Thallium based high temperature superconductors for microwave device applications

Abstract

The first large scale commercial application of high temperature superconducting (HTS) thin films has been in the area of passive microwave devices. Thallium based superconductors are particularly attractive for these applications because they have high transition temperatures (up to 127 K) and are relatively stable to atmospheric attack. Tl₂Ba₂CaCu₂O₈ films with high critical current density and low surface resistance can be fabricated by growing biaxially aligned films epitaxially on single crystal substrates. However, the processing of Tl based HTS thin films is particularly difficult due to the complexity of the crystal structure and the volatility and toxicity of thallium. Most Tl HTS films are fabricated using a two step process involving the deposition of an amorphous precursor followed by an ex situ thalliation anneal. The relative stability of the phases in the TBCCO system depends on the partial pressure of oxygen and thallous oxide, the processing temperature and the initial stoichiometry of the precursor. Unfortunately, it has not been possible to grow 'epitaxial' Tl HTS films directly on substrates such as MgO and sapphire which have dielectric properties almost ideal for high frequency applications, because the lattice mismatch is too great and/or chemical reaction occurs. Buffer layers can be deposited on MgO or sapphire substrates to reduce the lattice mismatch and/or prevent chemical interaction with the HTS film.