This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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Physics of thin-film ferroelectric oxides

An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

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Ferroelectric thin films: Review of materials, properties, and applications

Memristors are devices whose dynamic properties are of interest because they can mimic the operation of biological synapses. The demonstration that ferroelectric domains in tunnel junctions behave like memristors suggests new approaches for designing neuromorphic circuits.

A ferroelectric memristor

Fundamentals of Piezoelectricity- Thermodynamics of Ferroelectricity- Piezoelectricity and Crystal Symmetry- Crystal Chemistry of Piezoelectric Materials- Piezoelectric and Acoustic Materials for Transducer Technology- Lead-Based Piezoelectric Materials- KNN-Based Piezoelectric Ceramics- Bismuth-based Piezoelectric Ceramics- Electropolymers for Mechatronics and Artificial Muscles- Low-Attenuation Acoustic Silicone Lens for Medical Ultrasonic Array Probes- Carbon-Fiber Composite Materials for Medical Transducers- Transducer Design and Principles- Piezoelectric Transducer Design for Medical Diagnosis and NDE- Piezoelectric Transducer Designs for Sonar Applications- Finite Element Analysis of Piezoelectric Transducers- Piezoelectric Transducer Fabrication Methods- Piezoelectric Fiber Composite Fabrication- Composition Gradient Actuators- Robocasting of Three-Dimensional Piezoelectric Structures- Micropositioning- Piezoelectric Actuator Designs- Piezoelectric Energy Harvesting using Bulk Transducers- Piezocomposite Ultrasonic Transducers for High-Frequency Wire Bonding of Semiconductor Packages- Piezoelectric MEMS: Materials and Devices- High-Frequency Ultrasonic Transducers and Arrays- Micromachined Ultrasonic Transducers

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Piezoelectric and Acoustic Materials for Transducer Applications

The switching kinetics in ferroelectric thin films has been intensively studied during the past decade. It is widely accepted that this kinetics is basically governed by the dynamics of domain coalescence (the Kolmogorov-Avrami-Ishibashi model). This conclusion is mainly supported by fitting the time dependence of the switching currents to that predicted by this model, the fit being typically performed in a 1-2 decade interval of time. The present paper reports on a study of the switching kinetics in modified Pt/Pb(Zr,Ti)O-3/Pt thin films as a function of time and applied voltage, performed in time intervals from 10 ns to 1s. Our experimental data show that both the time and applied field dependences of the switching polarization (when monitored over a wide enough time interval) are in a strong qualitative disagreement with the predictions of the Kolmogorov-Avrami-Ishibashi approach. For the interpretation of our result, an alternative approach is forwarded. In contrast to Kolmogorov-Avrami-Ishibashi approach, we assume that the film consists of many areas, which have independent switching dynamics. The switching in an area is considered to be triggered by an act of the reverse domain nucleation. The switching kinetics is described in terms of the distribution function of the nucleation probabilities in these areas. The developed approach enables a good description of the polarization dynamics in typical ferroelectric thin films for memory applications.

Non-Kolmogorov-Avrami switching kinetics in ferroelectric thin films

The D - E hysteresis loop of ferroelectrics is theoretically studied on the basis of the extended Avrami theory. If the sideway velocity depends only on the instant value of the applied field, the volume fraction of the reversed area is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . This result is obtained irrespective of the waveform of the applied field and the field dependence of the sideway velocity, if the nucleation event is deterministic. For the stochastic nucleation due to thermal fluctuation, on the other hand, the above result is modified as q ( E )=1-exp (- f -( d +1) Φ( E )). The effect of the delay of wall motion is also discussed.

A Theory of D-E Hysteresis Loop Based on the Avrami Model

We measured the D - E hysteresis loop of TGS (triglycine sulphate), and transformed it to the q - E hysteresis loop by a method proposed in this paper, where q is the volume fraction of reversed domains. The experimental result was compared with a theory based on the Avrami model by the present authors [H. Orihara, S. Hashimoto and Y. Ishibashi: J. Phys. Soc. Jpn. 63 (1994) 1031], in which the volume fraction is expressed as q ( E )=1-exp (- f - d Φ( E )), where f and d are, respectively, the frequency of the applied field and the growth dimension of domains, and Φ is a function of E . We obtained a good agreement between them and confirmed the validity of our theory. The frequency dependences of the coercive field and the area inside the loop were measured and power laws were obtained with respect to them, which are discussed on the basis of the theory.

Shigeharu Hashimoto

Papers

A Theory of D-E Hysteresis Loop Based on the Avrami Model

Study on D-E Hysteresis Loop of TGS Based on the Avrami-Type Model