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

Citation

Stern, Jeffrey Aaron (1991) Fabrication and testing of NbN/MgO/NbN tunnel junctions for use as high-frequency heterodyne detectors. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/AZ4Q-DF35. https://resolver.caltech.edu/CaltechETD:etd-12202004-091326

Abstract

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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