FABRICATION OF ALL-NBN JOSEPHSON TUNNEL-JUNCTIONS USING SINGLE-CRYSTAL NBN FILMS FOR THE BASE ELECTRODES

All-NbN Josephson tunnel junctions with sputter-deposited magnesium oxide barriers have been fabricated using single crystal NbN films for the base electrodes. Fabricated Josephson junctions have shown good tunneling characteristics with large gap voltages(5.6-5.8 mV), narrow gap widths(0.1-0.2 mV, from 30 to 70%), and small subgap leakage currents(V(m) = 20-30 mV, measured at 3 mV). The result of a measurement of a subgap structure for a fabricated junction suggested that the excess leakage currents of fabricated junctions are due to multi-particle tunneling through locally thin areas in the MgO barriers.