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Direct current (dc) bias-induced acoustic resonance phenomena is observed in micromachined tunable thin-film capacitors based on paraelectric [Ba.sub.0.3][Sr.sub.0.7]Ti[O.sub.3] thin films. The frequency of the first-order resonance has shifted down with increasing bias, whereas the antiresonance frequency might shift up or down with increasing bias.

The epitaxial SrTi[O.sub.3] thin films were deposited on (001) LaAl[O.sub.3 single crystals by pulsed laser deposition to study the postannealing effect at 1100 degree Celsius in [O.sub.2] atmosphere for 4 h. The 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 effect of dilution of tunable ferroelectric with linear materials are theoretically addressed. The main impact of the dilution on the main dielectric parameters of the material is shown to be very sensitive to the composite type.

Structural and dielectric properties of strain-controlled epitaxial SrTiO3 (STO) films by two-step growth technique are investigated using in situ and ex situ observations. In the two-step grown film, the first layer accumulates defects, which leads to two effects of strain relaxation and lower loss of the film.

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]

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]