Multi-techniques characterisation of anti-reflective Ta2O5 and TaOxNy thin films deposited by reactive sputtering: coupling X-ray photoelectron spectroscopy, scanning/transmission electron microscopy and ion beam analysis

TaO_xN_y thin films were elaborated by reactive magnetron sputtering of a Ta target in an Ar/N₂/O₂ plasma mixture. These layers are intended to be used as antireflective coatings in systems for concentrated solar applications (operating temperature around 500 °C in air). The investigation of these layers using a set of complementary techniques, including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ion beam analysis (IBA), revealed the impact of nitrogen flux during deposition on the overall morphology and elemental composition. These characterizations also highlighted the various chemical environments constituting the thin films, ranging from oxidized environments (Ta₂O₅) to oxynitride and nitride environments (TaO_xN_y and Ta₃N₅). It was also shown that with a low nitrogen flow, nitrogen is present only near the substrate and is absent in the rest of the layer. The IBA technique was also carried out to quantify the concentration of light elements as a function of depth, in particular H, which is not detected by the other techniques used in this paper (XPS and TEM). It has been shown that the majority of hydrogen atoms is incorporated into thin films during deposition as hydroxyl groups or only close to the surface, depending on the samples.