Growth of NbO, NbO(2 )and Nb(2)O(5 )thin films by reactive magnetron sputtering and post-annealing

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 degrees 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 degrees 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.