Ultrathin rock-salt type NbN films grown on atomically flat AlN/sapphire substrates

•An ultrathin NbN film on AlN exhibits a step-and-terrace structure on the surface.•The NbN exhibited distinct streaky RHEED patterns and reasonable XRD diffraction.•The Tc of the 7-nm-thick NbN was 11.3 K. In this study, we grew NbN(111) films on AlN(0001)/sapphire substrates by sputtering and investigated their structural and electrical properties. It was found that a 7-nm-thick NbN film grown on an AlN/sapphire substrate at 850 °C had a step-and-terrace structure on the surface, similar to that of the underlying AlN surface. Reflection high-energy electron diffraction and X-ray diffraction revealed the high crystallinity of the ultrathin NbN film. The superconducting transition temperature of the 7-nm-thick NbN was 11 K, which is lower than that of the 27-nm-thick NbN film grown at the same temperature, but sufficient to be applicable to superconducting single-photon detectors.