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IEEE Transactions on Magnetics

Refractory material SIS junction structures

Victor Y. Belitsky, V. Gubankov, V. P. Koshelets, G. A. Ovsyannikov, I. L. Serpuchenko, S. V. Shitov, Michael A. Tarasov, Alexander N. Vystavkin

Published Mar 1, 1987• 6 citations
Publisher

Abstract

Niobium based SIS junction structures for microwave receiving devices were investigated. The possibility of the mixing efficiency improving for Nb-Al2O3-Nb junctions with a negative resistance on I-V curve near the gap voltage has been demonstrated. Comparative microwave investigation of the parallel and series DC biased arrays shows the advantage of the first one. SIS junction capacitance has been compensated with microstrip or lumped elements at the signal frequency. Such compensated structures were investigated at frequencies 38 and 76 GHz both in waveguide and quasioptic systems.

DOI
10.1109/tmag.1987.1065119

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Niobium based SIS junction structures for microwave receiving devices were investigated. The possibility of the mixing efficiency improving for Nb-Al2O3-Nb junctions with a negative resistance on I-V curve near the gap voltage has been demonstrated. Comparative microwave investigation of the parallel and series DC biased arrays shows the advantage of the first one. SIS junction capacitance has been compensated with microstrip or lumped elements at the signal frequency. Such compensated structures were investigated at frequencies 38 and 76 GHz both in waveguide and quasioptic systems.
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The possibility of overcoming the main drawbacks of the traditional series arrays by parallel connection of the SIS (superconductor-insulator-superconductor) junctions for DC and IF are demonstrated experimentally. The proposed mixing elements avoid the influence of the junction nonuniformity in the array, provide the optimal matching on both RF input and IF output, and tune out junction capacitance. Experimentally tested arrays contained up to eight high-quality Nb-AlO/sub x/-Nb junctions with R/sub n/=20-70 Omega at junction area 8/14 mu m/sup 2/. The best values of T/sub r/ (SSB) were measured to as low as 5 K on the mixer cold input flange in the 45-55-GHz bandwidth at an ambient temperature of 4.2 K. A mixer gain of up to 10 dB was obtained. The gain values were limited by bias stability at very high or even negative array output impedance. An experimental receiver noise temperature (SSB) of less than 20 K has been achieved.
Parallel connected for dc and IF, but in series for rf current SIS junction arrays are under investigation since 1982. Novel conception of the improved matching for both input and output mixer ports have been developed and confirmed successfully with experimental receivers in 40-180 GHz frequency range. Parallel and parallel-series combinations integrated up to 11 Nb-AlOx-Nb type SIS junctions have been tested in full height waveguide mixers. The receivers' noise temperatures Tr(DSB) I50K have been achieved in the laboratory tests across frequency range 40-150GHz. The Tr as low as 35K have been carefully measured at 145GHz. The parallel biased SIS arrays for integrated on a chip SIS receiver have been designed and tested successfully. The integrated receiver noise temperature as low as 85K was achieved at 140GHz. New SIS array design for mixer in frequency range 500-1000 GHz is presented.
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At the Smithsonian Astrophysical Observatory, we have been developing expertise in the construction of high-frequency heterodyne receivers using SIS junctions as their first mixing elements. This research has been carried out in support of the Submillimeter Array (SMA), a radio interferometer soon to be built on Mauna Kea that will require multiple reciever systems at a number of bands between 177 GHz and 920 GHz. In this paper, we describe research on the fabrication of all-refractory SIS junctions. To be useful as high-frequency mixers, SIS junctions must have small area and high critical current density. At the lower SMA frequencies, Nb/Al-Oxide/Nb ahs proven to be an ideal material system, and optical photolithography has sufficient resolution for junction definition. At the higher frequencies, NbN/MgO/NbN may enjoy an advantage due to its larger energy gap, and electron-beam lithography may be necessary for defining junctions much smaller than one micron in size. We report good initial results for Nb/Al-Oxide/Nb junctions, fabricated using a self-aligned optical technique, with area $AP 0.5 micrometers 2 and critical current density $AP 10,000 Amp/cm2.