Optical properties of epitaxial Na0.5Bi0.5TiO3 lead-free piezoelectric thin films: Ellipsometric and theoretical studies

Ultra-thin Na0.5Bi0.5TiO3 films were epitaxially deposited on (001)SrTiO3 substrate by pulsed laser deposition. From the ellipsometric spectra collected in the photon energy range 2÷8.7 eV, real and imaginary parts of the complex dielectric function were determined. It was found, that the electronic transitions related to the fundamental absorption edge are of indirect type, which is confirmed by ab initio band structure calculations. This finding is in contradiction to previous theoretical studies but in agreement with the most recent experimental works. Based on their determined energies, phonons involved in the band gap transition can be primarily associated with vibrations involving oxygen atoms. Contributions to the VUV absorption spectra come mainly from the electronic transitions from the O 2p valence band levels to Ti 3d conduction band levels and a little to the Bi p levels.