Stable fluoride based sputtering target for Tl-based cuprate superconducting thin film fabrication

•Stable Ba–Ca–Cu–O sputtering target for thallium based thin films fabrication.•Use of fluoride powders for sputtering target fabrication.•No need of fluorine reduction in sputtered Ba–Ca–Cu–O precursor films.•No degradation of superconducting properties.

Thin films of thallium based superconductors were prepared successfully using sputtering target fabricated as a mixture of barium fluoride, calcium fluoride and copper oxide. Such a target is stable, does not degrade and does not involve any special handling (vacuum or argon atmosphere storage). Precursor films sputtered from this target contained less than 5 at.% of fluorine so no fluorine content reduction is needed. Depending on the substrate, film thickness and synthesis conditions, Tl-2212 phase or a mixture of Tl-2212 and Tl-2223 phases was prepared. Although Tl-2212 phase has epitaxial character, Tl-2223 phase, growing on the top of Tl-2212, is only c-axis oriented. The highest values of TC0 were obtained using 300 nm thick precursor films thallinated at 860 °C. After the thallination the films contained a mixture of Tl-2212 and Tl-2223 phase with a critical temperature of TC0 = 106 K and a critical current density (at 77 K) up to 6 × 104 A/cm2. Lower TC0 and JC values suggest that further optimization of the film synthesis is needed; however, the first results show that the stable sputtering target prepared from fluorides is a good candidate for high quality Tl-based thin film fabrication.