INITIAL-STAGES OF OXIDATION OF SI(111) WITH CONDENSED O2 AND N2O AT 20-K

The oxidation induced by soft-x-ray illumination of condensed multilayers of O2 and N2O on cleaved Si(111) at 20 K has been investigated with high-resolution core-level spectroscopy. The results for O2/Si(111) show that a SiO2 phase is produced by illumination with as few as 4.3 x 10(13) photons cm-2 of energy 130 eV. This Si4+ oxide is characterized by a Si 2p binding energy that is shifted 3.4 eV relative to bulk Si, and its effective thickness increases with illumination. Intermediate oxides having Si1+, Si2+, and Si3+ bonding configurations are also observed, and their total thickness also increases. The enhanced formation of these intermediate oxides is due primarily to the growth of Si3+ bonding configurations throughout the SiO2 layer. Exposure of the oxidized Si surface to white light from the synchrotron-radiation source or annealing to 300 K produces structural changes as the SiO2 layer thickens at the expense of the intermediate oxides. Hence, the defectlike Si3+ configuration converts to a SiO2 configuration and a sharper interface develops. Studies of N2O/Si(111) interactions during photon irradiation show that the initial oxidation rate is slower than with O2. After more extended illumination, the oxidation rate with N2O approximates that with O2 as N2O molecules are dissociated. The intermediate oxide formed with N2O is thicker than with O2, and extended growth of the intermediate oxide reflects the formation of both Si2+ and Si3+. From these O2 and N2O results, we conclude that the oxidation rate for the submonolayer oxide is determined mainly by the availability of surface reaction sites and the cross section for electron capture by the condensed species. Further oxide growth is also limited by the existing oxide layer through which atomic oxygen must diffuse in order to react at the interface.