Effects of fabrication conditions on the properties of SIS tunnel junctions

Recent experiments on high-quality all-refractory Nb-Al 2 O 3 -Nb SIS tunnel junctions are reported. The authors have developed a highly reliable device processing route which has allowed the separate influences of the original whole-water deposition conditions and subsequent processing on the junction characteristics to be investigated. Results from several hundred devices, whose sizes vary between 4 and 250 mu m 2 , are presented. The post-processing has not been found to affect device characteristics significantly, but the conditions under which the whole-wafer deposition is performed have a strong effect on both the barrier resistance and the device quality. They report the first evidence that the normal-state resistance and critical current of junctions made under identical oxidation conditions are strongly affected by the temperature at which the base layer of niobium is deposited. Devices have been made with critical current densities greater than 1*10 7 A m -2 and V m values in excess of 50 mV. Differences in the resistances of separate devices on the same slide are evidence of statistical variations in the barrier itself.