The depth profile of the HfO2/Si interface grown by molecular beam epitaxy (MBE) has been investigated by high-resolution x-ray photoelectron spectroscopy using synchrotron radiation beam and low energy Ar+ sputtering. At the interfacial layer, the Hf 4f spectra show a Hf silicide state without a Hf silicate signal. The 0 Is and Si 2p spectra show weakly two silicon oxidation states of SiO and SiO2. The data suggest an inward reaction of the Hf and 0 species into the Si substrate during the MBE growth. The valence band offset (Delta Ev) of similar to 3.5 eV is obtained for the HfO2/Si interface by measuring the valence-band edges of HfO2 and Si. (c) 2005 American Vacuum Society.