A. K. Tagantsev

Room-temperature ferroelectricity in strained SrTiO3

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (Tc) is traditionally accomplished by chemical substitution—as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (εr) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust Tc in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in Tc is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high εr at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features

The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.

Domains in Ferroic Crystals and Thin Films

A Preview of Concepts and Phenomena.- Fundamentals of Ferroic Domain Structures.- Ferroic Materials.- Methods for Observation of Domains.- Static Domain Patterns.- Domain Walls at Rest.- Switching Properties: Basic Methods and Characteristics.- Switching Phenomena and Small-Signal Response.- Ferroelectric Thin Films.

Fundamentals of flexoelectricity in solids

The flexoelectric effect is the response of electric polarization to a mechanical strain gradient. It can be viewed as a higher-order effect with respect to piezoelectricity, which is the response of polarization to strain itself. However, at the nanoscale, where large strain gradients are expected, the flexoelectric effect becomes appreciable. Besides, in contrast to the piezoelectric effect, flexoelectricity is allowed by symmetry in any material. Due to these qualities flexoelectricity has attracted growing interest during the past decade. Presently, its role in the physics of dielectrics and semiconductors is widely recognized and the effect is viewed as promising for practical applications. On the other hand, the available theoretical and experimental results are rather contradictory, attesting to a limited understanding in the field. This review paper presents a critical analysis of the current knowledge on the flexoelectricity in common solids, excluding organic materials and liquid crystals.

Interface-induced phenomena in polarization response of ferroelectric thin films

This article reviews the existing theoretical models describing the interface-induced phenomena which affect the switching characteristics and dielectric properties of ferroelectric thin films. Three groups of interface-induced effects are addressed—namely, “passive-layer-type” effects, ferroelectric-electrode contact potential effects, and the poling effect of the ferroelectric-electrode interface. The existing experimental data on dielectric and switching characteristics of ferroelectric thin film capacitors are discussed in the context of the reviewed theories. Special attention is paid to the case of internal bias field effects.

Discrimination between bulk and interface scenarios for the suppression of the switchable polarization (fatigue) in Pb(Zr,Ti)O3 thin films capacitors with Pt electrodes

A simple and reliable method which allows one to distinguish between the two major microscopic scenarios for the suppression of the switching polarization (Prs), i.e., pinning of ferroelectric domain walls (DWs) through the Pb(Zr,Ti)O3 film (PZT) (bulk scenario) and inhibition of the growth of opposite domains due to the nucleus suppression at the electrode interfaces (interface scenario), is proposed. In addition, a new electric treatment able to significantly suppress Prs in Pt–PZT–Pt ferroelectric capacitors (FECAPs) of thicknesses above 1.4 μm, was discovered and studied. It consists of the application of an external alternating electric field (Ee) which cycles the polarization at very low frequency (1.7 mHz). After only 10–20 cycles, Prs can be suppressed by a factor of 10. The same FECAP, when subjected to Ee at higher frequency (30 kHz), endures at least 108 switches, before attaining an equivalent Prs suppression (hereafter called fatigue). The fatigued states obtained with the two different proce...

Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors

Stress-induced changes in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3 (PZT)-based capacitors have been studied using piezoresponse force microscopy. Visualization of polarization distribution and d33-loop measurements in individual 1×1.5-μm2 capacitors before and after stress application, generated by substrate bending, provided direct experimental evidence of stress-induced switching. Mechanical stress caused elastic switching in capacitors with the direction of the resulting polarization determined by the sign of the applied stress. In addition, stress application turned capacitors into a heavily imprinted state characterized by strongly shifted hysteresis loops and almost complete backswitching after application of the poling voltage. It is suggested that substrate bending generated a strain gradient in the PZT layer, which produced asymmetric lattice distortion with preferential polarization direction and triggered polarization switching due to the flexoelectric effect.

Injection-controlled size effect on switching of ferroelectric thin films

The effect of near-by-electrode charge injection on switching of a thin film ferroelectric capacitor is theoretically analyzed. We develop a model of switching affected by charge injection through a surface dielectric layer to calculate the coercive field of the capacitor as a function of both film thickness and maximal polarization of the switching cycle. The predictions of the model are verified by electrical measurements on sol–gel derived Pb(Zr, Ti)O3 thin films of thickness ranging from 100 to 1000 nm with Pt electrodes. The model gives a good description of the size effect on switching in the Pt/Pb(Zr, Ti)O3/Pt system and enables an explanation for a much smaller magnitude of this effect in Bi-containing and oxide–electrode thin films.

Direct evidence for Vögel–Fulcher freezing in relaxor ferroelectrics

Direct evidence for freezing in two relaxor ferroelectrics, PbMg1/3Nb2/3O3 and PbSc1/2Ta1/2O3, at a certain temperature Tf≠0 K was obtained from the temperature dependence of the maximum relaxation time, τmax. It was shown that τmax(T) follows the Vogel–Fulcher (VF) law, and tends toward infinity as temperature approaches Tf. The freezing temperature Tf derived from τmax(T) coincides with the value of Tf obtained from the fit of frequency, ω, dependence of the temperature, Tmax, of the dielectric permittivity maximum to the same VF law. Thus, this result solves the controversy that the VF law for ω(Tmax) may not correspond to the freezing of the spectrum.

Top-interface-controlled switching and fatigue endurance of (Pb,La)(Zr,Ti)O3 ferroelectric capacitors

Mechanisms of polarization switching and fatigue in (Pb,La)(Zr,Ti)O3 (PLZT) films are studied by comparative analysis of degradation and leakage conduction of PLZT capacitors with Pt, SrRuO3 (SRO), and layered Pt/SRO (80/5 nm) electrodes. It is found that the asymmetrical Pt/SRO/PLZT/Pt structure exhibits a good fatigue performance in combination with low leakage like that on identically processed Pt/PLZT/Pt capacitor. This asymmetrical structure exhibits very unusual dependence of the endurance of switching polarization on the driving alternating-current (ac) electric field amplitude. Specifically, for high ac electric field amplitude it shows good switching endurance similar to SRO/PLZT/SRO capacitors, whereas for amplitude lower than 80 kV/cm a pronounced polarization fatigue similar to that of the conventional Pt/PLZT/Pt capacitors is observed. Based on the analysis of our results on fatigue and leakage conduction we conclude that: (i) In asymmetrical Pt/SRO/PLZT/Pt system the physical mechanisms of p...