Domain Imaging, Polarization Hysteresis, and Switching in Nano-Size Ferroelectric Structures
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ABSTRACT A fundamental issue in the physics of ferroelectric memories and piezoelectric devices is to study how their properties scale with the size of the ferroelectric structure. In this work we present results concerning the ferroelectric behavior of structures with lateral sizes in the range 100 nm to I ýim. The ferroelectric characterization was achieved using the piezoresponse mode of a scanning force microscope. 100 nm patterned PZT polycrystalline structures as well as 200 nm epitaxial SrBi2Ta 20 9 grains exhibit distinct ferroelectric hysteresis loops. Fatigue measurements on 500 nm structures of PZT were also performed and are discussed.
INTRODUCTION Ferroelectric thin films are very attractive in view of their possible applications in nonvolatile memories and microelectromechanical devices. The decrease in lateral size of ferroelectric devices down to tens of nanometers requires an appropriate description of the basic polarization and electromechanical processes in ferroelectric nanostructures. Therefore, it is of both fundamental and practical interest to investigate how ferroelectric and piezoelectric properties are affected by the size [1]. This paper demonstrates that ferroelectric structures of only a few hundreds of nanometers in lateral size still exhibit piezoelectric and ferroelectric properties. It is shown that using piezoresponse scanning force microscopy (SFM) it is possible not only to image the ferroelectric domain structure, but also to perform local quantitative measurements on these nanostructures. The local piezoelectric coefficient can be estimated using the piezoelectric response of the sample. Measurements on SrBi 2Ta 2O 9 (SBT) and BiYi30 1 2 (BiT) crystallites revealed piezoelectric coefficients of 2 pm/V and 4 pm/V, respectively, under the poling voltages applied. It is also proven that patterned nanostructures of PZT and crystallites of SBT having lateral sizes down to only 100 nm exhibit ferroelectric properties.
EXPERIMENT PZT structures with lateral sizes between 100 nm and Iýtm were patterned using electronbeam direct writing as described previously [2]. Thin films of bismuth-layered perovskites were prepared using pulsed laser deposition. These films consist of a flat c-oriented matrix in which (100)orth and (I I O)orth-oriented crystallites are embedded. Details about the deposition regime and the microstructural characterization of these films were reported elsewhere [3]. The piezoresponse SFM technique used to study the ferroelectric behavior of these nano-size structures is based on the detection of local vibrations of a ferroelectric sample induced by a testing AC signal applied between the conductive tip of the SFM and the bottom electrode of the sample. The global deflection signal contains the mechanical oscillations of the sample underneath the tip.
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Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
measurement [11]. We used this procedure for the hysteresis measurements during the fatigue experiment on the PZT cells 500 nm in lateral s
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