Modification of Layer Structures of Superconducting Tunnel Junctions to Improve X-ray Energy Resolution

The layer structure of a Nb/Al-based superconducting tunnel junction (STJ) X-ray detector was modified to improve the energy resolution (\(\Delta E\)) of STJ X-ray detectors by suppressing the recombination of excited quasiparticles in the electrodes. A 100-pixel array of 100 \(\times \) 100 \(\upmu \)m STJs with a symmetric layer structure of Nb (300 nm)/Al-AlO\(_{x}\)(70 nm)/Al (70 nm)/Nb (300 nm) was fabricated. Fabrication yields of the 100 pixels were more than 90 %. The average leak current of the STJ array was 8 nA. The mean \(\Delta E\) of the STJ array was 6.7 \(\pm \) 1.0 eV for 400 eV X-rays. This \(\Delta E\) is the best value reported for Nb/Al STJs.