High quality Nb/Al-AlOx/Nb Josephson junction

We have improved the quality of Nb/Al-AlOx/Nb Josephson junctions and obtained the most excellent junction characteristics ever obtained for all refractory Josephson junctions. The Vm values (the product of the critical current and the subgap resistance measured at 2 mV) are 88 mV at the critical current density Ij =500 A/cm2 and 72 mV at Ij =1000 A/cm2. The Vm values are larger than 40 mV up to Ij of 2400 A/cm2. The high Vm values are important to obtain the low subgap leakage current and not to reduce the transfer current to the load in Josephson circuit. These characteristics have been achieved by improving the quality of the Nb film and optimizing the Al thickness. We have also confirmed that Ij can be controlled by both the oxidation pressure and time in the range of 40–4600 A/cm2.