Unexplored MBE growth mode reveals new properties of superconducting NbN

Accessing unexplored crystal growth conditions often reveals new regimes of physical behavior. In this work, performing molecular beam epitaxy growth of the technologically important superconductor NbN at temperatures greater than 1000 ^∘C reveals a growth mode that has not been accessed before. This mode results in persistent reflection high-energy electron diffraction (RHEED) oscillations through the entire growth, resulting in atomically smooth surfaces, normal metal resistivities of ∼37 μ Ω cm. We find that the superconducting critical temperature depends strongly on growth temperature, and report a maximum superconducting critical temperature of 15.5 K. Electron microscopy studies reveal a rich range of crystalline phases that depend on the growth temperature and correlate to the physical properties. Surprisingly, a reversal of the sign of the Hall coefficient from n-type to p-type is observed as the NbN films are cooled, indicating an electronic structure that has not been observed before in this material. In addition to this observation, the crystallinity of the high-temperature epitaxial NbN allows for an ordered Abrikosov vortex lattice to be imaged for the first time in this superconductor.