Temperature dependence of the optical properties of Ba 0.75 Sr 0.25 TiO 3 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 300K. 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 1.5–5.5eV 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.