Abstract
Epitaxial YBa2Cu3O7 (Y123) films were obtained on CeO2-buffered sapphire substrates by a fluorine-free MOD process. A 170-nm-thick homogeneous film revealed a high critical current density (Jc > 4 MA/cm2) at 77 K, whereas a 320-nm-thick film had a low Jc of 0.7 MA/cm2. Both of the films have neither a-axis orientation nor BaCeO3 phase by x-ray diffraction. In this article, the microstructures of these Y123 films were observed by crosssectional transmission electron microscopy. The epitaxial c-axis layers were observed successively from the interface with CeO2 buffer to the surface. Selected area diffraction (SAD) exhibited that the orientation relationship among the Y123, CeO2-buffer and sapphire substrate was: Y123(001)[110] ∥ CeO2(100)[100] ∥ Al2O3(-1012)[1-210]. The impurity phase on the surface was revealed to be a CuYO2 compound by energy dispersive X-ray spectroscopy and SAD. The Y123 grains on the surface of the 320-nm-thick film were somewhat inclined to the epitaxial Y123 layer; this is one of the reasons for degraded Jc in the thicker films.
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