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 potassium coverages on Si(111) have been investigated using XPS. Strong contrasts have been observed between the various cases studied: potassium monolayer, simultaneous oxygen-potassium adsorption, potassium multilayers exposed to high O2 pressure or low O2 pressure. For the latter case, our measurements reveal that four potassium oxides (K2O, K2O2, KO2 and K2O3) have been successively formed during increasing O2 exposure. Strong evidence suggest that the potassium overlayer is characterized by the Stranski-Krastanov (S-K) growth mode on Si(111) at 150 K. Surprisingly potassium islands seem to be leveled under low pressure of oxygen, while they are frozen under high pressure. For monolayer coverage of potassium, XPS results indicate the presence of two adsorbed states for O atoms, and no specific potassium oxide. Depending on the O2 pressure, various potassium oxides are observed for the simultaneous adsorption case, which gives the most promising results.