The effect of post-annealing on laser-deposited superconducting Bi–Sr–Ca–Cu–O thin films

Abstract

Superconducting Bi–Sr–Ca–Cu–O thin films were obtained from post-annealing partially crystallized and amorphous films grown on MgO(100) by pulsed laser deposition. The substrate temperature investigated was in the range of 350–750 °C, over a range of pressure 0.1 to 100 mTorr. The as-deposited films were annealed in 7.5 vol.% O₂/Ar or in air at 800–865 °C from several minutes to a few hours. Unlike the pure Bi₂Sr₂CaCu₂O_8+δ (2212) phase (T_c = 80 K) which is easily formed after a long continuous period of post-annealing at a temperature below 830 °C, the formation of (Bi, Pb)₂Sr₂Ca₂Cu₃O_10+δ (2223) phase from the as-deposited amorphous films requires repetitive annealing cycles of short duration in air at 850 °C to simultaneously achieve good crystal quality, small surface roughness, and sharp diamagnetic transition (T_c = 110 K). After annealing, the temperature is lowered down to ∼650 °C by quenching in air and then a slow-cooling step is employed. This procedure was found to enhance the volume fraction of the 2223 phase as compared with a direct slow-cooling process. The trade-off between annealing temperature and time was observed to affect the phase formation and the smoothness of the annealed films. To optimize the post-annealing conditions, Rutherford backscattering spectrometry, x-ray diffraction, and scanning electron microscopy were systematically used to examine the composition, structure, and morphology of the films, respectively.