Growth mode dependence of microstructures and superconducting properties of Nd1+xBa2-xCu3Oy thick films prepared by liquid-phase epitaxy

We have prepared c-axis oriented Nd1+xBa2-xCu3Oy (Nd123) films onto MgO(1 0 0) single-crystal substrates by the liquid-phase epitaxy (LPE) technique under two different modes of supersaturation. When the films are grown in the transient mode LPE (TMLE), the top surface of the films is characterized by structures with step heights varying over several to some ten unit cells along the c-axis and with many small-angle tilt boundaries. In contrast, when the films are grown in the step-cooling mode, spiral steps with the height of a mono-unit cell are observed at the top surface of the films and the number of small-angle tilt boundaries is greatly reduced relative to that in the films grown by the TMLE. The superconducting transition temperature Te of the films grown in the step-cooling mode reaches 93 K by annealing the films at 400 degrees C for 100 h in 10 atm O-2 atmosphere, whereas that of the films grown in the TMLE is limited to 83 K. The less easy accommodation of oxygen uptake but resulting better superconducting properties of the step-cooling film can be explained in terms of the compositionally well-ordered structure with less density of imperfections. (C) 1998 Elsevier Science B.V. All rights reserved.