Improved fabrication method for Nb/Al/AIOx/Al/Nb superconducting tunnel junctions as X-ray detectors

Niobium-based superconducting tunnel junctions (STJs) with aluminum trapping layers as X-ray detectors were fabricated. We applied a fabrication technique for reducing the leakage current and improving the energy resolution for X-rays at around 6 keV. The edges of two Al layers sandwiching a tunneling barrier were oxidized by plasma discharge. The performance of these new STJs was compared with other types of STJs to which thermal oxidization in an O-2 atmosphere had been applied. The oxidization process in O-2 plasma significantly improved the energy resolution for X-rays, which reached values (FWHM) of 41 eV, 58 eV, 65 eV, and 129 eV for STJs with the size of 20 x 20 mu m(2), 100 x 100 mu m(2), 200 x 200 mu m(2), and 500 x 500 mu m(2) respectively.