Fabrication of a tantalum-based Josephson junction for an X-ray detector

We have fabricated a tantalum-based Josephson junction for an X-ray detecto r. The tantalum layer was selected for the junction electrode because of it s long quasiparticle lifetime, large X-ray absorption efficiency and stabil ity against thermal cycling. We have developed a buffer layer to fabricate the tantalum layer with a body-centered cubic structure. Based on careful c onsideration of their superconductivity, we have selected a niobium thin la yer as the buffer layer for fabricating the tantalum base electrode, and a tungsten thin layer for the tantalum counter electrode. Fabricated Nb/AlOx- Al/Ta/Nb and Nb/Ta/WAlOx-Al/Ta/Nb Josephson junctions exhibited current-vol tage characteristics with a low subgap leakage current.