Fabrication and Testing of Superconducting Tunnel Junctions for AN SIS Mixer Operating at 265-280 GHZ.

This dissertation describes fabrication processes that were developed to make two types of Josephson tunnel junctions; lead-indium-gold alloy junctions and niobium -aluminum-oxide-niobium junctions. The processes were designed in particular to produce the low capacitance, small area junctions required for use in a superconductor-insulator -superconductor (SIS) mixer operating at high frequencies (265-280 GHz). The SIS mixer is part of a low-noise heterodyne receiver which is intended for use in millimeter wavelength spectroscopic studies of trace gases in the earth's stratosphere. High quality lead alloy junctions have successfully been fabricated with the low capacitance and sharp I-V characteristics (V_{m} = 25 mV at 4.2 K) required for use in a SIS mixer at high frequencies. These junctions were ultimately found to be unsuitable for use in a practical receiver, however, because of electrical characteristics which deteriorated over time. Niobium tunnel junctions with areas as small as 0.25 mum^2 have been fabricated with a quality comparable to the best niobium devices reported in the literature, with V _{m} > 1 volt at 2.1 K. These devices have proven very rugged over time, with no observed deterioration of quality. A series pair array of niobium junctions has been tested in a SIS receiver built at Stony Brook. The construction of the receiver is described, along with experiments undertaken to optimize its performance. The best result attained to date is a single sideband (SSB) receiver noise temperature of 258 K at 278 GHz. This result is within a factor of 1.5 of the lowest noise temperature achieved at this frequency in the laboratory. It is at least a factor of two lower than the best noise temperature of a receiver currently in use in the field for millimeter wavelength stratospheric observations.