The resistivity and superconducting transition temperature (T c ) of TMNs are dictated by their chemical composition, impurity concentration, and crystal structure. [4] While crystals of most TMNs are stabilized in a cubic rock-salt structure, certain TMNs crystallize in a hexagonal structure. [5] The (111) plane of the cubic lattice of some TMN superconductors possesses only a small lattice mismatch with the (0001) plane of nitride semiconductors. For instance, the lattice mismatches are −0.2% and +0.3% for NbN/AlN [6][7][8] and HfN/GaN, [9] respectively. These structural similarities make the integration of superconductor electronics into semiconductor optoelectronic devices feasible. [10][11][12][13] δ-NbN transitions to the superconducting state at ≈16 K, which is the highest T c among TMNs. It

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.

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The polarity control of AlN by epitaxial growth can enable the fabrication of wavelength conversion devices and innovative electronic devices based on nitride semiconductors. Conventional techniques for controlling the AlN polarity are based on oxygen-mediated growth mechanisms. In contrast, this study reports a technique to invert the polarity of wurtzite-type AlN using lattice-matched centrosymmetric NbN. The surface of AlN grown by sputtering on NbN/Al-polar AlN was atomically flat and highly crystalline. Structural analysis with scanning transmission electron microscopy revealed that the AlN grown on NbN/Al-polar AlN was N-polar. The proposed all-nitride epitaxial N-polar AlN/ NbN/Al-polar AlN heterostructure did not contain oxide materials, which typically degrade the optical and electrical properties of AlN. Furthermore, the stability of the N-polar AlN/ NbN interface was clarified by density functional theory calculations. These findings can contribute to the fabrication of structural defect-free vertical-and lateral-polarity structures based on AlN.