Intermittent diffusion on the reconstructed Si(111) surface

The diffusion of an extra-adatom on reconstructed Si(lll) surfaces is investigated using density-functional total-energy pseudopotential calculations. The diffusion path is characterized by a basin of attraction in which the extra-adatom is localized until an intermittent jump to a nearby basin of attraction occurs. The energy barrier between the neighboring basins is 1.12 eV, while the energy barrier within a basin is 0.56 eV. Our theoretical predictions for stable and activated configurations provide a natural explanation for the scanning tunneling microscopy images of an extra silicon atom on the Si(111)-(7 x 7) surface, which is radically different from the simple models proposed in the literature.