The paper presents results on an optical interface characterization of Ta2O5 stacks by spectroscopic ellipsometry. Inhomogeneous in depth interfacial layers were identified by a novel approach and proper algorithm for ellipsometric data interpretation. Simultaneous determination of a high-k oxide thickness, an optical dielectric function profile and the thickness of the interfacial layer was achieved. The interfacial constituents and their depth profiles were recognized. The profiles were used as an additional tool for exact determination of thicknesses. The identical interfacial layers were detected for samples produced by similar Si surface treatment, independent of the Ta2O5 thickness (<20 nm) and methods of Ta2O5 fabrication. High inhomogeneous amount of Si constituent phase in interfacial depth was detected in all investigated samples. The thickness reduction and homogenization of the interfacial layers was established after Hf-doping of Ta2O5. (C) 2012 Elsevier B.V. All rights reserved.