THE AS-TERMINATED SI(001) SURFACE AND ITS OXIDATION IN MOLECULAR-OXYGEN - AN SI 2P AND AS 3D CORE-LEVEL STUDY WITH SYNCHROTRON-RADIATION

The strong resistance of the As-terminated Si(001) surface to oxidation in molecular oxygen at room temperature is the subject of this study. As3d and Si2p photoemission spectra, as well as valence band spectra, excited with synchrotron radiation (135 eV less than or equal to hv less than or equal to 165 eV), have been recorded as a function of increasing O-2 exposures, over the range 2400-1.4 X 10(13) L (langmuir). Spectral changes, simultaneously observed in the As3d and Si2p core-levels near similar to 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 similar to 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 (similar to 24%) up to exposures as large as similar to 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 As4O6 molecules.