Final Technical Report

SIS Mixer Research

Arthur W. Lichtenberger, M. J. Feldman

Published Nov 1, 1990• 0 citations

Abstract

Abstract : Theoretical and experimental research has been conducted to elucidate the basic physics behind the properties of superconductor-insulator-superconductor (SIS) tunnel junction receiving devices. The properties of an SIS mixer using a slightly nonideal junction, with finite LO power, were determined by analytic expansion of the equations of the quantum theory of mixing. The resulting equations have a particularly simple form. The minimum noise temperature is controlled by the leakage current of the junction. Even the most nearly ideal junctions made today require a considerable LO for best sensitivity; nevertheless, even a comparatively large leakage current allows mixer noise to be only a small factor above the quantum limit. The saturation properties of SIS mixers subjected to broad-band thermal noise obey the equations derived for monochromatic saturating signals. The Josephson junction tuning inductor was analyzed with the conclusion that such a device is not feasible with present fabrication techniques. Computer calculations using synthetic SIS junction I-V characteristics predicted the performance of an optimized receiver over its entire operating frequency range. Many aspects of the operation of SIS mixers were clarified and unexpected new phenomena were predicted. A 'photon picture' interpretation of the quantum theory of mixing is in the early stages of development. Niobium nitride edge junctions with excellent current-voltage characteristics were fabricated using novel barrier formation processes. The role of surface damage in the quality of NbN edge junction electrical characteristics was investigated and unexpected results were obtained.

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SIS mixer research. Final technical report, 1 Feb 89-13 May 90

A. W. Lichtenberger, M.J. Feldman

Theoretica and experimental research has been conducted to elucidate the basic physics behind the properties of superconductor-insulator-superconductor (SIS) tunnel junction receiving devices. The properties of an SIS mixer using a slightly nonideal junction, with finite LO power, were determined by analytic expansion of the equations of the quantum theory of mixing. The resulting equations have a particularly simple form. The minimum noise temperature is controlled by the leakage current of the junction. Even the most nearly ideal junctions made today require a considerable LO for best sensitivity; nevertheless, even a comparatively large leakage current allows mixer noise to be only a small factor above the quantum limit. The saturation properties of SIS mixers subjected to broad-band thermal noise obey the equations derived for monochromatic saturating signals. The Josephson junction tuning inductor was analyzed with the conclusion that such a device is not feasible with present fabrication techniques. Computer calculations using synthetic SIS junction I-V characteristics predicted the performance of an optimized receiver over its entire operating frequency range. Many aspects of the operation of SIS mixers were clarified and unexpected new phenomena were predicted. A 'photon picture' interpretation of the quantum theory of mixing is in the early stages ofmore » development. Niobium nitride edge junctions with excellent current-voltage characteristics were fabricated using novel barrier formation processes. The role of surface damage in the quality of NbN edge junction electrical characteristics was investigated and unexpected results were obtained.« less

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SIS (Superconductor-Insulator-Superconductor) mixer research

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Theoretical and experimental research has been conducted to elucidate the basic physics behind the properties of superconductor-insulator-superconductor (SIS) tunnel junction receiving devices. The quantum theory of mixing was employedto calculate the saturation behavior of the SIS mixer and the SIS direct detector. The direct detector was found to saturate at far higher powers than previously believed, allowing the possibility of practical application of this device. Experiments verified the saturation theory. The mixer saturation expression led to the formulation of frequency scaling regions. The origin of quantum noise sources are completely minimized. The mixer quantum noise limit was shown to be reached in only two special cases. Computer calculations determined that the behavior of SIS receivers divides into two regimes, low and high frequencies, the cross-over frequency depending upon junction quality. The properties of these two regimes were delineated. All previous SIS receivers have operated in the low frequency regime. Plasma-etched niobium nitride edge junctions have been fabricated using a novel barrier formation process. These junctions have excellent current-voltage characteristics, but their precise physical nature has not yet been determined. (Author)

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Theoretical research has been conducted to elucidate the basic physics behind the properties of superconductor-insulator-superconductor (SIS) tunnel junction receiving devices. The properties of SIS mixers using nonideal junctions and finite LO power, were determined by analytic expansion of the equations of the quantum theory of mixing, and also by computer simulations of SIS receivers over the entire range of experimental parameters. The result is a new coherent and intuitive picture of SIS mixer behavior. Many of the outstanding mysteries and questions about SIS receivers are resolved, and this contributes greatly to the design and interpretation of SIS mixer experiments. Other calculations show how to achieve sub-quantum noise temperatures in the phase sensitive SIS mixer, an important step towards realization of ultra-low- noise detectors. A simplified model casts doubt on the superlative experimental results reported for the rf-series dc-parallel biased array SIS mixer. Superconductivity; SIS mixer; Submillimeter-wave detection.

