Residual strain and crystal structure of BaTiO₃-SrTiO₃ thin films prepared by metal organic chemical vapor deposition

The lattice parameters of BaTiO₃-SrTiO₃ films prepared by chemcal vapor deposition (CVD) were investigated. When the film thickness was 1500 nm, the lattice parameters were mainly determined by the thermal stress due to the thermal expansion difference between the film and the substrate. The lattice parameters of epitaxially grown SrTiO₃ films on (100)MgO substrates increased with decreasing film thickness, while those of polycrystalline films were independent of the film thickness down to 300 nm. They also increased with the Ti/(Sr+Ti) ratio for epitaxially grown 500-nm-thick films, while those of polycrystalline films were independent of Ti/(Sr+Ti) ratio. Large lattice parameters were also observed for the epitaxially grown (Ba_0.5Sr_0.5)TiO₃ film on a (100)MgO substrate but were not observed for the epitaxially grown BaTiO₃ film on (100)MgO substrate. These occurrences were closely related to the epitaxial growth of the film.