Switching Properties of Finite-Sized Ferroelectrics

Abstract

The characteristic property of ferroelectric materials, which is the reversal of polarization by an external electric field, is of technological importance in device applications, particularly in nonvolatile ferroelectric random access memories (NV-FeRAMs). These binary coded NV-FeRAMs can be fabricated by using ferroelectric materials in which the polarization direction can be switched between two stable states when a minimum electric field is applied. To fabricate good quality NV-FeRAMs to meet the demands of the current market, the ability to achieve low coercive field c E (the minimum external field required to reverse the direction of remnant polarization), short switching time s t and high packing density (Scott, 2000; Dawber et al., 2005) in the memory chips are great challenges. These challenging factors are closely knitted with the underlying physics on the switching properties of ferroelectric materials. Though the subject of interest has been elucidated both theoretically and experimentally over the past sixty years and the achievements are enormous, but the challenging factors mentioned are still current. Auciello, Scott and Ramesh (1998) have explicitly outlined four main problems in NV-FeRAMs fabrication which are related to basic physics. Firstly, what is the ultimate polarization switching speed? Secondly, what is the thinnest ferroelectric layer which sustains stable polarization? Thirdly, how do switching parameters, such as coercive field, depend on frequency? Lastly, how small can a ferroelectric capacitor be and still maintain in ferroelectric phase? These are fundamental problems which should be tackled through continuous experimental and theoretical efforts. From the theoretical perspective on this area of studies, a few models (Duiker and Beale, 1990; Orihara et al., 1994; Hashimoto et al., 1994; Shur et al., 1998) were proposed to study the switching properties of ferroelectric thin film based on the Kolmogorov-Avrami theory of crystallization kinetics (Avrami, 1939, 1940 and 1941). In these models, the authors focused on statistics of domain coalescence. Tagantsev et al. (2002) proposed another model based on the experimental work of a few groups (Lohse et al. 2001; Colla et al., 1998a; Ganpule et al., 2000). Their model also focuses on statistics of domains nucleation. Another approach, which is different from the classical nucleation reversal mechanism, is based on the Landautype-free energy for inhomogeneous ferroelectric system as discrete lattices of electric