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Being important for the antiresonance frequency tuning of tunable thin Film Bulk Acoustic wave Resonators (FBARs), the non-linear electrostrictive coefficient was for the first time calculated for BaTiO3 and SrTiO3 using ab initio methods. Further, taking into account the small difference of obtained values for BaTiO3 and SrTiO3 these results were linearly interpolated to the BaxSr1-xTiO3 (BST) compositions. The obtained values are consistent with previously made order-of-magnitude estimates. Using parameters obtained with first principles calculations, we simulated the resonance parameters of BST based tunable FBARs. Resulting antiresonance tuning was smaller than expected due to the compensation of two competing terms conditioned by linear and non-linear electrostrictions. Our calculations confirm that, for tunable FBAR modeling, it is important to use a polarization-based Landau free energy expansion taking into account both non-linear electrostriction and background permittivity. [ABSTRACT FROM AUTHOR]

A theoretical model for the dc bias dependence of induced acoustic resonances in paraelectric materials is presented. The field dependent piezoelectric constitutive equations were derived from the Landau free energy expansion with respect to the ferroelectric part of the polarization P. To derive the field dependent coefficients correctly, we demonstrate that it is important to take both linear and nonlinear electrostriction as well as the background permittivity into account. Two different resonator geometries, corresponding to the thickness excitation (TE) and the lateral field excitation (LFE) modes, are discussed and compared. In the TE mode the resonance frequency is expected to be much stronger dc bias dependent than the antiresonance frequency. In the LFE mode, both resonance and antiresonance frequencies may exhibit comparable dc bias dependences. In this case the antiresonance frequency shows a stronger tuning with increasing dc bias than the resonance frequency. We model the behavior of the field dependent acoustic resonances in BaxSr1-xTiO3 thin films addressing different compositions and orientations of the films. Our theoretical model corroborates the experimental results available in the literature. [ABSTRACT FROM AUTHOR]

A combination of microscopic and macroscopic methods enables an insight into polarization reversal in the copolymer of vinylidene fluoride and trifluoroethylene. Piezoforce microscopy together with switching charge measurements suggests that the polarization reversal is impeded by the restricted geometry of the domain growth. This effect limits the applicability of the conventional switching model of Kolmogorov-Avrami to the first stage of the polarization reversal. The interface-adjacent passive layer is found to impact the switching properties considerably resulting in a retardation of the polarization reversal. Furthermore, an extraordinary dielectric constant increase is observed in the films with a passive layer due to an additional domain wall contribution. [ABSTRACT FROM AUTHOR]

Direct current (dc) bias-induced acoustic resonance phenomena have been observed in micromachined tunable thin-film capacitors based on paraelectric Ba0.3Sr0.7TiO3 thin films. Both resonance and antiresonance frequencies are dc bias dependent. The antiresonance frequency is only weakly dc bias dependent and shifts slightly down with increasing dc bias. The resonance frequency shows a much stronger dependence on the applied dc bias than the antiresonance frequency, and also shifts down to lower frequencies with increasing bias. The resonance frequency shifted by 2.0% for a frequency of about 3 GHz and an applied field of 730 KV/cm. At the same time the effective electromechanical coupling constant kt,eff2 increased up to 3.6%. The tuning of the resonance frequency depends on the tunability of the film permittivity and on the mechanical load on the piezoactive layer. The constitutive equations controlling the tuning of the resonance and antiresonance frequency were derived from the Landau theory using the P-expansion of its free energies. The theoretical predictions show that, in the considered system, the frequency of the first-order resonance (λ/2 resonance) will always shift down with increasing bias, whereas the antiresonance frequency might shift up or down with increasing bias. The sense of the shift of the antiresonance frequency depends on the sign and the magnitude of the corresponding coefficients of the tensors of linear and nonlinear electrostriction. The theoretical predictions corroborate the experimental observations very well. [ABSTRACT FROM AUTHOR]

