Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba0.8Sr0.2TiO3 films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of approximate to 70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A(1)(TO) and E(TO) components of the soft mode. At film thicknesses of < 100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of similar to 18 and similar to 36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.