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

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Perovskite lead-free dielectrics for energy storage 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

This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

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Piezoelectric and ferroelectric materials and structures for energy harvesting applications

A bismuth ferrite and barium titanate solid solution compound can achieve good piezoelectric properties with a high Curie temperature when fabricated with low-temperature sintering followed by a water-quenching process, with no complicated grain alignment processes performed. By adding the super-tetragonal bismuth gallium oxide to the compound, the piezoelectric properties are as good as those of lead zirconate titanate ceramics.

High-Performance Lead-Free Piezoceramics with High Curie Temperatures

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.

Polarization Relaxation Induced by a Depolarization Field in Ultrathin Ferroelectric BaTiO 3 Capacitors

We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.

Domain switching kinetics in disordered ferroelectric thin films.

Effects of Bi nonstoichiometry on (Bi0.5+xNa)TiO3 (BNT) ceramics were investigated at x=−1–+2 mol % of Bi0.5 covering Bi deficiency and excess. At all compositions, rhombohedrally symmetric BNT perovskite formed without secondary phases. Increasing x caused smaller grains. Higher piezoelectric coefficient (d33) but lower depolarization temperature (Td) occurred at Bi excess than at Bi deficiency and vice versa. Leakage current at room temperature decreased with decreasing x. Electrical conductivity of the stoichiometric BNT (x=0) from 700 to 900 °C increased with decreasing partial oxygen pressure from 1 to 10−5 atm suggesting n-type conductivity at elevated temperatures.

Effects of Bi nonstoichiometry in (Bi0.5+xNa)TiO3 ceramics

Effects of Na nonstoichiometry in (Bi0.5Na0.5+x)TiO3 ceramics were investigated in the range of x=(−5)−(+1) mol % of Na0.5. A rhombohedral perovskite structure was maintained at all compositions with no secondary phases. Grain size decreased with Na nonstoichiometry. Piezoelectric coefficient (d33) increased from 74 pC/N at x=0 up to 91 pC/N at x=−3.5 then dropped with further Na deficiency. Depolarization temperature (Td), on the other hand, decreased from 190 °C at x=0 down to 112 °C at x=−3.5 then increased. Rhombohedral lattice distortion (90−α) decreasing with Na deficiency was a key factor for the variation in d33 and Td.

Tae Kwon Song

Papers

High-Performance Lead-Free Piezoceramics with High Curie Temperatures

Polarization Relaxation Induced by a Depolarization Field in Ultrathin Ferroelectric BaTiO 3 Capacitors

Domain switching kinetics in disordered ferroelectric thin films.

Effects of Bi nonstoichiometry in (Bi0.5+xNa)TiO3 ceramics

Effects of Na nonstoichiometry in (Bi0.5Na0.5+x)TiO3 ceramics