Epitaxial SrTiO3 thin films were deposited on (001) LaAlO3 single crystals by pulsed laser deposition. The postannealing effect was studied by annealing the deposited films at 1100 °C in O2 atmosphere for 4 h. It was found that the postannealing treatment enhances the in-plane permittivity of the film. An analysis of the film lattice parameters from selected area electron diffraction and high-resolution electron microscopy shows a relaxation of the compressive strain by annealing. In addition, the density of threading dislocations is significantly reduced, and stacking faults with translation vector R=1/2a<110> resulting from the dissociation of threading dislocations are formed. Both structural and dielectric characterizations indicate that the enhancement of permittivity is mainly due to the relaxation of the misfit strain between the film and substrate. The reduction of the threading dislocation density by annealing may be beneficial to the reduction of the extrinsic dielectric loss in the film. [ABSTRACT FROM AUTHOR]

The dielectric response of ferroelectric-dielectric composites is theoretically addressed. Dielectric permittivity, tunability (relative change of the permittivity driven by dc electric field), and loss tangent are evaluated for various composite models. The analytical results for small dielectric concentration and relative tunability are obtained in terms of the traditional electrostatic consideration. The results for large tunability are obtained numerically. A method is proposed for the evaluation of the tunability and loss at large concentrations of the dielectric. The basic idea of the method is to reformulate the effective medium approach in terms of electrical energies stored and dissipated in the composite. The important practical conclusion of the paper is that, for random ferroelectric-dielectric composite, the addition of small amounts of a linear dielectric into the tunable ferroelectric results in an increase of the tunability of the mixture. The loss tangent of such composites is shown to be virtually unaffected by the addition of moderate amounts of the low-loss dielectric. The experimental data for (Ba,Sr)TiO3 based composites are analyzed in terms of the theory developed and shown to be in a reasonable agreement with the theoretical results. [ABSTRACT FROM AUTHOR]

Structural and dielectric properties of epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates fabricated by a two-step growth technique using pulsed laser deposition were investigated by in situ and ex situ observations in terms of strain relaxation from the lattice mismatch between the film and the substrate. In a first step, a very thin STO layer of less than 10 nm was deposited at low temperature. The main part of the film was deposited in a second step at high temperature, as commonly used for epitaxial growth. In situ reflection high-energy electron diffraction observations showed that almost full strain relaxation was realized before the deposition of the second layer, whereas the normally grown film was not fully relaxed even when the film thickness exceeded 300 nm. The relaxation process of the two-step-grown film took place through the crystallization of the first layer with substantial misfit dislocation formation, which is therefore fundamentally different from that of the normally grown film. Since the normally grown film possesses compressive strain due to less strain relaxation, the two-step-grown film showed larger permittivity and larger tuning compared to the normally grown film. The estimation of the extrinsic loss contribution from the measured loss tangent implied that the two-step growth technique could suppress the extrinsic loss due to a redistribution of defects in the film. The first layer leads to two effects: (i) strain relaxation and (ii) lower loss of the film. [ABSTRACT FROM AUTHOR]

The phenomenon of polarization imprint consisting of the development of a preferential polarization state in ferroelectric films is known as one of the major issues impacting the development of high density ferroelectric memories. According to the commonly accepted scenario, the imprint is related to the charge injection and charge accumulation in the nearby-electrode passive layer of the ferroelectric film. Recent studies demonstrated that the coercive voltage shift induced by the imprint exhibits a nonlinear time dependence in a logarithmic scale. This result was interpreted as the presence of two different imprint mechanisms characterized by different activation energies. In the present work, an analytical theory of the injection scenario of imprint is developed. The charge accumulation at the interface is shown to provoke a voltage offset and polarization loss which are nonlinearly dependent on the time in logarithmic scale. This result is obtained for different charge injection mechanisms including Schottky, Pool-Frenkel, and tunneling scenarios. Thus, it is shown that a single imprint mechanism can be responsible for a nolinear (in logarithmic scale) time dependence of the voltage offset and polarization loss. Additionally, the temperature dependence of the logarithmic rate of imprint is shown to be nonexponential. The developed model ties together the time and temperature dependences of imprint. For the experimental verification of the model a study of imprint has been performed on (111) Pb(Zr,Ti)O3 film capacitors with temperatures ranging from 25 to 150°C and exposure times up to 1000 h. It has been found that the theory developed adequately describes the obtained experimental data. Based upon the theoretical and experimental results a test for ferroelectric memories is proposed, which enables the long-term prediction of polarization loss caused by imprint for a wide temperature range and for different operating voltages. [ABSTRACT FROM AUTHOR]

