Abstract: Ellipsometric spectra of Ba_0.9Sr_0.1TiO₃ (BST) thin films with various annealing
temperatures were measured in the range of photon energy from 2.1 to 5.2eV. Constructing appropriate fitting models and describing optical properties
of the BST with Cauchy dispersion model, their optical constants (refractive index n and extinction coefficient k) spectra and band
gap E_g were determined by means of an optimization. Compared these results, we obtained the variation of the refractive index n,
the extinction coefficient k and the band gap E_g with annealing temperatures.

Key words: spectroscopic ellipsometry, optical constant spectra, BST films