High current density NbN/AlN/NbN tunnel junctions for submillimeter wave SIS mixers

We report on the fabrication and properties of high current density NbN/AlN/NbN tunnel junctions for the application of superconducting SIS mixers in the submillimeter wave regions. Junctions having current densities as high as 50 kA/cm/sup 2/ exhibited a good Josephson tunneling behaviour, excellent terahertz response, and sensitive heterodyne mixing properties. Accurate values of junction capacitance were estimated by measuring dc-SQUID resonant steps for the design of tuning circuits. The measured specific capacitance gives values in the range of 20-200 fF//spl mu/m/sup 2/ for the current density range of 1-100 kA/cm/sup 2/. The NbN/AlN/NbN tunnel junctions were integrated with a NbN thin film antenna to investigate 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 heterodyne mixing was demonstrated in the 300-GHz band.