Origin of a-axis outgrowth in pulsed-laser-deposited YBa(2)Cu(3)O7-delta thin films from modified melt-textured grown targets

YBa2Cu3O7-delta (YBCO) thin films were deposited using pulsed laser deposit ion (PLD) from modified melt-textured grown targets. As the laser energy de nsity was increased, the surface of the films was covered with enhanced a-a xis outgrowths. In order to determine the origin of these formations, the m icrostructures of films deposited at 2 and 4 J cm(-2) were investigated usi ng x-ray diffraction, transmission electron microscopy and high-resolution electron microscopy, It was found that a significant number of Y2O3 inclusi ons were formed during the growth of c-axis-oriented films at 4 J cm(-2) Th ese inclusions formed nucleation sites for the a-axis outgrowths. The forma tion of Y2O3 inclusions was attributed to the Y-rich targets. In addition, the non-equilibrium characteristics of PLD and the enhanced ablation of Y-r ich phases attributed to the increase of laser energy density led to these Y2O3 inclusions. As for the a-axis outgrowth nucleation, it is considered t hat, due to the unstable growth conditions with a high flux density of inci dent vapour species and the strain induced by the surrounding c-axis films, the Y2O3 inclusions would prefer the nucleation of a-axis grains.