Ultra-high quality Nb/AlO/sub x//Nb tunnel junctions with epitaxial base layers

Using an ultra-high-vacuum DC sputtering system, Nb/AlO/sub x//Nb tunnel junction devices with epitaxial Nb base layers have been fabricated. Improvements in device quality that can be achieved by heating the substrates during the growth of the tunnel barrier are investigated. By measuring the subgap characteristic under magnetic field at temperatures down to 0.4 K, it is shown that for V< Delta /sub Nb/ the currents in devices with critical current densities in the range of 10/sup 5/-10/sup 6/ AM/sup -2/ follow closely the BCS prediction and show no extrinsic leakage current. The divergence of the curve for higher current densities and at higher voltages is discussed.