Infrared optical properties of Bi_3.25La_0.75Ti₃O₁₂ ferroelectric thin films using spectroscopic ellipsometry

Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that the BLT thin films are polycrystalline with the preferential orientation. The infrared optical properties of the BLT thin films have been investigated using the infrared spectroscopic ellipsometry in the wave number range of 800-4000 cm^-1 (2.5-12.5 μm). By fitting the measured ellipsometric parameter (ψ and Δ) data with a three-phase model (Air/BLT/Pt), and a derived classical dispersion relation for the thin films, the optical constants and thickness of the thin films have been obtained. The refractive index decreases as the wave number decreases, whereas the extinction coefficient increases. The absorption coefficient is very small and less than 100 cm^-1 for the wave numbers above 2027 cm^-1 (4.9 μm). However, it is higher than 1000 cm^-1 for the wave numbers under 830 cm^-1 (12.4 μm).