FABRICATION, STRUCTURE AND PROPERTIES OF TI-BASED HTS THIN FILMS

Thin films of the thallium-based high temperature superconductors are particularly suitable for passive microwave applications. The TI-2212, Tl-2223 and TI-1223 phases have critical temperatures in the range 100-120K; thus at the proposed operating temperatures of -80K, readily achievable with single-stage cryocoolers, their superconducting properties exhibit a slow variation with temperature, thus obviating the need for precise temperature control in maintaining device performance [1]. Device quality films of the above phases have been deposited on a variety of substrates, with and without buffer layers, by a number of groups and a range of practical microwave components have been successfully demonstrated [2]. Critical current densities at 77K and in zero field in excess ofGAni2, and surface resistance values, normalised to 80K and 10GHz, <0. Sm.Q have been achieved reproducibly in these films. At Oxford University a collaborative programme between the Departments of Materials and Engineering Science has, for the past six years, investigated the deposition of thalliumbased HTS thin films, studied the relations between deposition conditions, film structure, and superconducting properties, and has fabricated microwave devices from these films [35]. This paper describes the chemistry of the thallium compounds, the techniques used for thin films deposition and the relation between film structure and superconducting properties.