In this paper, we report the effect of high temperature annealing on the chemical and electronic structure of technologically relevant TiN/LaOx /HfSiON/SiON/Si gate stacks. Using medium energy ion scattering from the backside of the samples, a non-destructive compositional depth profile of La has been obtained, revealing the lanthanum diffusion in the SiON interface layer upon annealing. To complement this analysis, hard X-ray photoelectron spectroscopy with synchrotron radiation has been performed to investigate the chemical and electronic structure of the gate stacks. The results show clear changes in the Hf and Ti core level energy positions with respect to Si bulk, with changes in the thickness of the LaOx capping layer. We infer that La diffusion generates an internal electrical field at the La-silicate interface between HfSiON and SiON, and that its strength increases with the increase of LaOx thickness. These findings support the band alignment model based on a La-induced interfacial dipole. (C) 2015 Elsevier B.V. All rights reserved.