Structural difference of surface and sub-surface native oxides of evaporated amorphous silicon

Evaporated amorphous silicon (a-Si) films, oxidized in air or O₂ at room temperature, present two native oxides with different structures. The surface oxide is constructed from SiO₄-tetrahedron structural units with a 110° O---Si---O angle, which is the common structural unit of stable silicon oxides. The internal oxide has a different structure having a 120° O---Si---O angle. The results of molecular orbital (MO) calculations for (SiO₃)^m− and (SiO₄)ⁿ⁻ anionic clusters support the presence of the two stable structures of silicon oxides and also reveal the importance of the ionic character of the oxidized sites.