Microstructure of epitaxial $Ba0.7Sr0.3TiO3∕SrRuO3$ bilayer films on $SrTiO3$ substrates

The thickness evolution of the microstructure of epitaxial $Ba0.7Sr0.3TiO3$ thin films grown on $SrRuO3∕SrTiO3$ was investigated by means of transmission electron microscopy. Within the $Ba0.7Sr0.3TiO3$ layer, a layered structure (three sublayers) is distinguished as for the configuration of lattice strain and defects. The first sublayer extends for $3nm$ from the lattice-coherent $Ba0.7Sr0.3TiO3∕SrRuO3$ interface. The second $13-nm$-thick sublayer forms a semicoherent interface with the first sublayer due to the creation of a misfit dislocation network. The third sublayer extends beyond the second sublayer exhibiting a structure characterized by compact columnar features. Planar defects are formed at the boundaries between such features. The formation of a layered structure within the $Ba0.7Sr0.3TiO3$ films is discussed in the light of the growth modes of films on lattice-mismatched substrates.