High-resolution and energy-filtered transmission electron microscopy of YBa₂Cu₃O_7-x/PrBa₂Cu₃ O₇ superlattices

Structural disorder induced by epitaxial strain relaxation in c-axis oriented [YBa₂Cu₃O_{7-x n u.c}/PrBa₂Cu₃O_{7 5 u.c.}] superlattices on (100) SrTiO₃ substrates has been analyzed by high-resolution and energy-filtered transmission electron microscopy. Epitaxial strain shows up in ultrathin YBCO layers with thickness below 4 unit cells. Strain relaxation takes place when layer thickness increases above this value. Microscopy observations for epitaxially strained superlattices show a good morphology of the layers, with sharp and flat interfaces. However, in relaxed samples rougher interfaces are observed, together with microdomains showing the c-axis parallel to the substrate plane which preserve the superlattice compositional profile. Such microdomains of c-parallel growth provide a path for reduced lattice mismatch, conforming a very efficient mechanism for strain relaxation.