SI(100)-(2X1) SURFACE-DEFECTS AND DISSOCIATIVE AND NONDISSOCIATIVE ADSORPTION OF H2O STUDIED WITH SCANNING-TUNNELING-MICROSCOPY

We have investigated the adsorption characteristics of H2O on Si(100)-(2 X 1) at 300 K using scanning tunneling microscopy. Nondissociative adsorption is shown to occur at low coverage, as characterized by the formation of dark features in both occupied- and unoccupied-state images. Such features form straight chains, termed W defects, derived from two to six H2O molecules. The C-type defects commonly observed on Si(100)-(2 X 1) are shown to be identical to W defects with two H2O molecules. These results also show that most of the dark dimer features on Si(100)-(2 X 1) result from H2O adsorption, i.e., they are not dimer vacancies. Dissociative adsorption is observed at higher exposure. It is characterized by two-dimensional patches where Si-H and Si-OH states can be resolved as atomic features of different intensities. A 2 X 1 structure consisting of Si-H and Si-OH is observed at saturation, but there is no long-range order in the arrangement of H and OH. Through saturation, there is also evidence of oxidation.