SNS Junctions for AC Josephson Voltage Standards

We developed and fabricated series arrays of overdamped SNS Josephson junctions for AC Josephson voltage standards (S: superconductor, N: normal conductor). The junctions are based on NbxSi1-x barriers allowing nearly independent tuning of critical current density and characteristic voltage by varying niobium content and thickness of the barrier. We optimised the junction barrier deposition for the co-sputtered NbxSi1-x so that we are able to fabricate Josephson junction series arrays with up to 69,632 junctions for output voltages up to 10 V under 70 GHz microwave irradiation. The width of the constant-voltage steps at 10 V exceeds 1 mA for a typical critical current of about 3 mA.