MORPHOLOGY AND COMPOSITION OF YBACUO TARGETS AFTER LONG-PERIOD LASER-ABLATION

Both the surface morphology and chemical composition of sintered YBaCuO targets after long period pulsed laser ablation have been studied using a scanning electron microscope with X-ray energy dispersive spectroscopy. These targets were rotated during laser fluence and formed concentric trenches with distinctive surface morphologies. Contrary to a previous report [A. Hoffman, Supercond. Sci. Technol. 3 (1990) 1183], the chemical compositions in these concentric trenches have been found to be different from the original 1-2-3 stoichiometry. The correlation between morphology and composition in each region has been explained by both preferential evaporation and redeposition of the target material. The target surface composition changes from 1-2-3 (virgin surface), to Y-rich (after 30 000 shots), and finally to Y-rich and Cu-poor (after 60 000 shots). The film stoichiometry remained roughly constant during the target evolution but the growth rate decreased with the further erosion of the target. The in-situ sputtering effect from the film surface was proposed to bc the main mechanism in maintaining the film stoichiometry. In addition, elongated surface features on the ablated target have been confirmed to originate from the preferential erosion of target material by the laser beam rather than the movement of liquid droplets on the surface.