Systematic oscillation of peak positions in photoemission spectra during alternating caesium and oxygen exposures of silicon surfaces

Abstract

In studies of the oxidation of the Si(1 1 1) and Si(0 0 1) surfaces using Cs as a promoter, each surface is repeatedly dosed with caesium and oxygen at room temperature. During these cycles it is observed that valence band features and Si 2p, O 1s and Cs 3d core-level positions oscillate between two situations. Revealing the mechanism that causes this type of shifting could be important for understanding the complicated Cs–O system at the silicon surface and might ultimately lead to a better control of the Cs promoted oxidation process of silicon. The present observations are explained in a model suggesting the overlayer with caesium and oxygen to constitute an amorphous semiconductor that is switched between two alternating (Cs, O)-stoichiometries. The variations of the positions of the valence band edge and caesium and oxygen core-level peaks are then explained by changes in the Cs-oxide electronic structure and charge transfer between Cs and O. The Si 2p peaks shift due to band bending in the silicon crystal surface region, in response to charge transfer across the heterojunction from excess donors or to excess acceptors in the oxide.