High-resistivity superconductor-normal-superconductor junctions for an AC Josephson voltage standard

We report here our results on the development of Nb/Al/Nb Josephson junctions for a programmable voltage standard. The Al barrier is 5-120 nm thick. Its properties are quite different with respect to the single Al films because of diffusion between Al and the Nb counter-electrode. The resistivity of the Al barrier ranges from hundreds of microhm centimetres to a few milliohm centimetres, probably depending on the surface roughness of the Al layer, measured by atomic force microscopy (AFM). The junctions, with areas between 25 and 200 mu m(2) have critical currents I-C in the milliampere range, associated with normal resistances R-N of about 1 Omega. The very high ICRN product of these junctions allows us to use a microwave source of 70 GHz and above, as in present de voltage standards. Preliminary rf measurements on these devices are reported. Structural investigations by AFM and X-ray diffraction were also carried out to relate the electrical properties of the junctions to the Nb-Al interfaces.