Superconducting properties of magnetron sputtered high Tc thin films containing oxide compounds of yttrium, bismuth, or thallium

The authors have used multiple-source magnetron sputtering to prepare thin films of Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and Tl-Ca-Ba-Cu-O on (100) SrTiO₃, (100) MgO, and ZrO₂-9%Y₂O ₃ substrates. Y-Ba-Cu-O films grow best on SrTiO₃ with mostly an a-axis orientation. Stoichiometry, particularly the Ba/Cu ratio, must be within 2% of the correct value to obtain narrow Δ T_c transitions. Conversely, the 80 K phase of Bi-Ca-Sr-Cu-O films grows best on MgO substrates and has a predominant c-axis orientation. The requirements on composition are less stringent; however, the annealing temperature must be held within a narrow range around 865°C to obtain the best films. The best films of Tl-Ca-Ba-Cu-O compounds are grown on (100) oriented and polycrystalline ZrO₂ substrates. The highest zero-resistance transition temperature is about 114 K in the Tl₂Ba₂Ca₂ Cu₃O_x phase and 100-105 K in Tl₂Ba₂Ca₁Cu₂O_x and Tl₁Ba₂Ca₂Cu₃O_x phase samples. The upper critical-field measurements show high anisotropies in the critical-field slopes. A comparison is made among preparation conditions and superconducting properties, including T _c, ΔT_c, and dB_c2 /dT (parallel and perpendicular to film surface) of all three compounds