ATOMIC-SCALE DYNAMICS OF ATOMS AND DIMERS ON THE SI(001) SURFACE

The kinetics of adsorbed Si monomers and dimers, at submonolayer cover age, are measured using scanning tunneling microscopy (STM). Si monome rs are observed in empty-state STM images acquired between room temper ature and 115 degrees C. The monomers are trapped at the ends of rebon ded-SB type dimer rows. When monomers thermally escape from the traps, they rapidly diffuse along the substrate dimer row until they find an other unoccupied trap or return to their original trap. The binding ac tivation barrier at isolated traps is similar to 1.0 eV. A slightly lo wer barrier exists for monomers to hop between the ends of neighboring dimer rows - a process facilitating diffusion along segments of SE ty pe steps. In addition to monomers, the interactions of adsorbed Si dim ers with steps and islands on Si(001) are quantified using atom tracki ng STM. Diffusing dimers are reflected from steps, sides of islands, a nd certain surface defect structures. Site-specific free energies are extracted from measurements of lattice-site occupation probabilities o f dimers trapped between these reflecting barriers. Relative to the fr ee 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 s imilar to 0.03-0.06 eV. The binding decreases to half that at the seco nd lattice site, and is indistinguishable from the free-terrace value at a distance of three or more lattice sites. (C) 1997 Elsevier Scienc e B.V.