TA/W/ALOX-AL/TA/NB JOSEPHSON-JUNCTIONS FOR X-RAY-DETECTOR

We have fabricated a Josephson junction, with an approximately symmetrical gap structure which consists of a Ta/W counter electrode, a Ta/Nb base electrode and an AlOx-Al barrier for operation as a superconducting X-ray detector at 0.3 K. A Ta layer is selected due to its long lifetime of quasiparticles and high X-ray absorption efficiency. The W layer is selected as a buffer layer for a body centered cubic Ta counter electrode, and the Nb layer is chosen for a body centered cubic Ta base electrode The Al layer acts as an overlayer for fabricating the AlOx barrier. A fabricated junction shows a gap voltage of 1.3 mV, a sharp transition width of less than 50 mu V, and a small subgap leakage current of 1.5 mu A.