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.