Vladimir O. Sherman

Ferroelectric-dielectric tunable composites

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...

Relaxor ferroelectricity in strained epitaxial SrTiO3 thin films on DyScO3 substrates

The ferroelectric properties of 500A thick strained, epitaxial SrTiO3 films grown on DyScO3 substrates by reactive molecular-beam epitaxy are reported. Despite the near 1% biaxial tensile strain, the x-ray rocking curve full widths at half maximum in ω are as narrow as 7arcsec (0.002°). The films show a frequency-dependent permittivity maximum near 250K that is well fit by the Vogel-Fulcher equation. A clear polarization hysteresis is observed below the permittivity maximum, with an in-plane remanent polarization of 10μC∕cm2 at 77K. The high Tmax is consistent with the biaxial tensile strain state, while the superimposed relaxor behavior is likely due to defects.

Tuning of direct current bias-induced resonances in micromachined Ba0.3Sr0.7TiO3 thin-film capacitors

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...

Epitaxial/amorphous Ba0.3Sr0.7TiO3 film composite structure for tunable applications

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.

Strain relaxation of epitaxial SrTiO3 thin films on LaAlO3 by two-step growth technique

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.

Model of a low-permittivity and high-tunability ferroelectric based composite

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.

Can the addition of a dielectric improve the figure of merit of a tunable material

The influence of addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of the electrostatic consideration. The calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed by using three different models. On the basis of results obtained, the figure of merit of the composite material has been evaluated. No improvement of the figure of merit of composite material compared to the pure ferroelectric has been observed for the considered models.

Epitaxial growth of Ba0.3Sr0.7TiO3 thin films on Al2O3(0001) using ultrathin TiN layer as a sacrificial template

Epitaxial Ba0.3Sr0.7TiO3 (BST) films with significantly improved tunable performance were grown on c-plane sapphire [Al2O3(0001)] substrates using ultrathin TiN seed layers by pulsed laser deposition. A very thin (111) epitaxial TiN layer deposited at room temperature acted as a sacrificial epitaxial template for the BST film in the initial growth stage. The BST films grew with (111) orientation having BST[21¯1¯]‖Al2O3[112¯0] and BST[21¯1¯]‖Al2O3[21¯1¯0] epitaxial relationships. Planar capacitors fabricated on the epitaxial BST film showed significantly higher tunability as compared to those on a polycrystalline BST film directly deposited on the substrate.

Structural and dielectric properties of strain-controlled epitaxial SrTiO3 thin films by two-step growth technique

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 10nm 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 300nm. 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. S...

Annealing effect on dislocations in SrTiO3∕LaAlO3 heterostructures

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 4h. 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.