NbN/AlN/NbN tunnel junctions applied as terahertz SIS mixers

We report on progress in the development of high-current-density all-NbN tunnel junctions for application as submillimeter wave SIS mixers. A very high current density up to 54 kA/cm(2), roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with a thin aluminum nitride(AIN) tunnel barrier. Even though the junctions have a very high current density, they showed high-quality junction characteristics with a large gap voltage, sharp quasipartical current rise, and small subgap leakage current. The junctions also exhibited good Josephson tunneling behavior, excellent terahertz response, and sensitive heterodyne mixing properties. NbN/AIN/NbN tunnel junctions were integrated with a NbN thin-film antenna to investigate the terahertz responses and the heterodyne mixing properties in a quasioptical mixer testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1 THz, and low-noise heterodyne mixing was demonstrated in the 300-GHz band.