Development of Low Subgap Current Nb/Al STJ Detectors

We have developed a fabrication process for Nb-based STJ detectors which allows to fabricate junctions with subgap currents close to 20 pA at 300 mK, more than six orders of magnitude less than the subgap current at 4.2 K. The junctions are diamond-shaped devices, with areas between 20×20 and 100×100 µm2. We report the details of the fabrication process, together with an analysis of the grain structure of the films by AFM and XRD. We show the measured I - V curves at 300 mK and the temperature dependence of the subgap current between 300 and 1100 mK. We also report preliminary results on x-ray measurements at 300 mK with a 5 mC 55Fe source.