Spectroscopic ellipsometry of epitaxially stressed ferroelectric films

Abstract

Explicit knowledge of the optical properties of epitaxial perovskite oxide ferroelectric thin films is crucial for photonic applications and fundamental understanding of such films. Accurate assessment of these properties is difficult because of the presence of substrate and substrate-imposed stress, as well as small thicknesses of the films. Here, we explore capabilities of spectroscopic ellipsometry to establish the optical NIR–VIS–VUV dielectric function in epitaxial Ba0.5Sr0.5TiO3 films (thicknesses from 15 to 100 nm) subjected to substrate-imposed in-plane compression. The experimentally acquired data were processed assuming films which are either optically homogeneous or gradually varying out-of-plane, or containing two distinct phases. The obtained results were evaluated considering mathematical accuracy of the models and physical relevance of the extracted dielectric functions. It is shown that homogeneous approximation is valid for films with thicknesses of 15 and 100 nm. The homogeneous approximation is unsuitable, whereas the others are marginally acceptable, for films with thicknesses of 30 and 50 nm. The results are discussed in terms of substrate-induced inhomogeneous stress. The demonstrated approach can be useful for ellipsometric investigations of many other epitaxial films of ferroelectrics and related materials.