ZrO2 and Y2O3 thin films were deposited on Si substrates using pulsed laser deposition in vacuum or oxygen ambient. Angle resolved X-ray photoelectron spectroscopy investigations showed that there was always some oxidation of the Si substrate resulting in the formation of an interfacial layer that could have a deleterious effect on the overall capacitance of the resulting MOS devices. The positions and shape of the metal, the oxygen and the Si XPS peaks suggest that at the temperatures used for deposition (<650 degreesC) the SiO2 layer is physically mixed with the deposited oxide layer without forming silicate compounds. Xray reflectivity investigations confirmed the presence of the interfacial layer whose density was found to be higher than that of pure SiO2 but lower than that of the corresponding silicate, corroborating the XPS results. To slow down the substrate oxidation, a low temperature UV-assisted nitridation treatment was performed prior to the deposition of the oxide layer. This UV-assisted treatment resulted in thinner and denser surface passivation layers than those obtained without any treatments indicating an incorporation of nitrogen into the SiO2 network. High-k dielectric layers deposited on such samples exhibited thinner interfacial layers. (C) 2003 Elsevier B.V. All rights reserved.