High T_c Y-Ba-Cu-O Films Prepared by Multilayer, Reactive Sputtering from Separate Y, Cu, and Ba_0.5Cu_0.5 Targets

Abstract

In order to thoroughly investigate the phase diagram in films, and to perform destructive tests (e.g. various annealing procedures), a set (nearly ninety) of films having a composition Y_xBa₂Cu_yO_z with 0.6 < x < 1.9 and 2.5 < y < 4.5 (centered around the Y₁Ba₂Cu₃O_7-δ compound in the ternary phase diagram) were prepared on MgO. Multilayer deposition of the constituents at ambient temperature was performed by reactive de-maguetron sputtering from separate Y, Cu, and Ba_0.5Cu_0.5 targets in an atmosphere of 6×10^-5 Tarr O₂ and 1.0×10^-2 Torr Ar. Two hinds of post-deposition heat treatment were applied: rapid thermal annealing between 950–1000^°C and slow annealing between 840–860^°C. Superconducting films with T_c (R=0) between 80 K and 90 K were routinely produced by the rapid thermal annealing process, if the stoichiometry lay in the range of 0.9 < x < 1.4 and 3.2 < y < 3.5. The interrelation between stoichiometry, heat treatment, T_c (R=0), chemical reactions with the substrate, and the presence of phases; other than the known 1-2-3 superconductor was explored, Raman spectroscopy was employed to study the latter two properties. A stroug correlation was found between the Raman spectra and T_c (R=0), which suggests the potential utility of Raman scattering as an in-situ probe to characterize the growth of Y-Ba-Cu-O films. We also found that the rapid thermal annealing process is a very efficient way to reduce chemical reactions of the film with its substrate.