Effect of the critical current density and the junction size on the leakage current of Nb/Al-AlOx/Nb superconducting tunnel junctions for radiation detection

Nb/Al-AlOx/Nb superconducting tunnel junctions (STJ's) designed for X-ray detection have been fabricated. The behavior of the low-temperature subgap leakage current, which severely Limits the energy resolution obtained in such devices, is investigated. From trends in the dependence of the leakage currents on the critical current density and the size of the STJ, as well as from the low-temperature current-voltage characteristics, and an analysis of the base electrode surface morphology, it is concluded that physical defects in the barrier region are the most probable cause of the leakage currents. Suggestions are given for optimization of the device processing.