Very low surface resistance Tl2Ba2CaCu2O8 films of thickness 1.0 μm on (100) LaAlO3 were fabricated and characterized. The films are highly c-axis oriented (x-ray diffraction rocking curve full width half maximum of 0.68°) with good in-plane epitaxy with respect to the substrate. The Tc of the films was 107.6±0.5 K as measured by ac magnetic susceptibility. The surface resistance at 10 GHz was measured with the parallel plate resonator technique and found to be 23±5 μΩ at 4.2 K, 130±20 μΩ at 77 K, and 300±60 μΩ at 95 K. Using a 20 GHz TE011 end-wall replacement copper cavity, the surface resistance was found to be comparable to that of copper, about 18 mΩ, at 104±1 K. The surface resistance of the films is significantly lower than that of all films reported to date at temperatures above 70 K and only slightly higher than the best high-temperature superconductor films reported to date at 4.2 K.

A process has been developed for the fabrication of nearly single phase superconducting Tl2Ba2Ca2Cu3O10 thin films on (100) LaAlO3 substrates with a superconducting transition temperature Tc of 120 K and low microwave surface resistance at temperatures up to 110 K. Amorphous BaCaCuO precursor films were first deposited by rf magnetron sputtering and then thallinated at elevated temperatures. The double TlO layer phases (Tl2Ba2Ca2Cu3O10 + Tl2Ba2CaCu2O8) formed preferentially over the single TlO layer phases (TlBa2Ca2Cu3O9 + TlBa2CaCu2O7) at high Tl2O partial pressures. Thin films containing Tl2Ba2Ca2Cu3O10 and a small amount of CuO were prepared from Cu-rich precursor films (Cu/Ba > 1.7), while lower Cu content led to the formation of Tl2Ba2CaCu2O8 as a secondary phase. Tl2Ba2Ca2Cu3O10 film epitaxy was enhanced by carrying out the thallination in reduced oxygen partial pressures of 0.01–0.05 atm. Following the thallination step, the Tl2Ba2Ca2Cu3O10 thin films had a superconducting transition temperature Tc of only 106 ± 4 K. An additional 62 h anneal at 800 °C or an 8 h anneal at 850 °C in a Tl2O/O2 atmosphere increased the Tc to 120 K. The increase in Tc was accompanied by a decrease in the c-axis lattice constant, an enhancement in the long-range order in the c-direction, and the formation of a small amount of Tl2Ba2CaCu2O8 as a secondary phase. Minimization of surface resistance at high temperature (95–110 K) requires that the fraction of Tl2Ba2CaCu2O8 secondary phase in the films be kept low. Process routes are also described for the formation of nearly single phase TlBa2Ca2Cu3O9 and TlBa2CaCu2O7 thin films and the formation of a new ordered intergrowth phase, Tl4Ba4Ca3Cu5O18, which consists of alternating Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10 layers.

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