In this paper we use isotopic tracing experiments with (NH3)-N-15 and (NH3)-N-14 to investigate the nitridation mechanisms on both hafnium silicates films (40-175 angstrom) and HfO2 (50 angstrom) films deposited by MOCVD on silicon substrate covered by a 10-15 angstrom interfacial SiO2 layer. Nitrogen profiles in the films were obtained through nuclear resonance profiling (NRP) with the N-15(p,alpha gamma)C-12 resonance at 429 keV and the total amounts of atomic species and the overall stoichiometry were obtained by RBS and NRA. In the silicate films, nitrogen is incorporated both into surface and bulk regions. For HfO2, lowering the ammonia pressure favors the fixation of nitrogen in the near surface region of film. This phenomenon is not observed in the case of silicate films. The pressure dependence of near surface nitrogen incorporation in HfO2 films could be related to the formation of oxygen vacancies and opens a way to control the diffusion barrier needed in the gate dielectric. (c) 2006 Elsevier B.V. All rights reserved.