Raman Spectroscopy of Ferroelectric Thin Films
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Raman Spectroscopy of Ferroelectric Thin Films R. S. Katiyar, A. Dixit, M. Jain, A.A .Savvinov, and P.S. Dobal Department of Physics, University of Puerto Rico, San Juan, PR-00931-3343 ABSTRACT During a ferroelectric phase transition or domain rearrangement, ions or molecules in a ferroelectric material move in a highly cooperative manner from their initial lattice positions into the final positions they occupy and the collective response results into a “soft” lattice vibrational mode. Moreover, the structural changes are always accompanied by at least a few other changes in the normal mode behavior of the material. In the present work, Raman spectroscopy is conveniently employed to study such vibrational modes and other related phenomena in ferroelectric materials at the sub-microscale levels. We have investigated ferroelectric thin films of various lead and barium based perovskites prepared by sol-gel technique. The effect of processing conditions, A- and B- site substitutions, and size dependence on their Raman spectra were analyzed in terms of the structure-property correlations. INTRODUCTION Perovskites ferroelectric thin films are important materials because of their potential applications in a number of next generation of advanced and compact integrated electronic devices. These materials demonstrate remarkable ferroelectric, dielectric, piezoelectric, pyroelectric properties, and nonlinear optical behavior that can be tailored in the appropriate operating temperature region [1]. With the chemical formula of ABO3, these compounds are generally composed of a three-dimensional framework of corner sharing BO6 octahedra with the cation A in the center. Many perovskites are also well known for their phase transition behavior possibly affecting their physical and chemical properties significantly. The ternary systems formed by the compounds such as barium based or lead based compositions, are of particular interest in the field of dielectrics [2,3]. The ferroelectric characteristics of perovskites can be controlled by replacing A and B cations. PbxSr1-xTiO3 (PST) and BaZrxTi1-xO3 (BZT) systems classically illustrate ferroelectric solid solutions with a perovskite structure [4] with A and B site substitutions respecteively. Although perovskite materials have been studied for over half of a century the exact reason why similar, but chemically different perovskites, display very distinct ferroelectric behavior, remains unknown. Raman scattering, being nondestructive, can be suitably employed in-situ in the fabrication processes. A comprehensive review on Raman spectroscopy of ferroelectrics has been published recently by Dobal and Katiyar [5]. In the present work, Raman scattering results of two perovskite systems with different ferroelectric properties, namely PST and BZT in thin film forms, are discussed. PST is a typical ferroelectric with a tetragonal-to-cubic phase transition; where as BZT exhibits a sequence of phase transitions at lower concentrations of Zr contents, and with the increase of Zr to abov
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