Nature of epitaxial growth of high-T_c laser-deposited Y-Ba-Cu-O films on (100) strontium titanate substrates

Epitaxial YBa₂Cu₃O₇ films were fabricated on (100) SrTiO₃ substrates by the pulsed laser (λ =308 nm, τ=45×10^-9 s) evaporation of bulk targets in a controlled oxygen atmosphere (partial pressure 100-600 mTorr). The nature of epitaxial growth and orientation of the film was controlled by the oxygen partial pressure in the chamber and the substrate temperature during the deposition process. These parameters were also important in determining the superconducting properties of the thin films. The best films exhibited zero electrical resistance at about 90 K with very high critical current densities. Thin single-crystal films were grown in situ at substrate temperatures greater than 550 °C with the c axis perpendicular to the substrate, whereas films deposited at lower substrate temperatures showed a mixture of c- and a axes perpendicular to the substrate. The films deposited in high vacuum or low oxygen partial pressure (<1 mTorr) and further annealed at high temperatures (∼900 °C) in oxygen showed rectangular grains with the c axis parallel to the substrate. Higher-temperature (>920 °C) annealed films showed different surface morphology with the c axis perpendicular to the substrate. The epitaxial growth of YBa₂Cu₃O₇ films on (100) SrTiO₃ substrates is examined as a function of lattice mismatch between the thin film and the substrate. As the a' and c axes of the tetragonal unit cell (above transformation temperature) of YBa₂Cu ₃O_x varies both with the temperature and the oxygen content, a new method is proposed to calculate the lattice parameters of the superconducting phase. This method is based on the lattice parameters calculation at the tetragonal to orthorhombic transformation temperature which always occurs approximately at the same oxygen content regardless of the processing oxygen partial pressure.