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 junctio ns for a programmable voltage standard. The Al barrier is 5-120 nm thick. I ts properties are quite different with respect to the single Al films becau se of diffusion between Al and the Nb counter-electrode. The resistivity of the Al barrier ranges from hundreds of microhm centimetres to a few millio hm centimetres, probably depending on the surface roughness of the Al layer , measured by atomic force microscopy (AFM). The junctions, with areas betw een 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 ICR N 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 d iffraction were also carried out to relate the electrical properties of the junctions to the Nb-Al interfaces.