Growth of NbO, NbO2 and Nb2O5 thin films by reactive magnetron sputtering and post-annealing

Abstract

Niobium oxides are a complex materials system and synthesis of a specific crystal phase is challenging. Here, we demonstrate a synthesis route for polycrystalline films of cubic NbO, tetragonal NbO2 and monoclinic Nb2O5, based on reactive dc magnetron sputtering at moderate temperature and post-annealing in vacuum. Varying the O2 flow during deposition is used to tune the stoichiometry and subsequent annealing induces crystal formation. NbO is obtained by growth in 1–2 sccm O2 flow and post-annealing at 600–700 °C. NbO2 and Nb2O5 are formed by growth in 3–4 sccm O2 and 6–7 sccm O2 flows, respectively, followed by annealing at 800 °C. Prior to annealing the films are amorphous and atomically flat but crystallization during post-annealing results in a substantial increase in roughness and complex surface morphologies. The density of the amorphous films, determined by x-ray reflectivity prior to annealing, is a good predictor of the crystal phase obtained after annealing.