Mechanism of transition between a-axis and c-axis growth of YBa₂Cu₃O_x thick films grown on NdGaO₃ substrate

Abrupt transition from a-axis growth to c-axis growth was found in the liquid phase epitaxy (LPE) process of YBa₂Cu₃O_x (YBCO or Y123) on (110) NdGaO₃ substrate. Subsequently, the influence of supersaturation and liquid composition on the orientation of LPE thick films was systematically studied. Dissimilar to the thin film deposition, the a-phase growth was predominant at a low supersaturation (about 17 K), especially grown by using a Cu-rich melt. The investigation of the initial stage of YBCO growth was performed, in which we found the protruding epitaxial a-axis grains and preferentially a-phase nucleation on the c-dominated thick film. The energy dispersive x-ray spectrometry attached scanning electron microscope was employed to confirm the composition of the outgrowth of long-axis grains, as well as the microstructure around the dipping boundary line in the initial growth. Based on the “nucleation and growth” mode, a comprehensive description of the competition between a-axis and c-axis growth under various growth environments was given, which further unveiled the origin of a-c transition in YBCO thick films.