Binding and diffusion of a Si adatom on the Si(100) surface

The binding sites for adsorption of a single Si atom on the reconstructed Si(100) surface are identified using first-principles total-energy calculations. We establish several saddle points for the migration of the adatom by mapping out the total energy as a function of its position on the surface. For the diffusion parallel to the dimer rows on the surface, we find an activation energy of 0.6 eV; for diffusion perpendicular to the rows, the activation energy is 1.0 eV. One-dimensional hopping motion of individual adatoms should be observable by scanning tunneling microscopy at moderately low temperatures.