Epitaxial relations between in situ superconducting YBa₂Cu ₃O₇-x thin films and BaTiO₃/MgAl ₂O₄/Si substrates

In situ superconducting YBa₂Cu₃O_7-x films with T_c0 up to 87 K and J_c, 77 K up to 6×10 ⁴ A/cm² were prepared on Si substrates with MgAl ₂O₄ and BaTiO₃ double-buffer layers. The epitaxial relations between various layers were established by transmission electron microscopy. The MgAl₂O₄ layer is heavily faulted. The subsequent BaTiO₃ layer stops most of the faults, provides a template for the YBa₂Cu₃O_7-x growth, and partially screens off the stress due to different thermal expansion coefficients. The microstructure of the YBa₂Cu₃O _7-x layer is very similar to that of the films deposited directly on SrTiO₃, exhibiting a homogeneous heavily faulted single-crystal-like structure free from secondary phases and grain boundaries. The slight degradation of the transport properties is attributed to residual thermal stress.