Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature
Xiucheng Wei, Pinku Roy, Zihao Yang, Di Zhang, Zihao He, Ping Lu, Olivia Licata, Haiyan Wang, Baishakhi Mazumder, Nag Patibandla, Yong Cao, Hao Zeng, Mingwei Zhu, Q. X. Jia
Abstract
Ultrathin (5–50 nm) epitaxial superconducting niobium nitride (NbN) films were grown on AlN-buffered c-plane Al2O3 by an industrial scale physical vapor deposition technique at 400°C. Both X-ray diffraction and scanning electron microscopy analysis show high crystallinity of the (111)-oriented NbN films, with a narrow full-width-at-half-maximum of the rocking curve down to 0.030°. The lattice constant decreases with decreasing NbN layer thickness, suggesting lattice strain for films with thicknesses below 20 nm. The superconducting transition temperature, the transition width, the upper critical field, the irreversibility line, and the coherence length are closely correlated to the film thickness.