Fabrication and Testing of Niobium Nitride/magnesium Oxide/niobium Nitride Tunnel Junctions for Use as High - Heterodyne Detectors

This thesis describes the development and testing of NbN/MgO/NbN tunnel junctions for use as super-conductor -insulator-superconductor (SIS) mixers. SIS mixers are the most-sensitive heterodyne detectors in the millimeter -wavelength region. NbN/MgO/NbN tunnel junctions are being developed as an alternative to currently used Pb-alloy tunnel junctions. It is intended that these devices be used as mixers at and below millimeter wavelengths. This thesis describes the fabrication process involved in making high critical current density small-area NbN tunnel junctions. Additionally, it describes the detailed measurements of both NbN-film properties, and tunnel-junction properties. Among the film properties measured are superconducting transition temperature, resistivity, critical current density and magnetic penetration depth. Among the junction properties measured are current-voltage characteristics (sum gap, normal resistance area product and subgap- to normal-resistance ratios) and the specific capacitance. Current-voltage characteristics were also measured as a function of temperature. Finally, mixer tests were performed at 205 GHz. These tests were encouraging with double-sideband mixer noise temperatures as low as 133 K. Finally, the future utility of NbN/MgO/NbN mixers at high frequencies is analyzed based on the measurements performed.