Nb and NbN junction development for MM-wave mixers

Within the past few years all-refractory superconductor-insulator-superconductor (SIS) tunnel junctions based on Nb/Al/AlO_x/Nb technology have displaced lead alloy devices as the preferred junctions for SIS mixers. There is growing interest in materials with higher superconducting energy gaps. NbN has a high gap (~3 meV) and higher critical temperature than Nb. Its main drawback, apart from the greater complexity of the material itself, stems from its short superconducting coherence length which results in a decrease in the measured gap voltage in proximity barrier structures, as generally used for Nb-based junctions. For this reason, most recent work has concentrated on the direct deposition of MgO as a tunnel barrier. The authors summarise work on Nb devices for MM-wave mixers and then report recent work performed at Cambridge on the fabrication of high quality NbN device structures