VIS-UV-VUV optical functions of epitaxial ferroelectric PbTiO3/SrTiO3 thin films

Lead titanate PbTiO3 ferroelectric thin films were epitaxially grown by pulsed laser deposition on strontium titanate SrTiO3 substrates. Two samples of thicknesses 117 nm and 313 nm were investigated. The a/c polydomain structure, which results from a partial relaxation of the epitaxial strain, was evidenced in the thicker film. The real and imaginary parts of the complex dielectric function were determined in a broad photon energy range of 2-9.8 eV using spectroscopic ellipsometry. The energy gap values and critical point energies were determined for the both films. An effect of the epitaxy on the optical functions obtained is detected and discussed. The results showed, that even in the case of a relatively small mismatch between the layer and substrate lattice constants, the effect of epitaxy on the optical properties of ferroelectric films is not negligible.