Direct and Rb-promotedSiOx/β-SiC(100) interface formation

We investigate the formation of ${\mathrm{SiO}}_{\mathit{x}}$/β-SiC(100) interfaces by direct and Rb-promoted oxidation by means of photoemission spectroscopy using the Al Kα (1486.6-eV) and Zr ${\mathit{M}}_{\mathrm{\zeta }}$ (151.4-eV) x-ray lines at Si 2p, C 1s, O 1s, and Rb 3p core levels. Clean and stoichiometric β-SiC(100) surfaces have been obtained applying thermal annealing treatments only. Room-temperature Rb deposition on the clean β-SiC(100) surface induces a large decrease of the work function, reaching a minimum with ΔΦ=-3.3 eV. This work-function change corresponds to the Rb saturation coverage of one physical monolayer (1 Rb ML). Room-temperature exposure of the clean β-SiC(100) surface to molecular oxygen results in only little oxygen uptake and small amounts of silicon oxide, showing no evidence of bonding between carbon and oxygen atoms on the surface. When the β-SiC(100) surface is modified by a Rb monolayer, the oxygen uptake is dramatically enhanced by four orders of magnitude, leading to a large increase of the oxidation rate and forming high silicon oxidation states. The oxygen atoms appear to be more tightly bound to Si than to Rb atoms while, as in the case of direct oxidation, there is apparently no sign of carbon-oxygen bonding present on the surface, which can be explained by formation of CO or ${\mathrm{CO}}_2$ species desorbing into the vacuum. Upon thermal annealing at temperatures up to 700 °C, the oxide layer thickness is increased with a significant improvement of the stoichiometry, leading to the formation of a nonabrupt carbon-free ${\mathrm{SiO}}_2$/β-SiC(100) interface including lower oxidation states at the interface. The Rb overlayer is removed from the surface by thermal desorption below 780 °C.