Superconducting Transition Temperatures and Annealing Effect of Single-Crystalline NbN _x Films

Abstract

Among the many compounds known as high-transition-temperature superconductors, niobium nitride has the highest transition temperature in binary compounds with the rocksalt structure. According to our researches over the last several years, NbN films prepared by reactive sputtering techniques show a maximum T, value of 17.3°K,¹ an upper critical field of 290 kOe,² and a critical current carrying capacity of the order of 10⁵ A/cm² at 100 kOe.³ These polycrystalline films show only the NaCl structure. However, the phase diagram for the NbN _x system is quite complex. While it has been desired to get a single-crystal sample in order to obtain more detailed knowledge about the basic properties of NbN _x compounds, this has not been achieved except in the case of whiskers reported by Darnell. Recently we have succeeded in obtaining single-crystalline NbN films by using a vapor-phase growth technique. In the present report the relationship between the transition temperatures and the crystal structure of such films is described.