Characterization of adsorption properties of H2O on Si(001) and Si(111) byellipsometry and ab initio molecular orbital calculations

The ellipsometry was applied to estimate the thicknesses of the adsorption layer of water on tile surfaces of single crystals of silicon, Si(001) and Si(111) at a constant temperature of 20 degreesC. Water molecules adsorb mo re strongly on Si(111) than on Si(001) in the monolayer regime, whereas the opposit is tile case for the bilayer regime. The water adsorbed on Si(111) in the submonolayer regime does not desorb even when the vapor pressure of water is lowered down to 0.1 Pa, resulting in a strong hysteresis behavior . In contrast, water desorbs readily from Si(001) when the water vapor pres sure is lowered. The air initio molecular orbital (MO) calculations with cl uster models of the surfaces Si(111) and Si(001) were carried out to elucid ate the characteristics of the adsorption of water observed by the ellipsom etry, The calculations were executed for the physical adsorption and dissoc iative adsorption of water on Si(lll) and Si(001) in the monolayer regime, and the physical adsorption of water on the dissociative-adsorbed sites in the bilayer regime. The results from the ab initio MO calculations were qua litatively consistent with the experimental ones.