GAP VOLTAGE OF NB-AL ALOX-NB TUNNEL-JUNCTIONS

The gap voltage of Nb-Al/AlOx-Nb junctions was studied as a function of the Nb electrode thickness. The gap parameter increased with the electrode thickness. The I(V) curves of these junctions, employed as quasiparticle frequency mixers, are discussed. For 120-nm-thick Nb electrodes, a gap voltage 2DELTA/e of 2.95 mV at 3 K is found in junctions with a critical current density of 2.5 kA/cm2. This is close to the ideal bulk value of Nb. The corresponding gap frequency of 714 GHz is clearly above 691 GHz, an important radioastronomical frequency. Complementary T(c) measurements of Nb films have been carried out. The results agree with a model based on the proximity effect between the ideal metallic layer and a nonideal surface layer.