Spectroscopic ellipsometry was used to investigate the oxidation of pure Hf films on silicon for the formation of HfO2 (hafnium oxide) gate-dielectric films in advanced complementary metal-oxide-semiconductor field-effect transistors. Absorption coefficients near the absorption edge were extracted using the data inversion method, in which the optical constants for short wavelengths were calculated using the film thickness determined from long-wavelength data. The extracted optical band gap of 5.7 eV matches well with published data, and a curve shift due to crystallization was detected. In addition, an extra absorption peak corresponding to electron transition from the valence band to a defect energy level was observed in the range 4.5-5.0 eV above the valence-band edge. The 1.2 eV energy difference between the conduction-band edge and the edge of this extra peak is close to the electron trap energy level reported elsewhere. The intensity of the detected peak was clearly correlated with leakage current and near-interface trap densities. Based on the annealing condition dependence of the extra absorption peak, the defects are likely oxygen vacancies within the HfO2 film. (C) 2004 American Vacuum Society.