Growth model and the effect of CuO nanocrystallites on the properties of chemically derived epitaxial thin films of YBa ₂Cu ₃O _7-x

In metalorganic deposition using trifluoroacetates (TFA-MOD), CuO nanocrystallites in calcined film, which influence the critical current density (J _c) of the resulting film, have seldom been considered before. CuO nanocrystallites ripen and grow during the calcining process at 200-250°C and grow into large CuO grains in the YBa ₂Cu ₃O _7-x (YBCO) film. The final diameter of the grains is about 0.1 μm. When highly purified coating solution is used, suppressing the creation of nanocrystallites in the precursor film is an effective way to obtain high-J _c YBCO film because the nanocrystallites consist only of CuO. We obtained the highest-J _c YBCO film on LaAlO ₃ single crystal with a calcining process of 9h43m at 200-250°C. It was 141 nm thick and had a J _c of 7.5 MA/cm ² (77 K, 0 T), as measured by the four-probe method. The firing process of TFA-MOD still has several unsolved problems; why can H ₂O and HF gas diffuse quickly within the film? These phenomena can be explained by a model with a quasiliquid consisting of Y, Ba, Cu, O, H, and F. The H ₂O and HF can move quickly in the quasiliquid network within the film during the firing process. According to the model, we can get more conversion of CuO nanocrystallites into quasiliquid by increasing the amount of water vapor. J _c of the film was increased from 3.3 to 4.5 MA/cm ² (77 K, 0 T) by increasing the humidity from 4.2% to 12.1% during the long calcining profile of 66h40m at 200-250°C. These results confirm the above model. CuO nanocrystallites in precursor films induce nonstoichiometric metal contents in the quasiliquid. Isolated CuO grains in YBCO film indirectly cause nonstoichiometric quasiliquid in other areas. Such nonstoichiometric quasiliquid leads to non-YBCO materials such as BaO, Y ₂O ₃, and Y ₂Cu ₂O ₅. Thus, films derived from non-highly purified coating solution and ones prepared with an inappropriate calcining process both have lower J _c values. Micrographs obtained by transmission electron microscopy and J _c results support this hypothesis.