An understanding of the exact structural makeup of dielectric interface is crucial for development of novel gate materials. In this paper a study of the HfO2/Si interface created by the low-temperature deposition ultrathin stoichiometric HfO2 on Si substrates by reactive sputtering is presented. Analysis, quantification and calculation of layer thickness of an HfO2/Hf-Si-Ox/SiO2 gate stack dielectrics have been performed, using X-ray photoelectron spectroscopy (XPS) depth profile method, angle resolved XPS and interface modeling by XPS data processing software. The results obtained were found to be in good agreement with the high frequency capacitance-voltage (C-V) measurements. The results suggest a development of a complex three layer dielectric stack, including hafnium dioxide layer, a narrow interface of hafnium silicate and broad region of oxygen diffusion into silicon wafer. The diffusion of oxygen was found particularly detrimental to the electrical properties of the stack, as this oxygen concentration gradient leads to the formation of suboxides of silicon with a lower permittivity, kappa. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.