Alkali-metal-promoted oxidation of the Si(100)2×1 surface: Coverage dependence and nonlocality

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Abstract
We have studied the effect of alkali-metal (Na,Rb,Cs) coverages in the monolayer range on the oxidation of the Si(100)2×1 surface by core-level photoemission spectroscopy using synchrotron radiation. By comparing oxide thicknesses obtained using various coverages of Na and Cs, we find that coverages in excess of ≊0.5 monolayer are required to enhance significantly the oxidation of silicon. The existence of such a threshold demonstrates that the mechanism for alkali-metal-promoted oxidation is essentially nonlocal in nature, in strong contrast to the conclusions drawn in a recent Auger-electron-spectroscopy study. We also observe that the thickness of oxides grown with Na increases (by about 50%) upon annealing, while those grown with Cs do not change appreciably. Using Rb, we obtain intermediate results. In all cases, annealing results in higher-oxidation states. We also report spectral changes with time, which are to some extent analogous to those associated with annealing of the sample.