Bias-dependent scanning tunneling microscopy study of the oxygen-adsorbed Si(111)-(7×7) surface : Observation of metastable molecular oxygen

We have observed the initial stage of oxygen adsorption on a Si(111)-(7×7) surface using scanning tunneling microscopy. Among the bright sites observed after exposing the surface to oxygen in occupied state images, there are differences in the bias dependence of the brightness. Taking into account the local density of states of the oxygen-adsorbed Si(111)-(7×7) surface, we conclude that the sites appearing brightly only with a tip bias of >~+2.1 V are the molecular oxygen. The preferred adsorption site of this molecular species is a corner adatom, which has an oxygen atom adsorbed into its backbond, of the faulted half of the (7×7) unit cell