Superconductive thin films and devices : some developments and applications
This thesis begins by overviewing 'The Theoretical Base' of superconductivity and tunnelling. The development is attempted of a model by means of which the principles entering into the design of devices, and their resultant characteristics, can be understood. No attempt is made to present a full review of theory (ie aspects not relevant to some later practical point are omitted or are dealt with very briefly), and no new theoretical contributions of my own are presented. It is the objective of the first of three practical chapters, 'The Superconductive Tunnel Junction', to show how practical devices (in particular thin film tunnel junctions) can be produced whose current/voltage (IV) characteristics well approximate those obtained theoretically. After briefly reviewing superconductive devices in general the 'whole-wafer' technique of junction fabrication, now the basis of essentially all device fabrication within the group of which I am a part, is introduced and discussed. Practical thin film deposition and device fabrication are then detailed, after which the saga of the development of working junctions and reliable fabrication routes (which formed a major part of my own work) is related. IV characteristics typical of the reliable high quality devices now routinely available are reported. Finally some further results (stemming from the work of the group as a whole), expanding our understanding of the influences of wafer deposition conditions on device characteristics and quality, are presented, together with a discussion of some early special devices (series junctions and shunted junctions) that seem to be leading on to useful applications as development proceeds. It has long been known that superconductors can be used as the active elements in particle detectors. In collaboration with Dr Norman Booth and his colleagues at the Department of Nuclear Physics of the University of Oxford, 'first base' experiments to investigate, on a small scale, techniques of device fabrication that might be required in constructing large scale bulk detectors have been carried out; devices being fabricated by myself and tested collaboratively with the Oxford group. The initial stimulus for this work was the problem of the 'missing' solar neutrinos, which Booth and others believe might best be addressed via a superior experimental measurement performed using a solid state detector based on bulk indium. It is now believed that the general conclusions of the work and the techniques evolved in their finding ought to have general applicability to the high energy-resolution detection of a variety of forms of radiation, such as X- and gamma rays. Objectives were perceived to be the designing and building of reproducible, robust, high quality junctions, incorporating a 'quasiparticle trap' (a novel technique of signal concentration); the production of equivalent junctions in contact with a bulk single crystal of indium; and the investigation of the feasibility of detecting, via such a junction, a signal injected at a point in the bulk crystal. Chapter three, 'Devices for Particle Detection and The Quasiparticle Trap', explores the background to, and development of, work towards the attainment of these objectives, and, before going on to point out difficulties and suggest possible directions for future development, presents and discusses results obtained. One of the original objectives of my work was that once - or if - reliable routes to niobium based devices had been achieved, an attempt should be made to extend deposition and fabrication technologies to high critical temperature materials. Of course at the time this objective was established such materials as niobium germanium (Nb3Ge) and niobium nitride (NbN) were the ones in mind: alloys of the existing elemental superconductors. Work in this area had just begun (first films of niobium germanium having been made, using a composite target) when the news of the discovery by Bednorz and Muller of what turned out to be the first of a family (or family of families) of wholly new oxide superconductors, and all its consequences, burst upon the scene. Having had many years of experience with superconductive devices and also with the deposition of films of a variety of materials the group of which I am a part was well placed to attempt to produce films and devices using the new oxides. Chapter four begins with an outline of the Nobel prize winning work and discoveries of Bednorz and Muller, after which theoretical ideas that have emerged, and are emerging, to explain the behaviour of these materials, are very briefly outlined and discussed. The saga of the development of the first superconducting Y-Ba-Cu-O films is then related, in a semi-chronological manner; although, to avoid excessive un-clarity (...very much 'the way it was'(Missing data....) many facets of the deposition are discussed in the light of information only later becoming available. After this possible future courses for film development are outlined, most of which have been investigated to a certain extent, and their promise and problems discussed. A number of rudimentary device structures have been created using YBCO films, and the techniques of their fabrication, together with interpretations of the observed characteristics, are outlined. Finally future devices and applications are considered.