Chlorine oxidation is important methods for improvement of many properties such as passivation of mobile oxide charge, breakdown strength or enhancement of the minority-carrier lifetime in the underlying silicon. In this study we consider effects influencing the density of SiO2 layers formed by three different methods: thermal oxidation at 850 degrees C, low temperature oxidation method by use of nitric acid - HNO3 (NAOS) and HClO4 afterward passivated with KCN/HCN solutions. Thicknesses of SiO2 layers determined by both capacitance-voltage (C-V) and XPS revealed fast oxidation rate compared with samples prepared by thermal oxidation. FT IR measurement showed that all absorption spectra are almost similar. Higher absorption of the sample prepared in HClO4 was observed. No Si-Cl bonds were visible. Calculated atomic density of the SiO2 layer obtained from IR measurements was lowest for sample formed in HClO4. Chlorine oxidation results in higher oxidation rate (higher thickness) and formation of stoichiometric SiO2 layer with lower density. Following KCN/HCN passivation causes formation of Si-N, Si-O and Si-OH bonds at the expense of Si-Cl bonds. (C) 2012 Elsevier B. V. All rights reserved.