Crystalline orientation and twin formation in YBCO thin films laser ablated from a nanocrystalline target

Two sets of superconducting thin films were deposited on SrTiO 3 by laser ablation, the first using a nanocrystalline target (n-films) and the second using a commercial microcrystalline target (μ-films). The effect of film thickness on the structure of the films was investigated using detailed x-ray diffraction analysis. It was found that for both types of film growth is a three-phase process, where at the beginning some grains with their a -axis perpendicular to the substrate surface are formed. In the second phase a purely c -axis oriented film is formed and in the last phase some a -axis oriented grains are again observed. The last phase was observed only for the μ-films. Both types of film were found to have equal numbers of c -axis oriented grains oriented with their a -axis to the ϕ = 0° and to the 90° direction. It was also found that the twin structure observed in both types of film develops much faster in the n-films, which might be the reason for the observed high critical current densities in n-films.