Microstructure and properties of superconducting sputter deposited Y–Ba–Cu–O films

Abstract

Films of Y–Ba–Cu–O have been reproducibly prepared with metal stoichiometry near 1-2-3 by sputter deposition from a single sintered target. Twenty-seven depositions resulted in films with a standard deviation in composition of 1%. For 30 samples deposited on (100) SrTiO₃ and annealed, the mean and standard deviation of the critical temperature, T_c (10% above ρ = ο), was 86.2 ± 1.4 K with a transition width (10–90%) of 1.8 ± 1.1 K. The composition and T_c were independent of film thicknesses from 0.09 to 2.5 μm. Critical currents were calculated for samples as a function of thickness from transport and from magnetization measurements. Within the considerable scatter observed of ± half an order of magnitude, the two methods agreed with each other over the thickness range studied. They were both fit with an initial J_c independent of thickness, t, for films up to about 0.3 μm thick followed by a 1/t decrease in J_c for thicker films. Metallographic examination of these films indicated microstructural features consistent with this behavior. Planar TEM images of 0.10 μm thick films showed that high J_c correlated with a film which predominantly has its c-axis normal to the film plane. An alternative structure was also seen, in which regions of the firm had their c-axis along each of the three 〈100〉 SrTiO₃ directions, two in the film plane, and one normal to it. This type of film has a much lower J_c.