N-doped ZnO (ZnO: N) thin films, intended to be used as one of the layers in solar cell applications were deposited by r.f. sputtering, using ZnN target (99.9% purity), on silicon and fused silica substrates. In the gas flow composition, Ar was kept constant (50%) and the O-2/N-2 ratio was varied as: 40%/10%, 25%/25% and 10%/40%. After deposition, rapid thermal annealing (RTA) at 400 and 550 degrees C for 1 min in N-2 ambient has been performed. The RTA impact on the optical and microstructural properties of ZnO: N thin films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX), UV-vis-NIR spectroscopy, UV-vis-NIR spectroscopic ellipsometry (SE) and infrared ellipsometry (IR-SE). The as-deposited (ad) ZnO: N films are polycrystalline with preferentially oriented columnar crystals. After RTA we found ZnO: N films with improved crystallinity and fewer boundary defects. We report optical constants of ZnO: N from UV to IR spectral range as well as the infrared active phononic modes. (C) 2012 Elsevier B. V. All rights reserved.