Investigation of precipitate formation on laser-ablated YBa2Cu3O7- delta thin films.

Abstract

We have investigated systematically the particle formation on the surface of high-quality ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (YBCO) epitaxial films grown by pulsed laser deposition. Scanning-Auger-electron spectroscopy identified two types of precipitates, i.e., ${\mathrm{BaCuO}}_{2}$ and CuO, on the films deposited from a stoichiometric YBCO target. The coexistence of these two phases with a YBCO matrix film is well explained by the pseudoternary phase diagram of ${\mathrm{YO}}_{1.5}$-BaO-CuO, assuming a slight Y deficiency in the film. The precipitate formation was reduced when the films were grown under conditions slightly deviated from the optimum for the formation of well-equilibrated, highly crystalline, and high-${\mathit{T}}_{\mathit{c}}$ and high-${\mathit{J}}_{\mathit{c}}$ films. Under such thermodynamically off-equilibrated conditions, the phase separation into YBCO and secondary phases is presumed to be kinetically prevented to give solid-soluted films and off-stoichiometric compositions and inferior crystallinity and superconductivity. The film-thickness dependences of the precipitate density and size were quantitatively analyzed by scanning electron microscopy to indicate that the nucleation of precipitates occurred by the initial 50 nm growth of the film to be followed by the increase of the precipitate's size. The use of off-axis polished substrates could successfully suppress the nucleation of the precipitates. A possible mechanism of precipitate formation is proposed.