Fabrication of high quality Nb/AlO_x/Nb superconducting tunnel junctions for radiation detection

A fabrication of high quality Nb/AlO_x/Nb superconducting tunnel junctions (STJs) for radiation detection is presented. The fabrication process includes a MgO thin film, an underlayer method, a control of critical current, a nitrogen plasma passivation and a SiO₂ insulator. Subgap current characteristics of STJs have been investigated and compared with conventional STJs in the temperature range of 0.35K to 4.2K in order to evaluate the junction quality for radiation detection. The STJs made by present process showed extremely high quality junction characteristics of a subgap current density of 0.1pA/μm, a resistance ratio R_d/R_n of 6 × 10⁶ for 200 × 200μm² STJ with a critical current density of 200A/cm².