(001)-epitaxial (Ba0.5Sr0.5)TiO3 (BST) thin films with different magnitudes of compressive strain were fabricated on SrRuO3/SrTiO3 substrates by pulsed laser deposition, and their electro-optic (EO) properties were characterized by modulation ellipsometry at different temperatures. All fabricated films showed an increased paraelectric-to-ferroelectric phase transition temperature upon compressive strain and revealed c-domain structures in the ferroelectric phase. We experimentally clarified that the EO properties of compressively strained BST thin films are enhanced toward the phase transition temperature modified by the strain. The experimental results were compared with the theoretical prediction based on a phenomenological thermodynamic model. Although the measured EO coefficient r c was less than that theoretically predicted, the experimentally observed strain effect on the EO properties is in good qualitative agreement. [ABSTRACT FROM AUTHOR]

Antiphase boundaries (APBs) are unique domain walls that may demonstrate switchable polarization in otherwise non-ferroelectric materials such as SrTiO3 and PbZrO3. The current study explores the possibility of displacing such domain walls at the nanoscale. We suggest the possibility of manipulating APBs using the inhomogeneous electric field of an Atomic Force Microscopy (AFM) tip with an applied voltage placed in their proximity. The displacement is studied as a function of applied voltage, film thickness, and initial separation of the AFM tip from the APB. It is established, for example, that for films with thickness of 15 nm, an APB may be attracted under the tip with a voltage of 25V from initial separation of 30 nm. We have also demonstrated that the displacement is appreciably retained after the voltage is removed, rendering it favorable for potential applications. [ABSTRACT FROM AUTHOR]

Properties of ferroelectric domain walls are attractive for future nano- and optoelectronics. An important element is the potential to electrically erase/rewrite domain walls inside working devices. Dense domain wall patterns, formed upon cooling through the ferroelectric phase transition, were demonstrated. However, room temperature domain wall writing is done with a cantilever tip, one domain stripe at a time, and reduction of the inter-wall distance is limited by the tip diameter. Here, we show, at room temperature, controlled formation of arrays of domain walls with sub-tip-diameter spacing (i.e., inter-wall distance down to 10nm). Each array contains 100-200 concurrently formed walls. Array rewriting is confirmed. The method is demonstrated in several materials. Dense domain pattern formation through a continuous electrode, practical for potential device applications, is also demonstrated. A quantitative theory of the phenomenon is provided. [ABSTRACT FROM AUTHOR]

PbTiO3 thin films were epitaxially grown on (001) KTaO3 single crystal substrates by metalorganic chemical vapor deposition. The coherent epitaxial growth introduced a large in-plane tensile strain to the PbTiO3 film. This tensile strain increased TC and directed the polarization to one of the in-plane (100) axes below TC, resulting in the formation of perfect a1/a2/a1/a2 domain structure. We found that the polar distortion is appreciably suppressed in such a1/a2/a1/a2 domain structure while TC is enhanced due to the strain. [ABSTRACT FROM AUTHOR]

This article presents a correction to an article entitled "Electrical tuning of dc bias induced acoustic resonances in paraelectric thin films," published in the "Journal of Applied Physics," issue 104, 094102 of 2008.

