Characteristics of Nb/Al superconducting tunnel junctions fabricated using ozone gas

To improve the energy resolution (ΔE) of Nb/Al superconducting tunnel junctions (STJs), an ozone (O₃) oxidation process has been developed to fabricate a thin defect-free tunnel barrier that simultaneously shows high critical current J_C > 1000 A/cm² and high normalized dynamic resistance R_DA > 100 MΩ · μm², where A is the size of the STJ. The 50-μm² STJs produced by O₃ exposure of 0.26 Pa· min with an indirect spray of O₃ gas, which is a much lower level of exposure than the O₂ exposure used in a conventional O₂ oxidation process, exhibit a maximum J_C = 800 A/cm² and a high R_DA = 372 MΩ · μm². The 100-pixel array of the 100-μm² STJs produced using the same O₃ oxidation conditions exhibits a constant leak current I_leak = 14.9 ± 3.2 nA at a bias point around Δ /e (where e is half the energy gap of an STJ), and a high fabrication yield of 87%. Although the I_leak values are slightly larger than those of STJs produced using the conventional O₂ oxidation process, the STJ produced using O₃ oxidation shows a ΔE = 10 eV for the C-Kα line, which is the best value of our Nb/Al STJ x-ray detectors. (paper)