Fabrication and characterization of high-quality all-NbN Josephson tunnel junctions for superconductive quantum interference devices

We have developed high-quality NbN/AlN/NbN Josephson tunnel junctions for direct current-superconducting quantum interference devices (DC-SQUIDs). The junctions were fabricated with epitaxial NbN/AlN/NbN trilayers on single crystal MgO (100) substrates and showed excellent tunneling properties with a quality factor Rsg/Rn above 20, a large gap voltage about 5.7 mV, and a large IcRn product above 3.8 mV. The critical current density Jc of the NbN/AlN/NbN junctions is controlled by the AlN barrier thickness dAlN in the junctions. The performance of DC-SQUIDs comprising NbN junctions was measured at 4.2 K, and the white noise in a flux-locked-loop mode was 5 µΦ0/Hz1/2, thus demonstrating the feasibility of all-NbN junctions for SQUID development.