New Model of the Initial Stages of Si(111)-(7×7) Oxidation

By combining scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy, we show that the adsorption of single ${\mathrm{O}}_2$ molecules on Si(111)-(7 $\times$ 7) produces essentially molecular sites, seen as bright individual sites in STM topographs, which are stable at room temperature. Atom-resolved STM studies also reveal that these molecular sites react very efficiently with further ${\mathrm{O}}_2$ and ${\mathrm{H}}_2$O molecules. This is shown to account for the saturation at low exposure of the molecular site density and for the apparent decay lifetime of these molecular sites.