STRUCTURE-ANALYSIS OF O-2 AND H2O CHEMISORPTION ON A SI(100) SURFACE

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) has been applied to structural analysis of O-2 and H2O chemisorption on a Si{100} surface near room temperature. The H, O, and Si recoil intensities were monitored as a function of the beam incident angles and crystal azimuthal angles. Structural information was directly elucidated from the angular anisotropies of these recoil intensities. The results show that the Si(2 x 1) dimer structure is gradually destroyed due to oxygen adsorption in the initial oxidation stage; the resulting structure lacks long-range or short-range order with a 1 ML oxygen coverage. Chemisorption of N2O was also carried out in order to prove the dissociative nature of O-2 chemisorption. At all coverages, no short-range order for oxygen adsorbates was observed. A totally different behavior was observed upon H2O adsorption; the (2 x 1) structure was maintained even at saturation coverage. Concerning the adsorption sites, O atoms adsorb randomly at sites that are > 0.5 Angstrom above the 1st-layer Si atoms at low coverage (< 0.5 ML) and begin to adsorb at lower positions at higher coverages; the data are consistent with this lower position being the bridge site between 1st- and 2nd-Si layers. Water is dissociatively adsorbed on the Si{100} surface, resulting in H-(ad) and OH(ad) species. The data is consistent with both of the H and OH species being bound at the on-top site on through the Si dangling bonds.