GROWTH-KINETICS-INDUCED STRUCTURAL DISORDER IN BI-2(SR1-XCAX)(3)CU2OY THIN-FILMS STUDIED BY X-RAY-DIFFRACTION

Thin films of Bi2(Sr1-xCax)3Cu2Oy grown in situ by molecular-beam epitaxy have been studied by x-ray diffraction. The experimental results were interpreted using a general one-dimensional kinematic x-ray-diffraction model. An accurate determination of the lattice structure and of the distribution of the different atoms on the lattice sites is obtained. The model uniquely reconstructs a large substitution between Ca2+ and Sr2+ at the Ca and Sr lattice sites. The model also predicts an expansion of the CuO2-CuO2 and a contraction of the SrO-CuO2 interplanar distances compared with x-ray-diffraction data reported from bulk single crystals. The large atomic substitution between different lattice sites is attributed to the growth kinetics, which for molecular-beam epitaxy plays a major role in forming the growing epitaxial structure, due to the low interaction energy between the thermal atom beams and the substrate surface inherent to the molecular-beam epitaxy process. This structural disorder is interpreted as causing the suppressed superconducting transition temperatures, Tcs and the broad superconducting transition widths, Tcs observed for Bi2(Sr1-xCax)3Cu2Oy films grown by molecular-beam epitaxy compared with films fabricated by magnetron sputtering or laser ablation. The relation between the structural disorder and the superconducting properties is discussed. © 1995 The American Physical Society.