In this work atomic oxygen was created by the photochemical dissociation Of O-2 with a F-2 laser at 157 nm and of N2O with an ArF laser at 193 nm. With this technique ultrathin (< 6 nm) amorphous silicon oxide films (a-SiOx and a-SiO2) were grown onto hydrogen terminated Si(1 1 1), Si(I 0 0), and amorphous silicon (a-Si) substrates. The oxidation process was monitored in realtime by spectroscopic ellipsometry. Different ellipsometric models are applied to characterize the interface. The influence of temperature, atmosphere, and of the surface morphology of the substrate was studied. Although the kinetics of oxidation is affected by the gas phase chemistry and temperature, the oxidation conditions do not influence appreciably the structure of the growing interface. The interface formed by this low-temperature oxidation process seems to be characteristic for the structure of the substrate surface. It consists of amorphous silicon with a low content of oxygen (a-SiOx). The thickest a-SiOx interface layer grew on the Si(1 1 1):H surface with a thickness of (0.8 +/- 0.3) nm and a stoichiometric index x of (0.40 +/-0.15), whereas on Si(1 0 0):H a (0.40 +/- 0.15) nm thick SiOx interface of nearly pure amorphous silicon was observed. Additionally X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy measurements were carried out to confirm the ellipsometric results. (c) 2005 Elsevier B.V. All rights reserved.