Electronic promoters and semiconductor oxidation: Alkali metals on Si(111) surfaces

We examine the effect of thin (1--2 monolayers) overlayers of the low-electronegativity metals Cs and Na upon the oxidation of Si(111) surfaces. Synchrotron-radiation photoemission studies of oxygen chemisorption as a function of overlayer thickness and oxygen exposure indicate large oxidation-promotion effects and the formation of Si oxide phases where high Si oxidation states dominate. Comparison with promotion effects induced by transition-metal and noble-metal overlayers forces, for alkali metals, a reevaluation of the microscopic mechanisms proposed to explain overlayer-induced oxidation promotion.