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Thin dielectric films (∼4.0 nm) of Ta2O5 were prepared by ion beam deposition technique onto Si(1 0 0) without and with a buffer layer of silicon nitride and oxynitride of about 1 nm. The composition of the films, submitted to annealing at 850 °C in nitrogen/oxygen atmosphere was studied by X-ray photoelectron spectroscopy. The thickness and roughness of layers were determined by a best squares fitting routine, based upon the algorithm of Parrat, of the X-ray reflectivity spectra. From capacitance as a function of voltage measurements we analyze the dielectric properties of these samples. We observed that after annealing the intermediate layer of silicon nitride or oxynitride confers to the samples a greater thermal stability in composition and thickness together with an improvement of the electrical properties especially for the sample deposited onto pure silicon nitride that has a dielectric constant of about 18 and an equivalent thickness of 1 nm. [Copyright &y& Elsevier]