ULTRA-HIGH QUALITY NB/ALOX/NB TUNNEL-JUNCTIONS WITH EPITAXIAL BASE LAYERS

Using an ultra-high vacuum DC sputtering system we have fabricated Nb/AlO(x)/Nb tunnel junction devices with epitaxial Nb base layers. We have been investigating the improvements in device quality which can be achieved by heating the substrates during the growth of the tunnel barrier. By measuring the subgap characteristic under magnetic field at temperatures down to 0.4K, we show that for V < DELTA-Nb the currents in devices with critical current densities in the range 10(5)-10(6)Am-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.