The synthesis of evaporated thallium-based superconducting thin films using an improved thalliation process

Abstract

Superconducting Tl-Ca-Ba-Cu-O (TCBCO) thin films, about 1μm thick, have been prepared on (100) MgO substrates using a thermal evaporation and improved two-step annealing process involving final thalliation under controlled Tl₂O vapour pressure in a quartz reaction tube. The Tl₂O vapour pressure was changed by adjusting the position of a Tl₂O₃-containing boat, thus altering the boat temperature from 740–780°C when the furnace temperatures were between 830 and 850°C. The superconducting properties of TCBCO films are strongly dependent on the Tl₂O vapour pressure and oxygen partial pressure. Thalliated Tl₂Ca₁Ba₂Cu₂O_x and Tl₂Ca₂Ba₂Cu₃O_x films with T _{c o}'s of up to 110 and 117 K, respectively, and J _{c, 77 K}'s measured over large areas of up to 1×10⁴ Acm⁻², have been obtained. A low-temperature annealing process after thalliation does not seem to offer significant advantages for these films, which is thought to be due to the loss of thallium. It is also found that measuring J _c values on narrow microbridge structures can give a false impression of the overall quality of polycrystalline thin-film samples.