Epitaxial thin films of Ba2YCu3O7−x (BYC) were prepared on (001) LaAlO3 single‐crystal substrates by metalorganic deposition of metal trifluoroacetate precursors. This is an ex situ process that requires high‐temperature annealing in a humid atmosphere to produce stoichiometric BYC thin films. The chemically derived superconducting films were found to have high critical temperatures and high current densities when crystallized under low‐oxygen partial pressures. Superconducting films of 70 nm thickness with zero‐field critical current densities greater than 5×106 A/cm2 at 77 K and zero resistance at 92 K were prepared by annealing at 780 and 830 °C in 2.5 × 10−4–1 × 10−3 atm oxygen furnace atmospheres. As the film thickness was increased, the superconducting properties and surface smoothness of the films tended to degrade. This behavior was consistent with a microstructural model in which the films are composed of a dense slab of c‐axis normal BYC near the film/substrate interface with the overlying mater...

Effect of growth conditions on the properties and morphology of chemically derived epitaxial thin films of Ba2YCu3O7−x on (001) LaAlO3

The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post‐deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid‐phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c‐oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c‐oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical ...

Effect of oxygen pressure on the synthesis of YBa2Cu3O7-x thin films by post-deposition annealing

Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure 100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400012498

Reactive coevaporation of YBaCuO superconducting films

The temperature and oxygen pressure conditions were determined by thermogravimetry for decomposition of phase-pure YBa 2 Cu 3 O 7− x and YBa 2 Cu 4 O 8 from 3 Pa to 0.1 MPA O 2 . All extant data were reviewed to determine the conditions for thermodynamic dynamic stability of phase-pure YBa 2 Cu 3 O 7− x and for compositions lying between YBa 2 Cu 3 O 7− x and CuO . Thermodynamic values for YBa 2 Cu 4 O 8 were derived, as was the 123-CuO section of the Y 2 O 3 -BaO-CuO phase diagram.

Decomposition of YBa2Cu3O7−x and YBa2Cu4O8 for po2≤0.1 MPa

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.

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

The quatenary phase relations near YBa 2 Cu 3 O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2 O 2 . Nine invariant reactions were discovered, including decomposition of YBa 2 Cu 3 O 6+ x into Y 2 BaCuO 5 , BaCu 2 O 2 and YBa 3 Cu 2 O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2 Cu 3 O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2 O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2 Cu 3 O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

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Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures