Improved Fabrication Method for Nb/Al/AlO_x/Al/Nb Superconducting Tunnel Junctions as X-Ray Detectors

Abstract

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 20×20 µm 2 , 100×100 µm 2 , 200×200 µm 2 , and 500×500 µm 2 , respectively.