TWINNING OF YB2CU3O7 THIN-FILMS ON DIFFERENT SUBSTRATES AND MODIFICATION BY IRRADIATION

Bragg reflections of YBa2Cu3O7 (YBCO) films with c-axis orientation on (001) SrTiO3, (001) MgO and (110) NdGaO3 substrates prepared by MO-CVD, sputtering techniques and laser ablation were examined with Co Kalpha and synchrotron radiation (lambda = 0.56 angstrom, HASYLAB, Hamburg) using the q-scan technique at four circle diffractometers. One of the specimens was doped with Ni. The evaluation of positions and relative intensities of the splitted reflections shows a strong correlation between the structure of the substrate surface and the twinning geometry of the YBCO layer. Irradiation of the films on SrTiO3 substrate by heavy ions (1.2 GeV Bi-209, 1.4 GeV U-238, UNILAC, GSI Darmstadt) leads to a gradual parallel alignment of a, b, and a, b(f) (subscripts (s) denoting substrate. and (f) film) which is completed for a dose between 10(11) and 10(12) particles/cm2.