Anisotropy of persistent photoconductivity in oxygen-deficient YBa₂Cu₃O_x thin films

YBa/sub 2/Cu/sub 3/O/sub x/ (x approximately=6.6) thin films were deposited by pulsed-laser ablation on SrTiO/sub 3/ substrates, that were cut with a tilt angle alpha =10 degrees with respect to the [001] direction, inducing a steplike growth of the CuO/sub 2/ layers. The films appeared semiconducting along the projection of the c axis to the film surface, but metallic in the perpendicular direction. From the anisotropic resistance, the in-plane ( rho /sub ab/) and the out-of-plane ( rho /sub c/) resistivities were calculated. Photodoping with white light was performed at various temperatures from 70-305 K. We have found strongly temperature dependent photoinduced decreases of rho /sub ab/ and rho /sub c/. At low temperatures, the relative reduction of rho /sub c/ was smaller than that of rho /sub ab/, whereas, near room temperature, they were nearly the same. The electrical anisotropy rho /sub c// rho /sub ab/ increased monotonously at 70 K, but, at higher temperatures, a fast increase was followed by a long-term decay. This unusual behavior resembles that of the in-plane Hall mobility in a previous study, suggesting that both quantities are changed by the same physical process. The results are discussed within the models previously proposed for the photodoping effect in YBa/sub 2/Cu/sub 3/O/sub x/.