Ferroelectric Superlattices Research Articles

Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

Properties of (BaTiO$_3$)$_1$/(BaZrO$_3$)$_n$ ferroelectric superlattices (SLs) with $n = {}$1--7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the $\Gamma_{15}$ polar phonon and phonons at the $M$ point (for SLs with even period) or at the $A$ point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed $z$-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.

Hysteretic Internal Electric Fields and Polarization Reversal in Ferroelectric Superlattices

A thermodynamic model of interface intermixing in ferroelectric superlattices with appropriate electrostatic boundary condition is developed. Interface intermixing forms intermixed layers with properties different from its constituent layers. In this work, the polarization reversal in ferroelectric superlattices with “switchable” polarization in intermixed layers is examined. Our results reveal that polarization hysteresis in ferroelectric superlattices is accompanied by hysteresis in internal electric fields. The underlying physical mechanisms which induce the internal electric field dependence of applied electric field are discussed by looking at the polarization and internal field profiles.

Strain effect in PbTiO3/PbZr0.2Ti0.8O3 superlattices: From polydomain to monodomain structures

Ferroelectric symmetric superlattices consisting of alternating layers of PbTiO3 and PbZr0.2Ti0.8O3, were grown by pulsed laser deposition on SrTiO3 and SrRuO3-coated SrTiO3 substrates. The superlattices, with wavelengths Λ ranging from 20 Å to 200 Å, were analyzed using x-ray diffraction (θ−2θ diffraction scans, rocking curves, and reciprocal space mapping), high resolution transmission electron microscopy, and piezoforce scanning microscopy. For large-period superlattices, the strain is relieved by the formation of an a/c polydomain structure which propagates through the whole film. We investigate the influence of the wavelength on the a-domain volume fraction, the lattice parameters, the in-plane strain εxx, and the mosaicity of the samples. We show that with decreasing the wavelength, a reduction of the a-domain volume fraction is observed as well as a reduced tensile in-plane strain and a lower mosaicity. A concomitant improvement of the local ferroelectric response is detected. Below a critical wavelength of about 30 Å and a critical sample thickness of 500 Å, the formation of 90° a/c domains is inhibited and the superlattices are completely c oriented. Thus the reduced wavelength induces compressive strain which dominates over the tensile clamping due to the thermal expansion mismatch between the substrate and the superlattice. This compressive strain favors a c-oriented structure in the PbTiO3/PbZr0.2Ti0.8O3 superlattices.

Modulated internal electric field, dielectric susceptibility and polarization in ferroelectric superlattices

A thermodynamic model for describing interface intermixing in a superlattice combining a ferroelectric and a paraelectric is developed. Formation of intermixed layer at interfaces leads to a periodic modulation of ferroelectric properties in these superlattices. Spatially-varying internal electric field, dielectric susceptibility and polarization of these superlattices are calculated. Effects of modulation period and temperature on the internal electric field, dielectric susceptibility and polarization of these superlattices with inhomogeneous properties are examined. Correlation between these ferroelectric properties is established and discussed.

Engineering Polarization Rotation in a Ferroelectric Superlattice

A key property that drives research in ferroelectric perovskite oxides is their strong piezoelectric response in which an electric field is induced by an applied strain, and vice versa for the converse piezoelectric effect. We have achieved an experimental enhancement of the piezoelectric response and dielectric tunability in artificially layered epitaxial PbTiO(3)/CaTiO(3) superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice were changed from sample to sample we found evidence for polarization rotation in multiple x-ray diffraction measurements. Associated changes in functional properties were seen in electrical measurements and piezoforce microscopy. The results demonstrate a new approach to inducing polarization rotation under ambient conditions in an artificially layered thin film.

Influence of interface intermixing and periodicity on internal electric field and polarization in ferroelectric superlattices

A thermodynamic model is developed to study the effect of interface intermixing and modulation period on the internal electric field of a superlattice comprising alternate layers of ferroelectric and dielectric. The intermixed layer plays an important role in determining both the polarization and internal electric field modulation profiles of the superlattice. The presence of interface intermixed layer gives rise to inhomogeneous internal field and polarization, which enhances the depolarization field of the superlattices. Interface intermixing; however, does not have much influence on the polarization behavior of the superlattice. Ferroelectric volume fraction or thickness ratio of ferroelectric to dielectric is one of the key parameter for controlling the properties of a superlattice consisting alternate layers of ferroelectric and dielectric.

