INTERACTION OF OXYGEN WITH A CS-COVERED SI(111)7X7 SURFACE

The influence of submonolayer Cs coverages, theta(Cs), on Si(111)7 x 7 at varying surface temperatures, T(g), on the initial oxygen uptake and Si oxide growth has been studied by means of thermal desorption, work function and Auger electron spectroscopy. The experimental data have shown that the presence of Cs always causes an increase of the oxygen initial uptake whereas promotion of the Si oxide growth is observed only at theta(Cs) above half a monolayer. It was found that in the presence of Cs the oxygen initial sticking coefficient becomes independent of T(g) (in the range 300-700 K) and the oxygen uptake rate remains constant up to a certain oxygen coverage. The value of this oxygen coverage increases with increasing theta(Cs) and T(g). Experimental evidence is given from the thermal desorption data that at theta(Cs) > 0.5, Cs participates in the formation of an intermediate with oxygen which is thermally stable at temperatures below 500 K. The mechanism of the Cs-promoted Si oxide growth and the conditions when Cs can act as a catalyst for Si oxidation are discussed.