Initial stages of oxygen adsorption on Si(111). II. The molecular precursor

In the initial stage of oxygen adsorption on Si(111) surfaces, a metastable molecular state is found which precedes dissociation and insertion of oxygen atoms in bridging positions between first- and second-layer silicon atoms. This precursor can be observed in conventional photoemission experiments at low coverages and—for optimum retention—at low substrate temperatures. Combining the results from several high-resolution electron spectroscopies such as x-ray photoemission (XPS), polarization- and photon-energy-dependent uv photoemission, x-ray-induced Auger spectroscopy, and near-edge x-ray absorption, we are able to characterize the precursor as a negatively charged molecular species chemisorbed in a bridge configuration between dangling bonds of the surface. Its decay and conversion into the stable dissociated state could be monitored by XPS up to room temperature, showing a thermally or electronically activated behavior. For the reaction of ${\mathrm{O}}_2$ with Si(111), a two-stage adsorption model is proposed. It explains a number of observations and features of oxygen adsorption on differently prepared Si(111) surfaces. The present model may be applicable to other semiconductor surfaces as well.