Growth windows of epitaxial NbNx films on c-plane sapphire and their structural and superconducting properties

NbN films are grown on $c$-plane sapphire substrates by molecular beam epitaxy. The structural and superconducting properties of the films are characterized to demonstrate that growth parameters such as substrate temperature and active nitrogen flux affect the structural phase of films, and thereby the superconducting critical temperature. Four phases of NbN are identified for films grown in different conditions. We demonstrate that atomically flat and highly crystalline $\ensuremath{\beta}\text{\ensuremath{-}}{\text{Nb}}_{2}\text{N}$ films can be grown at substrate temperatures of ${1100}^{\ensuremath{\circ}}\mathrm{C}$ or higher, and that the superconducting critical temperature of phase-pure $\ensuremath{\beta}\text{\ensuremath{-}}{\text{Nb}}_{2}\text{N}$ films is $0.35<{T}_{c}<0.6$ K for films grown at different substrate temperatures.