Resonances in ferroelectric phononic superlattice

The periodically poled ferroelectric wafer is a two-dimensional phononic superlattice. The important applications of such a solid include ultrasonic transducers at the micro/nano-scale for low intensity ultra-sonography, ferroelectric data storage, and development of very high frequency chips for next generation communication and information technologies, and others. In this work, we show theoretically and experimentally that a ferroelectric phononic superlattice has two distinctive resonances in acousto-electric transformation. They are associated with a split acoustic mode at the boundary between the first and second acoustic Brillouin zone.

Direct determination of the effect of strain on domain morphology in ferroelectric superlattices with scanning probe microscopy

A variant of piezo force microscopy was used to characterize the effect of strain on polarization in [(BaTiO3)n/(SrTiO3)m]p superlattices. The measurements were compared to theoretical predictions based on phase-field calculations. When polarization is constrained to be perpendicular to the substrate, the measured polarization and domain morphology agree quantitatively with the predictions. This case allows the presence of an internal electric field in the thin film to be identified. The measured trend in piezoelectric response with strain state was in qualitative agreement with predictions, and the differences were consistent with the presence of internal electrical fields. Clear differences in domain morphology with strain were observed; and in some cases, the lateral anisotropic strain appeared to influence the domain morphology. The differences in magnitude and morphology were attributed to the internal electric fields and anisotropic strains.

FerroelectricPbTiO3/SrRuO3Superlattices with Broken Inversion Symmetry

We have fabricated PbTiO3/SrRuO3 superlattices with ultrathin SrRuO3 layers. Because of the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO3 layers. Therefore, along the ferroelectric direction, SrRuO3 layers can act as dielectric, rather than metallic, elements. We show that, by reducing the concentration of PbTiO3, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The results are significant as they represent a new class of ferroelectric superlattices, with a rich and complex phase diagram. By expanding our set of materials we are able to introduce new behaviors that can only occur when one of the materials is not a perovskite titanate. Here, compositional inversion symmetry breaking in bicolor superlattices, due to the combined variation of A and B site ions within the superlattice, is demonstrated using a combination of experimental measurements and first principles density functional theory.

Electrostatic coupling and interface intermixing in ferroelectric superlattices

A thermodynamic model is developed to study electrostatic coupling and interface intermixing in superlattices comprising alternate layers of ferroelectrics and paraelectrics. Interface intermixing leads to inhomogeneous internal electric field and polarization in superlattices. The spatial distribution of polarization extends into the layer over a distance governed by its correlation length. Periodic modulation of the internal electric field and polarization in superlattices are correlated. Interface intermixing enhances the depolarization field of superlattices; however, it has a negligible effect on polarization and transition temperature. The internal electric field, originating from the electrostatic coupling between ferroelectric layers, plays a dominant role in determining the properties of superlattices.

Design of the PTO/PZO Ferroelectric Superlattice Assembly

A series of ferroelectric superlattice films on ITO substrates have been artificially fabricated by radio frequency magnetron sputtering, starting from the PbZrO3 (PZO) and PbTiO3 (PTO) matrix targets. The design sequence of the layers has been changed via shifting sputtering target in the preparing processes. Diverse systems have been shaped and discussed by XRD, SEM and UV-Vis. UV-Vis absorption spectra have been paid more attention especially, due to the low-cost and non-destructive method to provide the information about the thicknesses of the films.

Recent Applications of Landau-Ginzburg Theory to Ferroelectric Superlattices: A Review

This article briefly reviews recent developments of Landau-Ginzburg theory to ferroelectric phase transitions in superlattices. An overview of the contributions of Landau-type theory to study ferroelectric superlattices is given. Recent findings from first-principles calculations and experiments on intermixing, local polarization coupling and polar discontinuity at interfaces that are not address in these contributions are highlighted. This is followed by a review of recent developments of Landau-Ginzburg theory that addresses these emergent phenomena at interfaces, which is the focus of this review article. The Landau-Ginzburg approach to ferroelectric superlattices with spatial distribution of polarization is outlined. It describes the formation of intermixed layer with properties different from those of both layers. These intermixed layers are mutually coupled through the local polarization at interfaces. Polarization continuity or continuity at interfaces is determined by the nature of the intermixed layer formed at the interface region. Recent results obtained in investigating superlattices comprised primarily of ferroelectric and paraelectric materials are discussed. The results include modulated polarizations, phase transitions, dielectric susceptibilities and switching behaviors.

Nano-domain engineering in ultrashort-period ferroelectric superlattices

Ultrashort-period PbZrO3/PbTiO3 (PTO/PZO) ferroelectric/antiferroelectric superlattices are proposed for nano-domain engineering. They behaved like single ferroelectric metamaterials with monodomain excluding a 90° domain boundary, irrespective of their total thicknesses. Further, nanosized 180° domains, as small as ∼12 nm, were achieved by applying a short pulse voltage. Such domain structures of superlattices result from the lowering of the symmetry of supercell by specific atomic stacking in superlattices, contrary to the domain structures of ferroelectric thin films developed by the strain-induced ferroelectric instability. We suggest that ferroelectric superlattices with an ultrashort-period have a clear advantage for nano-domain engineering with greatly enhanced long term stability of the domains.

