Adsorption of O2 on Si(111) 7X7 at 300 and 30 K studied by ion photodesorption and electron photoemission -: art. no. 035315

Molecular oxygen adsorption on a Si(111) 7 X 7 surface is investigated by ion photodesorption and valence-band photoemission from very low O-2 exposures up to the saturation of the adatom dangling bonds. It was found that, at 30 K, the O-2 molecule is adsorbed dissociatively on a silicon adatom, with one oxygen atom on top of the silicon atom and one oxygen atom inserted into the silicon backbond. This single adsorption configuration is not stable at 300 K. At 300 K, the O-2 molecule dissociates on a silicon adatom, with two, oxygen atoms inserted into the two silicon backbonds. Double-ad sorption configurations, resulting from the successive adsorption of two O-2 molecules on the same silicon adatom, have been identified as SiO4 tetrahedral-type configurations coexisting at 300 K with double adsorption configurations having four subsurface O atoms. Finally, the diffusion of O-2 molecules on the surface at 30 K, and their subsequent reaction on single adsorption configurations, are proposed to explain the observed time dependence of the adsorption configuration populations.