NEW MODEL OF THE INITIAL-STAGES OF SI(111)-(7X7) OXIDATION

By combining scanning tunneling microscopy (STM) and ultraviolet photo electron spectroscopy, we show that the adsorption of single O-2 molec ules on Si(111)-(7 x 7) produces essentially molecular sites, seen as bright individual sites in STM topographs, which are stable at room te mperature. Atom-resolved STM studies also reveal that these molecular sites react very efficiently with further O-2 and H2O molecules. This is shown to account for the saturation at low exposure of the molecula r site density and for the apparent decay lifetime of these molecular sites.