Growth kinetics of Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y thin films

Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y (Cu_1−xTl_x-1234) thin films have been found to be very attractive candidates in the cuprate family due to their low superconductor anisotropy, long coherence length and, consequently, high J_c. The method of preparation has been reported previously, however the kinetics of their formation has not yet been studied. In this paper, we report on the growth kinetics of superconducting Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y thin films. In the preparation, we use the amorphous phase epitaxy method, which is a thallium treatment of the amorphous phase at elevated temperatures. The amorphous phase was deposited on a SrTiO₃ substrate by rf-sputtering from a stoichiometric target with a composition of CuBa₂Ca₃Cu₄O_y. The thallium treatment of the amorphous phase was carried out in a gold capsule for 45 min. The mechanism of the growth kinetics has shown that the formation of Cu_1−xTl_xBa₂Ca₃Cu₄O_12−y thin films was accomplished from Cu_1−xTl_xBa₂Ca₁Cu₂O_8−y (Cu_1−xTl_x-1212) and Cu_1−xTl_xBa₂Ca₂Cu₃O_10−y (Cu_1−xTl_x-1223) by the successive introduction of CuO₂ planes in these phases. We also studied the effect of the time and temperature of the thallium treatment on the growth of Cu_1−xTl_x-1234 films. The best synthesis temperature for Cu_1−xTl_x-1234 films was found to be 910 °C, but this phase could also be isolated as a single phase at lower temperatures (∼890 °C). However, the low-temperature synthesis results in a higher thallium content in the final compound. From the x-ray diffraction measurements the c-axis length was found to increase with the increase of the thallium content. The pole figure measurements of the (103) reflection of the films have shown a-axis oriented crystals with Δϕ = 0.8°. The observed critical temperature (T_c) for Cu_1−xTl_x-1212, Cu_1−xTl_x-1223 and Cu_1−xTl_x-1234 are 78 K, 103 K and 110 K respectively. Current density measurements have shown a maximum J_c ∼ 2 × 10⁶ A cm⁻².