We have investigated the composition, atomic transport, chemical reactions, and electrical characteristics following annealing in O-2 of ultrathin HfO2 films deposited on Si substrates thermally nitrided in NO. The dielectric stack was probed by X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, nuclear reaction analysis, and transmission electron microscopy. It presented an intermediate layer containing silicon oxynitride, hafnium silicate, and possibly hafnium-silicon oxynitride. Capacitance-voltage characteristics were determined before and after annealing. (C) 2004 The Electrochemical Society.