Atomic-scale dynamics of atoms and dimers on the Si(001) surface

The kinetics of adsorbed Si monomers and dimers, at submonolayer coverage, are measured using scanning tunneling microscopy (STM). Si monomers are observed in empty-state STM images acquired between room temperature and 115 degrees C. The monomers are trapped at the ends of rebonded-SB type dimer rows. When monomers thermally escape from the traps, they rapidly diffuse along the substrate dimer row until they find another unoccupied trap or return to their original trap. The binding activation barrier at isolated traps is similar to 1.0 eV. A slightly lower barrier exists for monomers to hop between the ends of neighboring dimer rows - a process facilitating diffusion along segments of SE type steps. In addition to monomers, the interactions of adsorbed Si dimers with steps and islands on Si(001) are quantified using atom tracking STM. Diffusing dimers are reflected from steps, sides of islands, and certain surface defect structures. Site-specific free energies are extracted from measurements of lattice-site occupation probabilities of dimers trapped between these reflecting barriers. Relative to the free energy of isolated dimers on a terrace, dimers located at the first lattice site next to SA steps and the sides of islands are bound by similar to 0.03-0.06 eV. The binding decreases to half that at the second lattice site, and is indistinguishable from the free-terrace value at a distance of three or more lattice sites. (C) 1997 Elsevier Science B.V.