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 YBa2Cu3O7-x (YBCO) thin films synthesized by metalorganic chemical vapor deposition have been investigated. The formation of single cation films using tetramethylheptanedionate precursors was shown to be mass controlled, exhibiting a ratio of deposited to evaporated species in the increasing order Ba, Y, and Cu. The physical properties of off-stoichiometric YBCO films deposited on MgO substrates were measured in the compositional range 1.1 less-than-or-equal-to Ba/Y less-than-or-equal-to 2.3 and 1.5 less-than-or-equal-to Cu/Ba less-than-or-equal-to 4.6. While structural properties such as c-axis values and rocking curves appeared unaffected to variations in cation stoichiometry, morphology was observed to be extremely sensitive even to slight changes in composition. Off-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-resistivity 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 superconducting 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 and 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.