Determination of gas adsorption on native oxides formed on Si(100), Si(111) and Si(110) surfaces

We measured gas adsorption on native oxides formed on Si(100), Si(111) and Si(110) surfaces and found that the dependence of the gas adsorption on the surface orientation was insignificant. This fact suggests that the atomic structure of SiO₂ near the top surface is almost amorphous. For the physical adsorption of water vapour, we obtained a monolayer capacity of 0.0040 µg cm^-2 and a BET parameter of 4.5, which corresponds to 0.0065 µg cm^-2 water vapour adsorption at 50% humidity with an uncertainty of 0.0005 µg cm^-2 (k = 1). Thus, it can be concluded that the physical adsorption of water on a defect-free surface of a 1 kg silicon sphere amounts to 1.8 µg with an uncertainty of 0.14 µg (k = 1) at 50% humidity. In addition, measurement using ethanol vapour and n-octane vapour showed that gas adsorption decreased after hydroxylation of SiO₂/Si surfaces.