HIGH-CURRENT DENSITY NBN ALN/NBN TUNNEL-JUNCTIONS FOR SUBMILLIMETER-WAVE SIS MIXERS/

We report on the fabrication and properties of high current density Nb N/AlN/NbN tunnel junctions for the application of superconducting SIS mixers in the submillimeter wave regions. Junctions having current den sities as high as 50 kA/cm(2) exhibited a good Josephson tunneling beh avior, excellent terahertz response, and sensitive heterodyne mixing p roperties. Accurate values of junction capacitance were estimated by m easuring dc-SQUID resonant steps for the design of tuning circuits. Th e measured specific capacitance gives values in the range of 20-200 fF /mu m(2) for the current density range of 1-100 kA/cm(2). The NbN/AlN/ NbN tunnel junctions were integrated with a NbN thin film antenna to i nvestigate the terahertz responses and mixing tests in a quasi-optical testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1.02 THz, and a low-noise het erodyne mixing was demonstrated in the 300-GHz band.