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

Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.

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Domain wall nanoelectronics

Ferroelectric oxide materials have offered a tantalizing potential for applications since the discovery of ferroelectric perovskites more than 50 years ago. Their switchable electric polarization is ideal for use in devices for memory storage and integrated microelectronics, but progress has long been hampered by difficulties in materials processing. Recent breakthroughs in the synthesis of complex oxides have brought the field to an entirely new level, in which complex artificial oxide structures can be realized with an atomic-level precision comparable to that well known for semiconductor heterostructures. Not only can the necessary high-quality ferroelectric films now be grown for new device capabilities, but ferroelectrics can be combined with other functional oxides, such as high-temperature superconductors and magnetic oxides, to create multifunctional materials and devices. Moreover, the shrinking of the relevant lengths to the nanoscale produces new physical phenomena. Real-space characterization and manipulation of the structure and properties at atomic scales involves new kinds of local probes and a key role for first-principles theory.

http://science.sciencemag.org/content/sci/303/5657/488.full.pdf

Ferroelectricity at the Nanoscale: Local Polarization in Oxide Thin Films and Heterostructures

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

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Advanced scanning probe lithography

Ferroelectrics are materials exhibiting spontaneous electric polarization due to dipoles formed by displacements of charged ions inside the crystal unit cell. Their exceptional properties are exploited in a variety of microelectronic applications. As ferroelectricity is strongly influenced by surfaces, interfaces and domain boundaries, there is great interest in exploring how the local atomic structure affects the electric properties. Here, using the negative spherical-aberration imaging technique in an aberration-corrected transmission electron microscope, we investigate the cation-oxygen dipoles near 180 degrees domain walls in epitaxial PbZr(0.2)Ti(0.8)O(3) thin films on the atomic scale. The width and dipole distortion across a transversal wall and a longitudinal wall are measured, and on this basis the local polarization is calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls is reported.

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Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films

This article describes a new scanning technique for imaging the state of spontaneous polarization of a ferroelectric material by measuring the microscopic point‐to‐point variation of its nonlinear dielectric constants. First, the theory for detecting polarization is described. Second, the technique for measuring the nonlinear dielectric response is described. Finally, using this new microscope, area scans are obtained of the polarization of poled lead zirconate titanate ceramics, a lithium niobate single crystal, and of piezoelectric thin films of the copolymer of vinylidene fluoride and trifluoroethylene.

Scanning nonlinear dielectric microscope

Nanosized inverted domain dots in ferroelectric materials have potential applications in ultrahigh-density rewritable data storage systems. Here, a data storage system based on scanning nonlinear dielectric microscopy and thin films of ferroelectric single-crystal lithium tantalite is presented. Through domain engineering, nanosized inverted domain dots have been successfully formed at a data density of 1.50 Tbit/in.2.

Tbit/inch2 ferroelectric data storage based on scanning nonlinear dielectric microscopy

A very high-resolution scanning nonlinear dielectric microscope was developed for the observation of ferroelectric polarization. We demonstrate that the resolution of the microscope is of a nanometer order by measurement of the c–c domain wall of a BaTiO3 single crystal, and that this microscope is very useful not only for the domain observation of ferroelectric bulk material but also for that of thin films.

Scanning nonlinear dielectric microscopy with nanometer resolution

All eight electrostrictive, all three third‐order dielectric, all thirteen third‐order piezoelectric, and all fourteen third‐order elastic constants of lithium niobate have been determined. Measurements for the electrostrictive constants have been made by measuring the electric capacity in selected directions when the directed static stresses were applied to the crystal. The third‐order dielectric constants have been calculated from these measured electrostrictive constants and the measured capacitance as functions of the applied static electric field. The third‐order piezoelectric and elastic constants have both been determined from the above‐mentioned two kinds of measured constants and the measured velocities for small‐amplitude ultrasonic waves as functions of several applied stresses and static electric fields.

Nonlinear, elastic, piezoelectric, electrostrictive, and dielectric constants of lithium niobate

We have investigated microscale to nanoscale ferroelectric domain and surface engineering of a near-stoichiometric LiNbO3 crystal by using scanning force microscopy. The single crystals LiNbO3 fixed on metal substrates were polished to a 5 μm thickness. Artificial patterns of inverted-domain structures were fabricated in the samples, where polarization directions of the domains were switched by scanning the samples with a conductive cantilever while applying voltages. Furthermore, the negatively polarized surfaces in the patterns were preferentially etched in HF solution. As a result, cavity and mound-shaped surfaces were fabricated; these structures could be used to create functional templates and devices.

Yasuo Cho

Papers

Scanning nonlinear dielectric microscope

Tbit/inch2 ferroelectric data storage based on scanning nonlinear dielectric microscopy

Scanning nonlinear dielectric microscopy with nanometer resolution

Nonlinear, elastic, piezoelectric, electrostrictive, and dielectric constants of lithium niobate

Microscale to nanoscale ferroelectric domain and surface engineering of a near-stoichiometric LiNbO3 crystal