Infrared optical properties of Ba0.8Sr0.2TiO3 ferroelectric thin films

Infrared optical properties of barium strontium titanate Ba0.8Sr0.2TiO3 thin films grown on Pt/Ti/SiO2/Si substrates are studied by infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5–12.6 μm. The refractive index and extinction coefficient of Ba0.8Sr0.2TiO3 are determined by fitting the IRSE data using a classical dielectric function formula. As the wavelength increases, the refractive index decreases, but on the contrary, the extinction coefficient increases. The absorption coefficient is greater than 1000 cm−1 for the wavelength over 12.1 μm, but very small and less than 100 cm−1 under 5.3 μm. The effective static ionic charge has also been derived, which is smaller than it would be in a purely ionic material for Ba0.8Sr0.2TiO3 thin films.