Effects of thickness on the infrared optical properties of Ba0.9Sr0.1TiO3 ferroelectric thin films

Using infrared spectroscopic ellipsometry (IRSE), the optical properties of the Ba0.9Sr0.1TiO3 (BST) ferroelectric thin films with different film thicknesses on Pt/Ti/SiO2/Si substrates prepared by a modified sol-gel method have been investigated in the 2.5–12.6 μm wavelength range. By fitting the measured ellipsometric parameter (ψ and Δ) data with a three-phase model (Air/BST/Pt) and the classical dispersion relation for the BST thin films, the optical constants and thicknesses of the thin films have been obtained. The average thickness of the single layer decreases with increasing film thickness. The refractive index of the BST films decreases with increasing thickness in the wavelength range 2.5–11 μm, and increases with increasing thickness in the wavelength range 11–12.6 μm. However, the extinction coefficient of the BST films monotonously decreases with increasing thickness. It is closely associated with the crystallinity of the thin films, the crystalline size effect and the influence of the interface layer. The absorption coefficient of the BST films with different thicknesses decreases with increasing thickness.