PHYSICA ELSEVIER Physica C 282-287 (1997) 623--624 Differences in the nucleation rate ofYBa2Cu307-0 on patterned (001) LaAl03 substrates M. Gustafsson a , E. Olsson a , H. R. Yi b *, D. Winkler b and T. Claeson b aDepartment of Experimental Physics, Chalmers University of Technology, S-412 96 Goteborg, Sweden. bDepartment of Microelectronics and Nanoscience, Chalmers University of Technology, S-412 96 Goteborg, Sweden The microstructures of laser deposited c-axis oriented YBa2Cu307-o (YBCO) thin films have been characterised by scanning and transmission electron microscopy. Grain boundaries were produced using substrates with wavy step edge profiles on cubic (001) LaAl03. The wavy step edge profiles simulated a meandering profile which may arise due to uneven etching of mask and substrate during conventional pattering procedures. The nucleation rate of the a-axis oriented film in the step region varied along the wave profile. The results indicate that a well defined grain boundary geometry and behaviour require a straight step profile on a sub micron scale. In addition, the YBCO nucleation in the step region can be suppressed by choosing a step direction which deviates from the [100]LAO and [010] LAO. 1. INTRODUCTION Grain boundaries in high temperature superconducting YBa2Cu307-o (YBCO) thin films are frequently used to fabricate Josephson junctions. Epitaxial orientation relationships between the film and substrate can be utilised to introduce grain boundaries with desired misorientation between the two adjacent YBCO grains. One technique is to use LaAl03 (LAO) or SrTi03 substrates with patterned step edges where the step angle should exceed 40° [1,2]. It is not only the orientation of the YBCO but also the exact grain boundary morphology which is important since the grain boundary geometry determines the superconducting properties of the grain boundary Josephson junction [3]. In this paper, we have studied YBCO grain boundaries produced using substrates with wavy step edge profiles on cubic (001) LAO, see figure 1. The intention of using a wavy step edge profile was to simulate a meandering profile, which may arise due to uneven etching of mask and substrate during conventional patterning procedures. The aim of this work is to study the nucleation rate of YBCO and to see how the nucleation rate varies with the in plane orientation of the step edge with respect to the [001] and [010] axes of the substrate. Figure 1. A schematic figure showing the wavy step edge profile. The distance between two apexes is 2 /lm and the amplitude between the apex and the bottom of the wave is 1 /lm. The distance between two steps is about 10 /lm. The step height is 240 nm. Note that the figure is not in scale. 2. EXPERIMENTAL The steps were made by ion milling using an amorphous carbon film as a milling mask. The pattern was defmed by electron *Present address: Institut fUr Schicht- und Ionentechnik, Forschungszentrum Jiilich GmbH (KFA), D-524 25 Jiilich, Germany 0921-4534/97/$17.00 © Elsevier Science B.V All rights reserved. PH S0921-4534(97)00440-1