Spectroscopic evidence of NOx formation and band-gap narrowing in N-doped TiO2 films grown by pulsed magnetron sputtering

N-containing TiO2 (TiO2:N) films have been grown by reactive pulsed magnetron sputtering (RPMS) with different N2/O2 mixtures and substrate temperatures (Ts) up to 450 °C. In this way, highly-doped films (N between 5 and 8 at.%) have been produced, as derived by elastic recoil detection analysis. The structural properties have been studied by grazing-incidence X-ray diffraction (GIXRD), X-ray absorption near-edge structure (XANES) and X-ray photoemission spectroscopy (XPS). On unheated substrates, the films are X-ray amorphous and N incorporation induces a transformation from rutile-like to highly disordered anatase-like structures. The crystal growth is enhanced by increasing Ts, being the effect larger for doped films. In this way, nearly single-phase (nanocrystalline) anatase TiO2:N films are achieved. The latter can be related to the imprinted anatase-like character by N incorporation observed on unheated substrates. XANES reveals that N sites are preferentially in the form of N2 or NOx (nitrite-like) complexes for growth on unheated and heated substrates, respectively. The dominant formation of NOx structures is corroborated by XPS, together with a small contribution of N at substitutional sites. Spectroscopic ellipsometry shows an effective reduction of the optical band-gap for doped films. A maximum decrease of ~0.3 eV at Ts =450 °C is observed, which correlates with a rigid red-shift of the valence band XPS spectra. The spectroscopic results indicate that band-gap narrowing is due to N interstitials as NOx complexes.