GROWTH DIRECTION CONTROL IN YBCO THIN-FILMS

The crystallographic orientation of thin films of Y1Ba2Cu3O7 (YBCO) on MgO and SrTiO3 was investigated with theta-2-theta X-ray diffraction. The fraction of a-axis oriented material was measured as a function of substrate temperature during growth. Films prepared at substrate table temperatures yielding maximum T(c) were purely c-axis oriented up to a critical film thickness (less-than-or-equal-to 5500 angstrom), above which also a-axis material was observed. Our hypothesis was that the substrate temperature is lowered during deposition due to the increasing emissivity of the film as it reaches a thickness comparable with the IR penetration depth, and that this lowered substrate temperature makes a-axis growth possible. We estimated this cooling effect, and found that it was too small to allow for a-axis oriented growth. The cooling hypothesis is contrasted to a-axis oriented growth caused by an increased density of lattice imperfections with growing film thickness.