BULK AND SURFACE-STRUCTURE OF EPITAXIAL YBA2CU3O7-DELTA THIN-FILMS GROWN ON MG2TIO4 (001) SUBSTRATE LAYERS .2. ORIENTATION AND SURFACE-STRUCTURE

Thin films of YBa2Cu3O7-delta (YBCO) have been grown on Mg2TiO4 substrate layers by pulsed laser deposition. The substrate temperature T(s) has been varied between 600 and 800-degrees-C in order to optimize the YBCO film growth. All samples were characterized by means of X-ray diffraction, scanning electron microscopy (SEM) and scanning tunneling microscopy (STM). We have found an optimum T(s) in the range of 675-700-degrees-C where the YBCO thin films show perfect (001) orientation. SEM and STM reveal that the (001) oriented YBCO films have very flat surfaces. STM shows a large number of spiral growth hills and steps of unit cell height. These films exhibit a superconducting transition temperature T(c)(onset) at 88.5 K and a zero resistance at 86.5 K. Films grown at lower substrate temperatures show no (001) orientation, but (100) and (110)/(103) orientations. At higher T(s) additional (100) oriented grains occur.