Electrostatic Coupling and Local Structural Distortions at Interfaces in Ferroelectric/Paraelectric Superlattices

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.

Raman scattering spectra of ceramics, films, and superlattices of ferroelectric perovskites: A review

Raman investigations of the crystal lattice dynamics in classical ferroelectric perovskites SrTiO3, PbTiO3, and BaTiO3 have been analyzed. The specific features revealed in the behavior of soft modes during the phase transitions occurring in ceramics and powders of these compounds, as well as in several related solid solutions, have been described. Particular attention has been paid to the investigations of ferroelectric thin films and superlattices in which the sequences of structural distortions can be radically different from those known for the initial bulk materials.

Dipole spring ferroelectrics in superlattice SrTiO3/BaTiO3 thin films exhibiting constricted hysteresis loops

Ferroelectric superlattice heterostructures have recently been explored for potential applications in electronic devices. In this letter, we employed the phase-field approach to simulate the domain structure and switching of a (BaTiO3)8/(SrTiO3)3 superlattice film constrained by a GdScO3 substrate. A constricted ferroelectric hysteresis loop was observed with a high saturation polarization but a small coercive field. The shape of the hysteresis loop is understood by analyzing the ferroelectric polarization distributions during switching. It is demonstrated that the multilayers stack behaves as dipole spring ferroelectric, named in analogy to exchange spring magnets in magnetic multilayers that show similar loops.

Infrared Spectroscopy of Nanoscopic Epitaxial BaTiO3/SrTiO3 Superlattices

Artificially layered ferroelectric superlattices have enormous appeal from both a technological and a fundamental standpoint. However, it is difficult to measure and monitor their properties on nan...

Quasi-two-dimensional ferroelectricity in KNbO3/KTaO3 superlattices

First-principles density functional theory is used to calculate the phonon spectrum in the paraelectric phase, the ground-state structure and polarization distribution in the polar phase, and energies of ferro- and antiferroelectrically ordered phases of free-standing (KNbO3)1(KTaO3) n ferroelectric superlattices with n = 1–7. It is established that quasi-two-dimensional ferroelectricity with polarization oriented in the layer plane, which weakly interacts with polarization in neighboring layers, appears in potassium niobate layers with a thickness of one unit cell in the superlattices. The possibility of using of such ferroelectric superlattices as a medium for three-dimensional information recording is shown.

Pyroelectric properties of a ferroelectric superlattice with surface transition layers

The transverse Ising model within the framework of the mean-field theory is used to investigate a ferroelectric superlattice composed of two different alternating ferroelectric slabs A and B, with the surface transition layer within each slab and an antiferroelectric interfacial coupling between two slabs. The combined influence of the surface transition layer and antiferroelectric interfacial coupling on the pyroelectric properties is discussed in detail. The results show that the pyroelectric properties of a ferroelectric superlattice present some interesting phenomena because of the existence of surface transition layer.

Preparation of a BiFeO3/LaNiO3 multiferroic oxide superlattice structure by RF magnetron sputtering

Artificial superlattices consisting of multiferroic BiFeO3 (BFO) and conductive LaNiO3 (LNO) were grown epitaxially on a Nb-doped SrTiO3 (STO) (001) single-crystal substrate at temperatures in a range 560–810°C with a RF magnetron sputtering system. The superlattice contained 30 periods of symmetric BFO/LNO bilayers with a thickness of 2nm of each individual layer. Ferroelectric and conductive superlattice materials of this type can serve for an investigation of the strain dependence of ferroelectric properties of BFO layers in superlattice structures.Measurements of X-ray reflectivity and X-ray diffraction at high resolution were employed to characterize the microstructure of these films. The formation of a superlattice structure was confirmed by the appearance of Bragg peaks separated by Kiessig fringes in the X-ray reflectivity curve and a diffraction pattern. The clearly discernible main feature and satellite features on both sides of the substrate feature about the (002) STO Bragg peak indicate the high quality of the BFO/LNO artificial superlattice structure formed on a STO substrate at all deposition temperatures.X-ray measurements show that these superlattice films become subject to greater compressive strain in the in-plane direction, and possess increased crystalline quality when increasing the deposition temperature to 660°C. The measurement of hysteresis loops shows that the largest remanent polarization (Pr) occurs at 660°C.