PROXIMITY EFFECT IN Nb/Al,AlOxide,Al/Nb JOSEPHSON TUNNEL JUNCTIONS

Regions with reduced energy gap induced by the proximity effect give rise to quasi-particle loss in Josephson junction X-ray detectors, but may also be used advantageously for quasi-particle collection. The influence of the thickness of the Al proximity layers in Nb/Al-1,Alox,Al-2/Nb Josephson tunnel junctions on the electrical characteristics has been investigated theoretically and experimentally. Theoretically it is found that the strength of the proximity effect is mainly determined by the proximity parameters gamma(M1) (gamma(M2)) of the electrodes. Good fits of the measured IV -curves with theory were obtained for junctions with thicknesses d(A11) ranging from 4 to 25 nm and d(A12)=3nm, with gamma(M2 approximate to)0.12 and gamma(M1)/gamma(M2)=d(A11)/d(A12) For all junctions the proximity knee remains more pronounced than predicted. The measured I-c(T) and V-g (T) curves could be fitted, taking Delta(0S)/k(B)TcS approximate to 1.92 for Nb.