Spectroscopic ellipsometry studies on the optical constants of Bi4Ti3O12:xNa thin films grown by metal-organic chemical vapor deposition

We have deposited undoped and Na-doped epitaxial Bi4Ti3O12 thin films on SrTiO3(001) substrates using the liquid-delivery spin metal-organic chemical vapor deposition technique. High resolution x-ray diffraction and Raman spectroscopy measurements were employed to investigate the structural perfection and incorporation of Na ions into the film. The ellipsometric measurements were carried out in the energy range 1.49–2.75 eV and the corresponding experimental data were fitted. Two different dispersion relations, Cauchy's absorbent and Tauc-Lorentz, have been used to determine the optical constants of the films. It is observed that there is a decrease in optical band gap for increasing sodium content. Furthermore, it has been found that the refractive index and extinction coefficient also depend on the sodium content. The refractive index dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters were determined. The optical constants tend to decrease with increasing doping content.