Growth and characterization of Ba₂YCu₃O_7−δ films in reduced oxygen partial pressures using the BaF₂ post-annealing process

Abstract

Epitaxial films of Ba₂YCu₃O_7−δ (BYCO), as thin as 250 Å 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 Å. 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 °C. The BYCO melt line is seen to be the upper limit for T_a. The use of lower p(O₂) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low p(O₂) process allows the formation of smooth films with greater microstructural disorder than for films grown in p(O₂) = 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)] ∝ T_a ⁻¹, independent of growth method (in situ or ex situ) over a range of five orders of magnitude in p(O₂).