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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Niobium nitride devices for SIS mixers

Edward J. Cukauskas, William L. Carter, Marty Nisenoff

Published Dec 1, 1985• 0 citations
Publisher

Abstract

The superconductor-insulator-superconductor (SIS) tunnel junction is an attractive candidate for millimeter wave mixers offering conversion gain and quantum limited noise.1SIS mixers have been operated at 36 GHz with a single sideband noise temperature of 3.7K which is within a factor of 2 of quantum limited performance.2These results were obtained with devices fabricated from lead alloy technology. While this technology is very useful for the demonstration of quantum limited noise an all refractory junction technology would be desirable as it would have the advantages of being mechanically rugged, cycle well between room temperature and 4K be resistant to radiation damage and have enhanced high frequency response.

DOI
10.1109/irmm.1985.9126723

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•Oct 2, 1992•0 citations
The superconductor-insulator-superconductor (SIS) tunnel junction is an attractive candidate for millimeter wave mixers offering conversion gain and quantum limited noise.1SIS mixers have been operated at 36 GHz with a single sideband noise temperature of 3.7K which is within a factor of 2 of quantum limited performance.2These results were obtained with devices fabricated from lead alloy technology. While this technology is very useful for the demonstration of quantum limited noise an all refractory junction technology would be desirable as it would have the advantages of being mechanically rugged, cycle well between room temperature and 4K be resistant to radiation damage and have enhanced high frequency response.
The development of ultra-low-noise receivers at frequencies of >200 GHz is primarily driven by astronomical applications. The most sensitive millimeter-wave receivers that have presently been constructed make use of the strong mixing afforded by superconductor-insulator-superconductor (SIS) tunnel junctions. In a collaborative project between UKC and RAL, a 230 GHz laboratory receiver has been built using a lead alloy SIS junction as the mixing element, mounted in 4:1 reduced-height waveguide, terminated at one end with a circular cross-section, adjustable backshort. Best results indicate a double-sideband receiver noise temperature of T/sub R/(DSB) approximately=120 K, which represents state-of-the-art performance at this frequency. Initial difficulties in achieving good mixing performance prompted an investigation into the importance of the 'quality' of mixer block backshort tuning elements. An analysis of the mixer equivalent circuit is presented which allows for loss in a non-ideal mixer block, and 3-port mixer model calculations are discussed which incorporate this analysis into the Tucker quantum theory of mixing.
We have fabricated and tested submillimeter-wave superconductor–insulator–superconductor (SIS) mixers using very high-current-density Nb/AlN/Nb tunnel junctions (Jc≈30 kA cm−2). The junctions have low-resistance-area products (RNA≈5.6 Ω μm2), good subgap-to-normal resistance ratios Rsg/RN≈10, and good run-to-run reproducibility. From Fourier transform spectrometer measurements, we infer that ωRNC=1 at 270 GHz. This is a factor of 2.5 improvement over what is generally available with Nb/AlOx/Nb junctions suitable for low-noise mixers. The AlN-barrier junctions are indeed capable of low-noise operation: we measure an uncorrected double-sideband receiver noise temperature of TRX=110 K at 533 GHz for an unoptimized device. In addition to providing wider bandwidth operation at lower frequencies, the AlN-barrier junctions will considerably improve the performance of THz SIS mixers by reducing rf loss in the tuning circuits.
Single-particle (quasi-particle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is being used to make very-low-noise detectors and mixers for millimeter wavelengths. The nonlinearity of theI-Vcurve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon-assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasi-particle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limitT_{M} =\bar{h}\omega/k. Both of these predictions have been experimentally realized at 36 GHz using superconductor-insulator-superconductor (SIS) junctions. It appears probable that these quasi-particle detectors and mixers will supercede the corresponding Josephson-effect devices at millimeter wavelengths.
Within the past few years all-refractory superconductor-insulator-superconductor (SIS) tunnel junctions based on Nb/Al/AlO/sub x//Nb technology have displaced lead alloy devices as the preferred junctions for SIS mixers. There is growing interest in materials with higher superconducting energy gaps. NbN has a high gap ( approximately 3 meV) and higher critical temperature than Nb. Its main drawback, apart from the greater complexity of the material itself, stems from its short superconducting coherence length which results in a decrease in the measured gap voltage in proximity barrier structures, as generally used for Nb-based junctions. For this reason, most recent work has concentrated on the direct deposition of MgO as a tunnel barrier. The authors summarise work on Nb devices for MM-wave mixers and then report recent work performed at Cambridge on the fabrication of high quality NbN device structures.