GROWTH AND CHARACTERIZATION OF BA2YCU3O7-DELTA FILMS IN REDUCED OXYGEN PARTIAL PRESSURES USING THE BAF2 POSTANNEALING PROCESS

Epitaxial films of Ba2YCu3O7-delta (BYCO), as thin as 250 angstrom and with J(c)'s approaching those for the best in situ grown films, can be formed by coevaporating BaF2, Y, and Cu followed by a two-stage anneal. These results extend the work of R. Feenstra et al. [J. Appl. Phys. 69, 6569 (1991)] for film thicknesses >2000 angstrom. This involves using low oxygen partial pressure [p(O2) = 4.3 Torr] during the high temperature portion of the anneal, which we vary from T(a) = 600 to 950-degrees-C. The BYCO melt line is seen to be the upper limit for T(a). The use of lower p(02) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low P(02) process allows the formation of smooth films with greater microstructural disorder than for films grown in P(02) = 740 Torr at higher T(a), resulting in higher J(c) values by a factor of four. The relationship between the T(a) required to grow films with the strongest pinning force and P(O2) is log [P(O2)] is-proportional-to T(a)-1, independent of growth method (in situ or ex situ) over a range of five orders of magnitude in p(O2).