THIN-FILM GROWTH AND COMPOSITIONAL EFFECTS IN YBA2CU3O7-X LAYERS PREPARED BY METALORGANIC CHEMICAL-VAPOR-DEPOSITION

Thin-film growth and compositional effects of c-axis oriented YBa2Cu3O 7-x (YBCO) thin films synthesized by metalorganic chemical vapor depos ition have been investigated. The formation of single cation films usi ng tetramethylheptanedionate precursors was shown to be mass controlle d, exhibiting a ratio of deposited to evaporated species in the increa sing order Ba, Y, and Cu. The physical properties of off-stoichiometri c YBCO films deposited on MgO substrates were measured in the composit ional range 1.1 less-than-or-equal-to Ba/Y less-than-or-equal-to 2.3 a nd 1.5 less-than-or-equal-to Cu/Ba less-than-or-equal-to 4.6. While st ructural properties such as c-axis values and rocking curves appeared unaffected to variations in cation stoichiometry, morphology was obser ved to be extremely sensitive even to slight changes in composition. O ff-stoichiometric layers with Cu/Ba > 1. 5 were observed to exhibit Cu -rich precipitates embedded in a 1:2:3 YBCO film matrix. The zero-resi stivity temperatures were above 77 K for all cation film compositions measured. However, sharp ac-susceptibility transitions were restricted to a more narrow compositional range (1.9 < Cu/Ba < 3.6). The best su perconducting properties [T(c) = 85 K, J(c) (77 K) > 10(6) A/cm2] were observed for films with relatively rough surface morphologies (Ba/Y = 1. 6 and Cu/Ba = 3.5). An optimum trade-off between smooth surfaces a nd superconducting properties was found for Ba/Y = 1.5 and Cu/Ba = 1.9 , yielding T(c) = 81 K and J(C) (77 K) = 3 X 10(5) A/cm2.