We have shown that, for thermally evaporated Ta2O5 or ZrO2 thin films on Si(100), O-2 annealing at 300-500 degreesC causes the formation of an interfacial silicon oxide layer as thin as 1-2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta2O5 and ZrO2 with Si(100) before and after the O-2 annealing at 500 degreesC. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(100) interface states over nearly entire Si bandgap. (C) 2002 Elsevier Science B.V. All rights reserved.