Dynamic Response of the Electro-Optic Effect in Epitaxial Ferroelectric Thin Films
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ABSTRACT The dynamic response of the electro-optic coefficients and the electronic polarizability for epitaxial thin films of KNbO 3 and BaTiO3 are measured. For these two systems a logarithmic dependence of the electro-optic response and the polarization on time was observed after removal of an applied electric field. The dynamic response of the electro-optic effect and the polarization of the films are attributed to the same physical mechanism, which we associate with the dynamic response of ferroelectric nanodomains.
INTRODUCTION Ferroelectric thin films are of considerable promise for use as electro-optic, non-linear optic, and photo-refractive materials. Electro-optic (EO) waveguide devices fabricated from thin films offer several advantages over bulk materials such as lower driving voltages, higher modulation speeds, and the potential for monolithic integration. While the EO effect in several ferroelectric thin film systems has been previously investigated'-3 at low frequencies (_• 1 Hz), the response at higher frequencies is not well understood. 4 Furthermore, a model describing the dynamic EO response of the thin films and its underlying mechanism has not been developed. In this paper the dynamic response of the EO effect and the polarization were investigated for two thin film ferroelectric oxide systems, KNbO, and BaTiO 3. A power law dependence of both the EO response and polarization on time is observed after the removal of an applied electric field. The observed time response is attributed to motion of ferroelectric nanodomains. EXPERIMENTAL Epitaxial films of KNbO3 and BaTiO 3 for this study were prepared by low-pressure metal-organic chemical vapor deposition. The growth and structural characterization have been detailed elsewhere. 5 The films deposited on MgO and MgA12 0 3 substrates are epitaxial as determined by x-ray diffraction and transmission electron microscopy. Film thicknesses ranged from 100 to 300 nm. Index of refraction, n, measurements yielded values comparable to those of bulk materials. For EO and polarization measurements the films were patterned with coplanar surface electrodes with dimensions 2.0 mm x 1.0 mm separated by 5 lrm. The electrodes were aligned parallel to the cubic axes of the substrate. Electrodes were deposited using electron-beam evaporation and consisted of a bilayer of 150 nm of Cr adhesion layer covered by 150 nm of Au. *
Current address: Department of Physics, Illinois Wesleyan University, Bloomington, Il 61702.
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Also with: Department of Electrical and Computer Engineering, Northwestern University. 157
Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
The apparatus used to measure the EO coefficient and its transient response from 10' to The transverse EO coefficient was measured with focused light from a He-Ne laser beam that passes through the electrode gap, normal to the film surface. Polarizers oriented at +45' and -45' with respect to the electrode gap are placed before and after the sample to convert from phase to intensity modu
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