β-TaON thin films: production by reactive magnetron sputtering and the question of non-stoichiometry

In this work we report on the structure, morphology, composition, chemical bonding and optical properties of tantalum oxynitride (Ta-O-N) thin films prepared by reactive direct current magnetron sputtering. The films have been investigated by grazing incidence x-ray diffraction, x-ray reflectivity, grazing incidence small angle x-ray scattering, time-of-flight elastic recoil detection analysis, x-ray photoelectron spectroscopy and spectroscopic ellipsometry. The composition and the partial pressure of the reactive atmosphere (N-2 + O-2) have been varied in order to find conditions suitable for the beta-TaON phase production. As prepared thin films are amorphous and annealing at T-a >= 800 degrees C is necessary to promote crystallization. We discuss the role of N-2 gas on the kinetics of sputtered particles and its influence on the oxidation rate and porosity of the growing film. The anion composition in Ta-O-N films strongly depends on the reactive gas condition during the deposition. We found that the O/N ratio in beta-TaON films increases with more N-2 or O-2 gas in the chamber, whereas the corresponding absorption onset and valence band maximum show a blue shift of up to 0.5 eV. These results were related to the non-stoichiometry in beta-TaOxNy crystallites with x > y.