SURFACE-MORPHOLOGY OF YBA2CU3O7-DELTA THIN-FILMS ON SRTIO3(100) STUDIED BY SCANNING-TUNNELING-MICROSCOPY

Thin films of YBa2Cu3O7-delta (YBCO) have been prepared by pulsed lase r deposition on single-crystalline SrTiO3(100) substrates at different substrate temperatures T(s). Depending on T(s), different orientation s of the films are revealed by X-ray diffraction: (001) orientation be tween 660 to 800-degrees-C, (100) orientation between 610 and 660-degr ees-C, and no YBCO reflections for films prepared at lower T(s). In-pl ane orientation (PHI-scan) and out-of-plane orientation data (mosaic s pread) of the grains are compared to scanning tunneling microscopy (ST M) images. The images show rectangular-shaped growth hills. Their grow th steps are aligned along the [100] and [010] directions of YBCO and are one c-axis distance in height (1.2 nm). Occasionally screw disloca tions are observed. Highest critical transition temperatures are reach ed at about T(s) = 750-degrees-C. The relationship between film thickn ess and surface morphology has been studied using YBCO films with a th ickness gradient prepared by a half-shadow technique. Thus, films of d ifferent thicknesses can be obtained simultaneously under identical co nditions. STM images give evidence of different growth stages such as nucleation and coalescence of two-dimensional islands, their growth an d, finally, the formation of growth hills.