Spectroscopic ellipsometry studies of ferroelectric thin films

Abstract

Spectroscopic ellipsometry (SE) has been used to depth profile the optical properties and the microstructural inhomogeneities present in ferroelectric thin films. It was found that many film deposition techniques engender remanent microstructural defects, including voids distributed inhomogeneously throughout the film thickness and surface roughness. In this paper, depth profile information from SE studies is combined with mathematical modeling to describe the way in which microstructural inhomogeneities influence the observed electrical and optical properties of ferroelectric thin films. Many of the differences between the electrical properties of thin films and bulk ferroelectrics of the same composition can be attributed to such defects. In particular, it was shown that high values for the coercive field, low values for the remanent polarization and the dielectric constant, and the loss of the marked maximum in the dielectric constant as a function of temperature can be attributed in part to common microstructural heterogeneities. The effect of such incorporated porosity on the bulk properties overwhelms any changes which might occur due to intrinsic size effects. SE has also been utilized to follow the annealing of films to determine at what point during the processing the inhomogeneities are generated. It was found that for both sol-gel and ion beam sputtered films which were homogeneous as deposited, crystallization of the perovskite phase led to generation of significant levels of surface roughness. Consequently, in preparation of thin films for electrooptic applications, control of the nucleation process may be essential for production of smooth surfaces.