Reduction of Twin Boundary in NbN Films Grown on Annealed AlN

Abstract

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.

Crystal‐Phase Controlled Epitaxial Growth of NbNx Superconductors on Wide‐Bandgap AlN Semiconductors

“…The growth conditions and method of preparation of the AlN templates have been described elsewhere. [7,8,35,36] The crystallinity of NbN x was characterized by XRD, RHEED, EBSD, and HAADF-STEM. In addition, the surface morphology of NbN x was observed using AFM.…”

Epitaxial Junction of Inversion Symmetry Breaking AlN and Centrosymmetric NbN: A Polarity Control of Wide-Bandgap AlN

“…Prior to epitaxial growth of NbN, the AlN templates were annealed at 1700 °C to eliminate atomic steps on the AlN surface that could lead to the formation of an NbN twin boundary. Details of the surface treatment of AlN are reported elsewhere . γ-NbN x layers were epitaxially grown at 1100 °C on the annealed AlN templates, followed by the growth of 400 nm thick AlN at 800 °C.…”

“…In other words, information about the polarity of the lower AlN layer is not propagated to the upper AlN layer. We succeeded in epitaxial growth of NbN on AlN substrates and clarified the basic properties and growth mechanism of NbN thin film. − In this study, the polarity of AlN was successfully converted from Al- to N polarity by epitaxially inserting ultrathin NbN into AlN.…”

“…Details of the surface treatment of AlN are reported elsewhere. 29 γ-NbN x layers were epitaxially grown at 1100 °C on the annealed AlN templates, followed by the growth of 400 nm thick AlN at 800 °C. Both the NbN and AlN layers were grown by sputtering.…”