Ta/W/AlO(x)-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 AlO(x)-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 quasi-particles 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 AlO(x) barrier. A fabricated junction shows a gap voltage of 1.3 mV, a sharp transition width of less than 50 micro-V, and a small subgap leakage current of 1.5 micro-A. (Author)