Silicon dioxide films are deposited on Si and unstrained Si0.83Ge0.17 from O2/tetraethylorthosilicate plasmas in a helicon reactor operated at low pressure (2 mTorr). The effect of the negative dc self-bias voltage (0 to -200 V) on the film properties is investigated. X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry measurements have been performed on ultrathin (∼5 nm) films to gain better insight into the quality of the dielectric/semiconductor interface. We observed that the ion bombardment energy is responsible for the amorphization of the substrate, which is in agreement with the TRIM (transport and range of ions in matter) simulation results. In the case of SiGe samples, a GeO2 phase is detected in the XPS spectra which increases with the applied bias. Changes on the vibrational properties are observed on thick films (500 nm) while refractive index and p-etch measurements are only slightly sensitive to the voltage applied to the substrate. Complementary electrical measurements have been carried out on metal–oxide–semiconductor capacitors. For films deposited on Si substrates, C–V measurements indicated a degradation of the electrical properties with increasing energy of the impinging ions. The results obtained on SiGe samples exhibit typical negative fixed charges in the oxide with a rather low density of interface states (Dit∼5×1011 cm-2 eV-1). © 2002 American Vacuum Society.