It is well-known that conventional field effect transistors (FETs) require a change in the channel potential of at least 60 mV at 300 K to effect a change in the current by a factor of 10, and this minimum subthreshold slope S puts a fundamental lower limit on the operating voltage and hence the power dissipation in standard FET-based switches. Here, we suggest that by replacing the standard insulator with a ferroelectric insulator of the right thickness it should be possible to implement a step-up voltage transformer that will amplify the gate voltage thus leading to values of S lower than 60 mV/decade and enabling low voltage/low power operation. The voltage transformer action can be understood intuitively as the result of an effective negative capacitance provided by the ferroelectric capacitor that arises from an internal positive feedback that in principle could be obtained from other microscopic mechanisms as well. Unlike other proposals to reduce S, this involves no change in the basic physics of the FET and thus does not affect its current drive or impose other restrictions.

Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices

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

Due to the nature of domains, ferroics, including ferromagnetic, ferroelectric, and ferroelastic materials, exhibit hysteresis phenomena with respect to external driving fields (magnetic field, electric field, or stress). In principle, every ferroic material has its own hysteresis loop, like a fingerprint, which contains information related to its properties and structures. For ferroelectrics, many characteristic parameters, such as coercive field, spontaneous, and remnant polarizations can be directly extracted from the hysteresis loops. Furthermore, many impact factors, including the effect of materials (grain size and grain boundary, phase and phase boundary, doping, anisotropy, thickness), aging (with and without poling), and measurement conditions (applied field amplitude, fatigue, frequency, temperature, stress), can affect the hysteretic behaviors of the ferroelectrics. In this feature article, we will first give the background of the ferroic materials and multiferroics, with an emphasis on ferroelectrics. Then it is followed by an introduction of the characterizing techniques for the loops, including the polarization–electric field loops and strain–electric field curves. A caution is made to avoid misinterpretation of the loops due to the existence of conductivity. Based on their morphologic features, the hysteresis loops are categorized to four groups and the corresponding material usages are introduced. The impact factors on the hysteresis loops are discussed based on recent developments in ferroelectric and related materials. It is suggested that decoding the fingerprint of loops in ferroelectrics is feasible and the comprehension of the material properties and structures through the hysteresis loops is established.

Decoding the Fingerprint of Ferroelectric Loops: Comprehension of the Material Properties and Structures

Piezoelectric materials, which respond mechanically to applied electric field and vice versa, are essential for electromechanical transducers. Previous theoretical analyses have shown that high piezoelectricity in perovskite oxides is associated with a flat thermodynamic energy landscape connecting two or more ferroelectric phases. Here, guided by phenomenological theories and phase-field simulations, we propose an alternative design strategy to commonly used morphotropic phase boundaries to further flatten the energy landscape, by judiciously introducing local structural heterogeneity to manipulate interfacial energies (that is, extra interaction energies, such as electrostatic and elastic energies associated with the interfaces). To validate this, we synthesize rare-earth-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), as rare-earth dopants tend to change the local structure of Pb-based perovskite ferroelectrics. We achieve ultrahigh piezoelectric coefficients d33 of up to 1,500 pC N-1 and dielectric permittivity e33/e0 above 13,000 in a Sm-doped PMN-PT ceramic with a Curie temperature of 89 °C. Our research provides a new paradigm for designing material properties through engineering local structural heterogeneity, expected to benefit a wide range of functional materials.

Ultrahigh piezoelectricity in ferroelectric ceramics by design

Ferroelectrics are essential components in a wide range of applications, including ultrasonic transducers, sensors, and actuators. In the single crystal form, relaxor-PbTiO3 (PT) piezoelectric materials have been extensively studied due to their ultrahigh piezoelectric and electromechanical properties. In this article, a perspective and future development of relaxor-PT crystals are given. Initially, various techniques for the growth of relaxor-PT crystals are reviewed, with crystals up to 100 mm in diameter and 200 mm in length being readily achievable using the Bridgman technique. Second, the characterizations of dielectric and electromechanical properties are surveyed. Boundary conditions, including temperature, electric field, and stress, are discussed in relation to device limitations. Third, the physical origins of the high piezoelectric properties and unique loss characteristics in relaxor-PT crystals are discussed with respect to their crystal structure, phase, engineered domain configuration, macr...

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High performance ferroelectric relaxor-PbTiO3 single crystals: Status and perspective

The importance of a high shear coefficient d15 (or d24) to the piezoelectric properties of domain-engineered and polycrystalline ferroelectrics is discussed. The extent of polarization rotation, as a mechanism of piezoelectric response, is directly correlated to the shear coefficient. The terms “rotator” and “extender” are introduced to distinguish the contrasting behaviors of crystals such as 4mm BaTiO3 and PbTiO3. In rotator ferroelectrics, where d15 is high relative to the longitudinal coefficient d33, polarization rotation is the dominant mechanism of piezoelectric response; the maximum longitudinal piezoelectric response is found away from the polar axis. In extender ferroelectrics, d15 is low and the collinear effect dominates; the maximum piezoelectric response is found along the polar axis. A variety of 3m, mm2, and 4mm ferroelectrics, with various crystal structures based on oxygen octahedra, are classified in this way. It is shown that the largest piezoelectric anisotropies d15∕d33 are always fo...

