NbNx-NbOy-PbInzJosephson junctions with R.F. oxidised tunneling barriers

We report the fabrication of high quality Josephson tunnel junctions made by the r.f. sputter-deposition method. The characteristics of these junctions were determined at 4.2°K as a function of the following fabrication parameters: the substrate temperature, the sputtering power during base electrode deposition, and the r.f. oxidation time for the barrier. We have obtained critical current densities from 90 to 800 Amperes/cm2which depended exponentially on the barrier formation time, a gap energy of 3.3meV, and hysteretic current-voltage characteristics. Thermal cycling of a few junctions resulted in small changes in the current-voltage curves. Auger Electron Spectroscopy and ESCA of the films show that x=1.02, z=0.25, and that the niobium oxide is a mixture of Nb2O5and the lower oxides, NbO and NbO2, that are present at the oxide-base electrode interface.