An expression that can be used to determine the thickness of an interfacial oxide layer between a high-K dielectric and Si substrate has been derived using the photoelectron intensity ratio of high resolution X-ray photoelectron spectroscopy (HRXPS). Experimental results show the interfacial silicon oxide thickness obtained with the expression fits very well with Ellipsometer measurements. We also studied the formation of an interfacial layer between a high-k HfO2 film and a Si substrate as well as its thermal stability. The results have revealed that: (i) the interfacial oxide formation requires the existence of thermally energized oxygen in an environment to break the H-Si bonds first for hydrogen terminated Si surface and (ii) the thermodynamic stability of the oxide interfacial layer depends on its initial formation process. The valence band structures of Si, SiO2, and HfO2 have been analyzed using HRXPS and an energy band diagram of HfO2/SiO2/Si has been constructed. (C) 2004 Elsevier Ltd. All rights reserved.