A silicone-based one-transistor nonvolatile memory cell has been implemented by integration of a ferroelectric polymer gate on a standard n-type metal oxide semiconductor field effect transistor. The polarization reversal in the gate results in a stable and reproducible memory effect changing the source-drain current by a factor 102–103, with the retention exceeding 2–3 days. Analysis of the drain current relaxation and time-resolved study of the spontaneous polarization via piezoforce scanning probe microscopy indicates that the retention loss is controlled by the interface-adjacent charge injection rather than the polarization instability. A semiquantitative model describes the time-dependent retention loss characterized by an exponential decay of the open state current of the transistor. The unique combination of properties of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene, including an adequate spontaneous polarization and low dielectric constant as well as rather benign processing demands, makes this material a promising candidate for memories fully compatible with silicon technology. [ABSTRACT FROM AUTHOR]

A phenomenological Landau-theory model, where a crystal on cooling from a state with polar symmetry exhibits a maximum of dielectric permittivity and polarization-field hysteresis loops, is offered. The model is equivalent to that of a true ferroelectric in a weak external dc bias. Permittivity maximum occurs as a result of crossing the Widom line. In the model, the role of the external bias plays an intrinsic internal bias originating from a weak polarity of the high temperature state of the material. The results obtained show that the ferroelectriclike phenomena reported for doped ZnO are compatible with a Landau-theory description. [ABSTRACT FROM AUTHOR]

It is shown that a prominent feature of ferroelectric-dielectric composites—the possibility of reducing their permittivity while keeping their tunability—may be substantially enhanced when using a ferroelectric matrix with “flatten” dielectric inclusions (inclusions with one dimension much smaller than two others). The composite containing flat inclusions oriented normal to the applied electric field is expected to yield a dielectric performance similar to that of the composite with nonflatten inclusions, albeit at much smaller concentrations of the dielectric. This effect is demonstrated from small-signal dielectric nonlinearity in diluted composite with spheroidal dielectric inclusions. It is proposed to use the composites of the considered structure in microwave systems in which low-permittivity tunable materials are required. [ABSTRACT FROM AUTHOR]

Different stages of polarization reversal in a Pb(Zr,Ti)O3 ferroelectric film have been directly observed using a technique of cross-sectional piezoelectric force imaging. Analysis of the local piezoelectric response measured across the cross section strongly suggests that polarization reversal occurs via oblique domain growth rather than by growth of domains perpendicular to the film plane. The proposed approach represents a valuable extension to the piezoelectric force microscopy (PFM) and contributes to a better understanding of results observed using more standard techniques. Particularly, oblique domain growth explains the gradual variation of piezoelectric response amplitude commonly observed in planar PFM images. [ABSTRACT FROM AUTHOR]

A Ba0.3Sr0.7TiO3 (BST) thin film composite structure was fabricated by means of a selective epitaxial growth process. The epitaxial growth of BST on SrRuO3 electrode surface was selectively achieved at 485 °C using a prepatterned ultrathin amorphous BST layer that locally prevented crystallization. This self-buildup mechanism resulted in a columnar composite structure, where epitaxial and amorphous BST columns are electrically connected in parallel. The effective permittivity of the composite capacitors decreased linearly upon increasing the amorphous BST concentration, while the tunability stayed fairly unchanged until it reached 70%. The results agree with the theoretical ferroelectric/dielectric parallel composite model. [ABSTRACT FROM AUTHOR]

Using pulsed-laser deposition, a two-step growth technique was applied to epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates providing a way to obtain an effective strain relaxation in these films otherwise strained due to lattice mismatch between film and substrate. By changing the thickness of a first layer, deposited at a temperature as low as 100 °C before the deposition of the main layer at 750 °C, different strain relaxation states of the films could be systematically realized. With a 10-nm-thick first layer, an almost full strain relaxation at the deposition temperature of the main layer was achieved, suggesting a strong impact of this method on strain relaxation. The in-plane dielectric measurements displayed that the ferroelectric transition temperature increases with strain relaxation during the growth. This trend is correct and compatible with the theoretical prediction of the behavior of strained STO derived from Landau theory. [ABSTRACT FROM AUTHOR]

