Reduction of Twin Boundary in NbN Films Grown on Annealed AlN

Superconducting nanowire single-photon detectors (SNSPDs) based on ultrathin niobium nitride (NbN) films have attracted much attention owing to their high superconducting transition temperature and fast response time. AlN is a suitable substrate for obtaining high-quality single-crystal NbN films because of the small lattice mismatch between them. However, NbN(111) grown on AlN(0001) suffers from the formation of high-density twin boundaries. In this study, we reduce the NbN twin boundaries using atomically flat AlN substrates annealed at 1700 °C. The twin-boundary-free region of the NbN grown on the annealed AlN is extended to ∼1 × 1 μm2. The epitaxial growth elucidated in this study may contribute to the fabrication of large-area NbN/AlN SNSPDs without structural defects.