiting ca. a hundred sites at room temperature, before falling back down into an optimal binding site. This top-of-row mechanism becomes more important the lower is the temperature. The calculations are carried out by finding minimum energy paths on the energy surface obtained from density functional theory within the PBEsol functional approximation followed by kinetic Monte Carlo simulations of the diffusion over a range of temperature from 200 to 900 K. While the activation energy for the direct diffusion mechanism, both parallel and perpendicular to the dimer rows, is calculated to be 0.84 eV, the effective activation energy for the indirect mechanism parallel to the rows is on average 0.56 eV.