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

Atsushi Kobayashi, Shunya Kihira, Takahito Takeda, Masaki Kobayashi, Takayuki Harada, Kohei Ueno, and Hiroshi Fujioka

The structural similarity between NbNx and AlN facilitates the integration of superconductors into semiconductor optoelectronic devices. The abrupt interface between wide-bandgap AlN and superconducting NbNx is essential for future nitride quantum devices, such as Josephson junction-based quantum bits. In this study, the fundamental properties of NbNx 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 NbNx 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 NbNx 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 NbNx films with flat interfaces and surfaces on AlN can contribute to the integration of nitride semiconductors and superconductor hybrid devices.

Advanced Materials Interfaces: