Crystal‐Phase Controlled Epitaxial Growth of NbN<i><sub>x</sub></i> Superconductors on Wide‐Bandgap AlN Semiconductors
Atsushi Kobayashi, Shunya Kihira, Takahito Takeda, Masaki Kobayashi, T. Harada, Kohei Ueno, Hiroshi Fujioka
Abstract
Abstract The structural similarity between NbN x and AlN facilitates the integration of superconductors into semiconductor optoelectronic devices. The abrupt interface between wide‐bandgap AlN and superconducting NbN x is essential for future nitride quantum devices, such as Josephson junction‐based quantum bits. In this study, the fundamental properties of NbN x epitaxially grown at various temperatures on atomically flat AlN/sapphire template substrates are investigated. It is found that with increasing epitaxial growth temperature, the nitrogen content of NbN x films grown on AlN decreases, resulting in a change in crystal structure from δ‐ to β‐ via ε‐ and γ ‐type. Microstructural analysis reveals that NbN x thin films epitaxially grown at high temperatures crystallize into a pure hexagonal β‐type and have atomically flat surface and the same in‐plane lattice constant as AlN. The technology for growing high‐purity NbN x films with flat interfaces and surfaces on AlN can contribute to the integration of nitride semiconductors and superconductor hybrid devices.