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1199-F01-06

Structural Response of BaTiO3/CaTiO3 Superlattice to Applied Electric Fields Ji Young Jo1, Rebecca J. Sichell, Ho Nyung Lee2, Eric Dufresne3, and Paul G. Evans1 1 Department of Materials Science and Engineering and Materials Science Program, University of Wisconsin-Madison, WI 53706, U.S.A. 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. 3 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, U.S.A.

ABSTRACT The structural response of a ferroelectric BaTiO3/dielectric CaTiO3 superlattice to the bipolar applied electric field was studied using time-resolved x-ray microdiffraction. Structural results were compared to the polarization-electric field hysteresis curve obtained from electrical measurements. The superlattice x-ray reflections were found to have a broad distribution of intensity in reciprocal space under applied electric fields exceeding the nominal coercive electric field. The broad distribution of the lattice constant at high electric fields is compared with a model in which the constituent layers of the superlattice have different coercive fields for the polarization switching.

INTRODUCTION Ferroelectric/dielectric superlattices can be designed to control the polarization of their dielectric components using the electrostatic boundary conditions at the interfaces [1]. The lattice constant of superlattices is electromechanically coupled to the polarization, and depends on the electrostatic state of the superlattice [2]. The structural and electrical properties of the superlattice at zero external electric field have been well established both experimentally [3] and theoretically [4] based on the electrostatic energy argument. How both the overall polarization and the lattice parameter of the superlattice response to applied electric fields, however, remains an important open question. The polarization switching process in the superlattice is affected by the electrostatic energy of neighboring domains with differing polarization states. Nano-scale domain can form within superlattices due to the bound charge at interfaces [5]. Polarization switching results in a coupling between the structural properties and the polarization because the lattice distortion is proportional to electric polarization [6]. By probing the structural and electrical properties of a superlattice during polarization switching we obtain insight into how these electrostatic conditions vary in applied electric fields. Here we combine in-situ studies of lattice distortion and electrical displacement current measurements for a ferroelectric/dielectric superlattice capacitor in applied electric fields. A broad distribution of lattice constant was found in electric field region higher than the nominal coercive field.

EXPERIMENT Each period of the superlattice for this study consisted of 2 unit cells of ferroelectric BaTiO3 and 4 unit cells of dielectric CaTiO3. The superlattice thin film with a nominal thickness of 200 nm was deposited on a 4 nm-thick