O 1S INVESTIGATION OF SIO2/SI INTERFACE FORMATION USING AN ALKALI-METAL PROMOTER

We have investigated the role of pre-adsorbed potassium in the promotion of silicon surface oxidation by core level and valence band photoemission experiments using AlKalpha (1,486.6 eV) and Zr Mzeta (151.4 eV) X-ray lines. We dose between 10(+15) and 10(+18) oxygen molecules onto a Si(100)2 x 1 surface modified by a K overlayer. Under these conditions the oxidation rate of silicon is enhanced by about 3 to 4 orders of magnitude with formation of a silicon oxide layer having high oxidation states (Si3+ ) at room temperature. The alkali metal catalyst is removed by thermal annealing (at about 600-degrees-C), leading to the formation of a SiO2/Si interface at lower temperatures than by direct thermal oxidation. The O 1s core level line clearly displays two components which suggests that the oxygen atoms have different bond configurations including binding to the substrate and the adsorbate, as also indicated by the corresponding valence band results. Interestingly, the respective intensities of the O 1s components behave very differently upon increasing oxygen exposure. The Si related component grows proportionally to the logarithm of the number of dosed oxygen molecules, whereas the K related one is already saturated at the smallest dose and yields a constant value for the whole exposure range. This feature demonstrates that the potassium atoms do not ''store'' oxygen, but only a small fraction of oxygen atoms is bound weakly to the catalyst. The micromechanisms of promotion will be described and compared to the latest state-of-the-art ab initio theoretical calculations using the local density functional approach.