Niobium nitride thin films for use in Josephson junctions

The properties of rf diode and magnetron reactively sputtered NbN films have been studied under a variety of preparation conditions. The aim of this investigation is to achieve high transition temperature, low resistivity films under conditions suitable for use in all refractory tunnel junction fabrication. We have systematically varied the relative amounts of Ar, N2and CH4gases, and the substrate temperature used during film growth. The transition temperature, resistivity, lattice parameter and crystal structure have been studied and correlated with the partial pressure of methane used during sputtering. The crystal structure was investigated using diffractometer and Read camera photographic X-ray techniques. We have prepared NbN films using both rf diode and magnetron sputtering with resistivities less than 70 μΩ-cm and transition temperatures greater than 16 K. The lattice parameter for our NbN films ranges between 4.39 Å and 4.45 Å and is dependent upon the amount of nitrogen and carbon used in the film preparation. We are currently investigating all refractory tunnel junctions with artificial barriers using these films as base electrodes and niobium counter electrodes.