Temperature dependence of the optical properties of Ba0.75Sr0.25TiO3 thin films

Optical properties of Ba0.75Sr0.25TiO3 (BST(75/25)) ferroelectric thin film have been investigated by spectroscopic ellipsometry in a broad temperature interval from 10 to 300 K. The polycrystalline perovskite-phase BST films were prepared by pulsed laser deposition technique. The complex dielectric functions and optical parameters of the films were determined in the 1.5-5.5 eV spectral range. The spectra can be divided into two distinctive regions according to the magnitude of absorption coefficient separated by a band gap. An apparent reduction of the band gap was observed on increasing temperature. This can be ascribed to the renormalization of electronic band structure due to the electron-phonon interaction, which follows the Bose-Einstein model.