Influence of target composition on the quality of YBCO films fabricated by Nd:YAG pulsed laser deposition

The influence of target composition on the formation of surface particles and the crystallinity of YBCO epitaxial films is studied by pulsed laser ablation-deposition method using the fourth harmonic of a Nd:YAG laser and by X-ray diffraction, scanning electron microscope, atomic force microscope, and electron probe X-ray microanalyzer analysis. Six kinds of targets in which one of the atomic constituents is deviated by +/-5% to +/-1% from stoichiometry were used in addition to the stoichiometric target YBa2Cu3Oy. All films deposited are highly c-axis oriented. Films fabricated from stoichiometric targets have a high transition temperature (T-c) of 90.0 +/- 0.5 K and a small number of surface particles (mainly droplets), as low as 1.5 particles/10 mum(2). By constrast, the films deposited from non-stoichiometric targets exhibit a large number of outgrowths (CuO and oxides mainly of Ba and Cu) in addition to droplets and a low T, in the range of 75-89 K. The number and the sort of the outgrowths do not depend largely on the kinds of the non-stoichiometric targets nor on the extent of deviation in the range of +/-1% to +/-5%. These results have been reasonably understood as mainly due to the targets inhomogeneity from the following experimental evidence. (1) The non-stoichiometry of the targets produces much more extent of segregation of impurity phases than that expected from the imbalance of +/-5% to +/-1%. (2) The extent of segregation is almost identical for every non-stoichiometric target. The present study indicates clearly that the target should have the stoichiometric ratio (1:2:3) in order to prepare high quality YBCO epitaxial films. (C) 2001 Elsevier Science B.V. All rights reserved.