Thin films of the Tl-based high-temperature superconducting (HTS) phases are beginning to find application in a number of practical devices, especially in the field of microwave communication systems. This review describes the issues surrounding the fabrication of thin films of the thallium-based HTS materials. The influence of thermodynamics and kinetics, the various potential thalliation techniques and the selection of an appropriate substrate material are all important considerations when designing a process for the fabrication of thin films of these highly reactive materials. The properties of the resulting films are critically dependent on the microstructures developed during thalliation. Microstructural evolution during thalliation, the effect of structural defects on the properties of such films and the importance of achieving biaxial grain alignment in optimizing the properties of the films are all discussed.

thin films on (001) with excellent alignment suitable for microwave applications at 77 K have been fabricated using an ex situ anneal step in argon atmospheres at temperatures of . Surface resistance values as low as 400 (79 K, 10 GHz) and large-area critical current densities up to (77 K) have been achieved. In order to understand the relationship between the microstructure and electrical properties the films have been characterized by a variety of techniques, but especially transmission electron microscopy and allied methods. Microstructural features such as a-axis oriented grains, secondary phase particles, grain boundaries and surface outgrowths (especially of non-superconductor phases) have been found to have a significant effect on the surface resistance.

TI-Ba-Ca-Cu-0 thin films have been grown on a variety of substrates, both with and without the use of buffer layers. Differences in processing routes required for the various film-substrate combinations are highlighted and the effect of the buffering layers on the microstructure, transport and microwave properties of the films are discussed.

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