The change in permittivity of bismuth zinc niobate (BZN) films with the cubic pyrochlore structure under an applied electric field was measured as a function of temperature. Dielectric measurements were performed using parallel-plate capacitor structures with Pt electrodes on sapphire substrates. The electric field tunability of the permittivity was weakly temperature dependent and increased with decreasing temperature up to the onset of dielectric relaxation. At temperatures below the onset of the dielectric relaxation (∼150 K at 1 MHz), larger electric fields were required to achieve the highest tunabilities. A simple model of hopping, noninteracting dipoles was not suited to describe the dielectric tunability of BZN thin films. A better description of the experimentally observed behavior at temperatures above the onset of the dielectric relaxation was obtained using a simple random-field model with hopping dipoles in a uniform distribution of random fields. [ABSTRACT FROM AUTHOR]

The region-by-region polarization switching in ferroelectric Pb(Zr,Ti)O3 thin films sandwiched between Pt electrodes has been directly observed using piezoelectric scanning probe microscopy. A resolution improved by one order-of-magnitude compared to the standard piezoelectric response imaging technique for ferroelectric capacitors was achieved by reducing the top electrode thickness to 10–15 nm through polishing. It was demonstrated that the individually switched regions correspond to single grains or clusters of grains where the grain boundaries act as frontiers limiting the propagation of the switched state. The study of the propagation of the reversed polarization state as a function of voltage applied shows a rather discontinuous growth of the switched areas, the movement of the domain walls being triggered abruptly by different threshold voltages. This result agrees with the earlier proposed nucleation-limited switching model. The observation of the frozen regions that do not switch even at higher voltages provides significant insight into the “bits-failure” problem in submicron ferroelectric capacitors used for nonvolatile memory applications. [ABSTRACT FROM AUTHOR]

It is demonstrated that for classical perovskites, such as BaTiO3, SrTiO3, and PbTiO3, electrostrictive strain induced by an electric field may not obey traditionally considered 'extension along the field and contraction perpendicular to it' behavior if the applied electric field is directed obliquely to the cubic crystallographic directions. A remarkable behavior is predicted for bars of BaTiO3, SrTiO3, and PbTiO3 cut along the [111] crystallographic cubic direction. In this case, in the paraelectric phase in an electric field parallel to the bar, some expansion along all its three dimensions is expected. The situation with the [110] cut is also unusual; the electrostrictive effect in a field along the bar will result in a contraction in one transversal direction and an expansion in the other. In terms of the effective 'polarization-strain' electrostrictive coefficients of the sample, this means a positive transversal electrostriction coefficient for some sample orientations. [ABSTRACT FROM AUTHOR]

Static flexoelectric effect in a finite sample of a solid is addressed in terms of phenomenological theory for the case of a thin plate subjected to bending. It has been shown that despite an explicit asymmetry inherent to the bulk constitutive electromechanical equations which take into account the flexoelectric coupling, there exists a situation where electromechanical response for a finite sample is 'symmetric.' 'Symmetric' means that if a sensor and an actuator are made of a flexoelectric element, performance of such devices can be characterized by the same effective piezoelectric coefficient. This behavior is consistent with the thermodynamic arguments offered earlier, being in conflict with the current point of view on the matter in literature. This result was obtained using standard mechanical boundary conditions valid for the case where the polarization vanishes at the surface. It was shown that, for the case where the polarization at the surface is not zero, the aforementioned symmetry of electromechanical response may be violated if standard mechanical boundary conditions are used, leading to a conflict with the thermodynamic arguments. It is suggested that this conflict may be resolved when using modified mechanical boundary conditions. It is also shown that the contribution of surface piezoelectricity to the flexoelectric response of a finite sample is expected to be comparable to that of the static bulk contribution (including materials with high values of the dielectric constant) and to scale as the bulk value of the dielectric constant (similar to the bulk contribution). This finding implies that if the experimentally measured flexoelectric coefficient scales as the dielectric constant of the material, this does not imply that the measured flexoelectric response is controlled by the static bulk contribution to the flexoelectric effect. [ABSTRACT FROM AUTHOR]