The As-terminated Si(001) surface and its oxidation in molecular oxygen: an Si 2p and As 3d core-level study with synchrotron radiation

Abstract

The strong resistance of the As-terminated Si(001) surface to oxidation in molecular oxygen at room temperature is the subject of this study. As 3d and Si 2p photoemission spectra, as well as valence band spectra, excited with synchrotron radiation (135 eV ≤ hv ≤ 165 eV), have been recorded as a function of increasing O 2 exposures, over the range 2400–1.4 × 10 13 L (langmuir). Spectral changes, simultaneously observed in the As 3d and Si 2p core-levels near ∼ 10 12 L are indicative of the onset of oxidation. However a modification of the valence band, due to the contribution of O 2p states, is clearly evident from ∼ 10 11 L, pointing to the presence of weakly bound oxygen species at that exposure. These results are confronted with a previous Auger study reporting an oxygen saturation coverage (0.5 monolayer) already in the 400–2000 L range, interpreted as the insertion of an oxygen atom between two dimerized arsenic atoms. On the contrary our data show that surface oxidation is a slow process, spatially inhomogeneous, as some dimerized As remain intact (∼ 24%) up to exposures as large as ∼ 10 13 L. Oxidation is not limited to the outermost layer, as three arsenic oxidation states and the four silicon oxidation states (indicative of subsurface oxidation) are detected simultaneously. Moreover metallic arsenic shows up: this suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As 4O 6 molecules.