Optical Properties of BST Thin Films by Spectroscopic Ellipsometry and Optical Reflectivity

Abstract

Optical properties of Ba 0.25 Sr 0.75 TiO 3 thin films fabricated by pulsed laser deposition (PLD) have been investigated using variable angle spectroscopic ellipsometry (VASE) and near-normal spectroscopic reflectivity (NNSR). The measurements were carried out from 5 K to 450 K in the spectral range 1–6 eV by VASE, and in the range 1–12 eV by NNSR at the room temperature. Both types of the experimental data (VASE and NNSR) were simultaneously fitted to obtain the optical functions (e.g. complex refractive index, dielectric function) and thicknesses of the films. To determine the optical response functions and parameters of the material we used several models to fit the experimental data. The Cauchy-Urbach formula was applied in the transparent spectral range and the direct fit procedure and the Cody-Lorentz model were applied around and beyond absorption edge. In the entire spectral range the reflectivity spectra were analyzed by Kramers-Kronig analysis. The behavior of the refractive index obtained in this way was studied and some anomalies were found. The x-ray diffraction was used for a study of sample texture. Some correlation with the optical study was found. The optimal deposition temperature can be found from both studies.