Effects of aluminum over-layer thickness on characteristics of niobium tunnel junctions fabricated by DC magnetron sputtering

We have fabricated Nb/AlO/sub x//Nb Josephson tunnel junctions using a sputtering apparatus with a load-lock system. The junctions that had 50 /spl mu/m/spl times/50 /spl mu/m area showed a V/sub m/ value (the product of the critical current and the subgap resistance at 2 mV) as high as 50 mV at a current density of 160 A/cm/sup 2/. Moreover, junctions having different thicknesses of the Al overlayer were concurrently fabricated on one wafer to study the dependence of the current-voltage characteristics on this Al over-layer. The I-V characteristics were also calculated by McMillan´s tunneling model and were compared with the measured I-V characteristics.<>