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Fabrication and testing of NbN/MgO/NbN tunnel junctions for use as high-frequency heterodyne detectors

Jeffrey A. Stern

This thesis describes the development and testing of NbN/MgO/NbN tunnel junctions for use as superconductor-insulator-superconductor (SIS) mixers. SIS mixers are the most sensitive heterodyne detectors in the millimeter wavelength region. Most SIS mixers use Pb alloy tunnel junction. These tunnel junctions have several problems associated with the soft nature of Pb and its low superconducting transition temperature. NbN-based tunnel junctions are being developed to overcome these difficulties. These devices are intended to be used as mixers at millimeter and submillimeter wavelengths. This thesis describes the fabrication process involved in making NbN junctions, and the results of measurements on these devices. The purpose of these measurements is to determine the future possibilities of NbN tunnel junctions as high-frequency mixers. The first chapter is an introduction to SIS mixers and explains how tunnel junction properties affect mixer performance. The basic theory of tunneling and mixing in SIS mixers is first presented. A description of quantum mixer theory is included in this presentation. This theory makes several interesting predictions that cannot be explained using classical theories. This is followed by a description of how real SIS tunnel junctions differ from ideal junctions. The physical origin of these differences is discussed, along with how they affect SIS mixer performance. Finally, the advantages and disadvantages of the various superconducting materials available are discussed. The decision to develop NbN devices is based on these material properties. The second chapter describes the methods used to fabricate small-area NbN/MgO/NbN tunnel junctions. The chapter begins with a description of the various methods that have been used to deposit NbN films. Reactive magnetron sputtering is chosen as the best method for tunnel junction fabrication. Details on the vacuum systems and the methods used for depositing superconducting NbN are discussed. Next, the process used for depositing junction trilayers (NbN/MgO/NbN) is described. The probable growth mode of MgO on NbN is presented. The importance of this growth mode to the junction quality is explained in some detail. Next, standard junction processing steps are reported. The details and limitations of each step are put forth. The standard process allows for the fabrication of 1 [square micron] tunnel junctions. Finally, this chapter discusses several methods of fabaricating submicron junctions that are being pursued. The status of this work is given. The third chapter describes the characterization of NbN films and NbN/Mg0/NbN tunnel junctions. Film properties are described first. The correlation of these properties to deposition conditions is discussed in some detail. Next, values for the various features of the I-V characteristic are given; typical and exceptional values are noted. How these features limit mixer results is described in detail. Several important device attributes were measured using superconducting-quantum-interference-devices (SQUIDs). These attributes are the junction's specific capacitance and the film's magnetic penetration depth. The theory and results of measuring specific capacitance and penetration depth are presented. Following the SQUID results is a large section on RP testing. Mixer tests were made at 205 GHz. The receiver design used, integral inductive tuning circuit used and results are discussed. These results are well understood, with the exception of the temperature dependence of the mixer performance. Finally, measurements of the uniformity of many junctions on a single wafer are presented. The importance of junction uniformity is also described. The final chapter discusses the ultimate limits on NbN mixers, and tells what future work must be done to achieve these limits. The primary high-frequency limit on NbN junctions is the capacitance of NbN/MgO/NbN junctions. The limit imposed by the junction capacitance and circuits used to tune out this capacitance are discussed. Also, the low-frequency limits on NbN junctions are discussed. The status of submicron devices is presented. Junction area is the most immediate limitation on NbN mixer results. The possibility of using other barrier materials to increase the RC speed of NbN junctions is presented. Finally, the possibility of operating NbN junctions at temperatures above 4.2 K is discussed.

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It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network.< >

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