Preparation and characterization of Tl2Ba2CaCu2O8 films on (100) LaAlO3

A two-step process for the fabrication of 0.12-1.20-μm-thick Tl ₂Ba₂CaCu₂O₈ films on (100) LaAlO₃ has been developed. The process involves the deposition of a BaCaCuO precursor film at low temperature by RF magnetron sputtering, followed by annealing in the presence of Tl₂ O vapor to convert the precursor film into Tl₂Ba₂ CaCu₂O₈. Over a distance of 4 cm, film composition was Tl:Ba:Ca:Cu=2.05±0.09:2.05±0.06:1.01±0.03:1.94 ±0.06, and thickness was uniform to within 6%. The films contain only trace quantities of secondary phases and are epitaxially oriented, with the c-axis of Tl₂Ba₂CaCu₂O₈ perpendicular to the surface and the two a-axes aligned with the pseudocubic a-axes of the underlying LaAlO₃ substrate. Films had a T_c for zero DC resistivity of 106.5±0.5 K, a transport critical current density for continuous current of 2.9×10⁶ A/cm² at 4.2 K and 2.7×10⁵ A/cm² at 77 K, and a sharp (<0.3°C) AC inductance transition with an onset temperature of 105±2 K. Surface resistance at 20 GHz of less than 2 mΩ, about 10× lower than Cu, was routinely achieved for temperatures below 90 K and, for the best films, at temperatures up to 100 K. Surface resistance at about 10 GHz on one film was 0.067 mΩ at 4.2 K, 0.36 mΩ at 77 K, and 0.74 mΩ at 90 K. Films have been reproducibly fabricated on substrates up to 2.54 cm²