STUDIES OF YBA2CU3O(7-X) FORMATION IN COEVAPORATED Y-BAF2-CU-O THIN-FILMS ON SAPPHIRE SUBSTRATES

Degradation mechanisms when films of YBa2Cu3O7-x (YBCO) were formed on sapphire substrates have been studied. The films were synthesized by post-deposition annealings in a wet oxygen atmosphere of coevaporated layers of Y-BaF2-Cu-O. X-ray diffraction revealed the existence of imp urity phases in the annealed samples, the most significant being BaAl2 O4, Y2BaCuO5, CuO and Y2O3. Redistribution of the constituents as a fu nction of time was examined by secondary ion mass spectrometry and Rut herford backscattering spectrometry, giving evidence of a Ba redistrib ution against the interfacial region and an agglomeration of Cu within the film. While zero-resistivity temperatures of 1 mum thick films ty pically were between 80 and 88 K, magnetic characterization indicated weak coupling between the superconducting grains, resulting in low tra nsport critical currents less-than-or-equal-to 10(3) A cm-2 at 77 K. T he film properties of YBCO/sapphire were compared with those of YBCO p rocessed in parallel on SrTiO3 or LaAlO3.