Fabrication of Nb/Al Superconducting Tunnel Junction Using Ozone Gas.

An ozone (O $$_{3})$$ oxidation process was introduced for Nb/Al-based superconducting tunnel junctions (STJs) in order to form defect-free tunnel barriers at high critical current and to improve the energy resolution ( $$\Delta E$$ ) for X-rays. The dependence of critical current ( $$J_\mathrm{C})$$ and leak current ( $$I_\mathrm{leak})$$ on the O $$_{3}$$ exposure was measured to optimize the oxidation condition. The 50-square- $$\upmu $$ m STJs produced by the O $$_{3}$$ oxidation process exhibited an extremely small $$I_\mathrm{leak}$$ of less than 50 pA. As expected, the lower or shorter the O $$_{3}$$ exposure, the higher $$J_\mathrm{C}$$ and the smaller the normal resistance ( $$R_\mathrm{N})$$ . However, the maximum $$J_\mathrm{C}$$ was 8 A/cm $$^{2}$$ at an O $$_{3}$$ exposure of 0.72 Pa min, which is much smaller than those of STJs with the conventional O $$_{2}$$ oxidation process. It is expected that the high $$J_\mathrm{C}$$ of 1,000 A/cm $$^{2}$$ , at which a 9-eV-energy resolution for 277 eV photons is predicted, can be reached by an O $$_{3}$$ exposure of 3.5 $$\times $$ 10 $$^{-4}$$ Pa min. [ABSTRACT FROM AUTHOR]