Si(100)-(2×1) surface defects and dissociative and nondissociative adsorption ofH2O studied with scanning tunneling microscopy

We have investigated the adsorption characteristics of ${\mathrm{H}}_2$O on Si(100)-(2×1) 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 ${\mathrm{H}}_2$O molecules. The C-type defects commonly observed on Si(100)-(2×1) are shown to be identical to W defects with two ${\mathrm{H}}_2$O molecules. These results also show that most of the dark dimer features on Si(100)-(2×1) result from ${\mathrm{H}}_2$O 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 2×1 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.