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Rotator and extender ferroelectrics: Importance of the shear coefficient to the piezoelectric properties of domain-engineered crystals and ceramics

The importance of a high shear coefficient d15 (or d24) to the piezoelectric properties of domain-engineered and polycrystalline ferroelectrics is discussed. The extent of polarization rotation, as a mechanism of piezoelectric response, is directly correlated to the shear coefficient. The terms "rotator" and "extender" are introduced to distinguish the contrasting behaviors of crystals such as 4mm BaTiO3 and PbTiO3. In "rotator" ferroelectrics, where d15 is high relative to the longitudinal coefficient d33, polarization rotation is the dominant mechanism of piezoelectric response; the maximum longitudinal piezoelectric response is found away from the polar axis. In "extender" ferroelectrics, d15 is low and the collinear effect dominates; the maximum piezoelectric response is found along the polar axis. A variety of 3m, mm2 and 4mm ferroelectrics, with various crystal structures based on oxygen octahedra, are classified in this way. It is shown that the largest piezoelectric anisotropies d15/d33 are always found in 3m crystals; this is a result of the intrinsic electrostrictive anisotropy of the constituent oxygen octahedra. Finally, for a given symmetry, the piezoelectric anisotropy increases close to ferroelectric-ferroelectric phase transitions; this includes morphotropic phase boundaries and temperature induced polymorphic transitions.

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The orientation dependence of the longitudinal piezoelectric coefficient, d(33)(*), is investigated as a function of temperature in BaTiO3 and PbTiO3 crystals using the Landau-Ginsburg-Devonshire theory. We show that a presence of the ferroelectric-ferroelectric phase transitions in BaTiO3 leads to enhanced d(33)(*) along nonpolar directions. The reason for this is that in the vicinity of a phase transition temperature at which a polarization vector changes its direction (tetragonal-orthorhombic/monoclinic, orthorhombic/monoclinic-rhombohedral), the shear piezoelectric coefficients become high. It is shown for all ferroelectric phases of BaTiO3 that the shear stress deforms the crystal cell and changes the polarization direction in a similar way as the corresponding temperature-induced phase transition. The influence of the piezoelectric shear effect on the anisotropy of d(33)(*) is particularly pronounced in the orthorhombic/monoclinic phase where the piezoelectric shear coefficients are determined by the presence of both the high-temperature tetragonal and the low-temperature rhombohedral phases. In PbTiO3, which does not exhibit ferroelectric-ferroelectric phase transitions, the shear piezoelectric effect is weak and d(33)(*) has its maximum along the polar axis at all temperatures. These results can be generalized to include phase transitions induced by electric-field and composition variations and are valid for all perovskite materials, including complex relaxor-ferroelectric perovskites that have recently attracted attention for their exceptionally large piezoelectric properties. (C) 2003 American Institute of Physics.

Piezoelectric anisotropy–phase transition relations in perovskite single crystals

The question of the origin of the piezoelectric properties enhancement in perovskite ferroelectrics is approached by analyzing the Gibbs free energy of tetragonal $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$, and $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ in the framework of the Landau-Ginzburg-Devonshire theory. The flattening of the Gibbs free-energy profile appears as a fundamental thermodynamic process behind the piezoelectric enhancement. The generality of the approach is demonstrated by examining the free-energy flattening and piezoelectric enhancement as a function of composition, temperature, electric field, and mechanical stress. It is shown that the anisotropy of the free-energy flattening is the origin of the anisotropic enhancement of the piezoelectric response, which can occur either by polarization rotation or by polarization contraction. Giant enhancement of the longitudinal piezoelectric response $({d}_{33}\ensuremath{\propto}{10}^{3}\phantom{\rule{0.3em}{0ex}}\mathrm{pC}∕\mathrm{N})$ is predicted in $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ under uniaxial compression.

Piezoelectric response and free-energy instability in the perovskite crystals Ba Ti O 3 , Pb Ti O 3 , and Pb ( Zr , Ti ) O 3

Using recently published experimental data we calculated the piezoelectric response of rhombohedral 067Pb(Mg1/3Nb2/3)O3–033PbTiO3 monodomain single crystals along arbitrary directions The calculations indicate that the value of the longitudinal piezoelectric coefficient along the [001] axis of the pseudocubic system ([111] axis of the rhombohedral system) of a monodomain crystal is close to the value determined experimentally along the same direction of a multidomain crystal with a so-called “engineered domain state” The results thus show that the large piezoelectric response in this material is dominated by intrinsic crystal lattice effects and that the multidomain state has a relatively minor effect ( 1900 pC/N for certain directions of the monodomain crystal

Marko Budimir

Papers

Rotator and extender ferroelectrics: Importance of the shear coefficient to the piezoelectric properties of domain-engineered crystals and ceramics

Rotator and extender ferroelectrics: Importance of the shear coefficient to the piezoelectric properties of domain-engineered crystals and ceramics

Piezoelectric anisotropy–phase transition relations in perovskite single crystals

Piezoelectric response and free-energy instability in the perovskite crystals Ba Ti O 3 , Pb Ti O 3 , and Pb ( Zr , Ti ) O 3

Monodomain versus polydomain piezoelectric response of 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals along nonpolar directions