K OVERLAYER OXIDATION STUDIED BY XPS - THE EFFECTS OF THE ADSORPTION AND OXIDATION CONDITIONS

In the context of the alkali metal promotion of Si oxidation, various combinations of oxidation processes (pressure and exposure) and potass ium coverages on Si(lll) have been investigated using XPS. Strong cont rasts have been observed between the various cases studied: potassium monolayer, simultaneous oxygen-potassium adsorption, potassium multila yers exposed to high O-2 pressure or low O-2 pressure. For the latter case, our measurements reveal that four potassium oxides (K2O, K2O2, K O2 and K2O3)have been successively formed during increasing O-2 exposu re. Strong evidence suggest that the potassium overlayer is characteri zed by the Stranski-Krastanov (S-K) growth mode on Si(111) at 150 K. S urprisingly potassium islands seem to be leveled under low pressure of oxygen, while they are frozen under high pressure. For monolayer cove rage of potassium, XPS results indicate the presence of two adsorbed s tates for O atoms, and no specific potassium oxide. Depending on the O -2 pressure, various potassium oxides are observed for the simultaneou s adsorption case, which gives the most promising results.