Measurement of internal stresses via the polarization in epitaxial ferroelectric films

The relations between electrical and mechanical properties of constrained ferroelectric films are analyzed. It is shown that the internal stresses and the elastic constants can be determined through the measurement of the electrical response. The change in the polarization is proportional to internal stresses due to film-substrate misfit, whereas the linear electrical and electromechanical responses to external field do not depend on the misfit and are determined by the film constraint. The theoretical results are successfully applied to PbZr 0.2Ti 0.8O (3) films on (001) LaAlO (3) substrate which exhibit a considerable increase in the saturation polarization due to epitaxial stresses. Significant recovery in the piezoelectric constant and susceptibility is theoretically predicted and experimentally verified for specific film configurations which reduce the degree of constraint. The concept presented in this Letter can be expanded to constrained ferromagnetic and superconductor films.     

Stability of polarization vortices within two interacting ferroelectric nanoparticles

The state of polarization vortices within a ferroelectric nanoparticle could be affected by the long-range electrostatic and elastic interactions from the neighboring particles. Phase field simulations were conducted to investigate the stability of polarization vortices within two interacting ferroelectric nanodots embedded in a non-ferroelectric medium. The interaction between these two nanodots can be neglected if the intermediate distance is larger than a critical distance, whereas the interaction becomes visible once the distance is smaller than that. Two separate vortices were observed to merge into one single vortex once an extreme state of perfect contact condition has been virtually achieved.     

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.     

Weak dependence of spontaneous polarization on epitaxial strain in ferroelectric BiAlO3: A first-principles study

We investigate the effects of epitaxial strain on the ferroelectric properties of BiAlO₃ (BAO) using first-principles calculations. We find a large polarization (P) of 72.3 μC/cm² as well as a small energy barrier of ∼0.1 eV/formula unit for coherent ferroelectric switching of unstrained BAO. Thus, this material is suitable for applications in high-speed nonvolatile memory. We study the modification of P due to epitaxial strains of −5 to +5%, where the minus and plus signs represent the compressive and tensile strains, respectively. The polarization of BAO is insensitive to epitaxial strain; the polarization change is limited to below ∼6%. The weak sensitivity of the polarization is attributed to the stereochemical activity of Bi 6s² lone pairs as well as the inertness of the B-site Al to the polarization. The strain application modifies the switching energy barrier by up to 30%, which provides guidelines for design of devices based on BAO.     

Threading dislocations in epitaxial ferroelectric PbZr0.2Ti0.8O3 films and their effect on polarization backswitching

The existence of threading dislocations in ferroelectric heterostructures has been frequently reported. However, their origin and impact on the ferroelectric properties are not sufficiently understood. PbZr_0.2Ti_0.8O₃/SrRuO₃ heterostructures were epitaxially grown by pulsed-laser deposition (PLD) onto vicinal SrTiO₃(001) substrates. The threading dislocations exhibited by the PbZr_0.2Ti_0.8O₃ films were investigated by cross-sectional and plan-view (high-resolution) transmission electron microscopy. Many threading dislocations were dissociated into dipoles spanning a, most probably lead-rich, stacking fault. It is likely that these dislocations are able to pin 180° ferroelectric domains, as suggested by a comparison between piezo-force microscope and transmission electron micrographs obtained on identical samples. Local backswitching of the polarization was observed in the vicinity of such threading dislocations.     

Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

Unusual thermodynamic properties and nonergodicity in ferroelectric superlattices

The properties of [Pb(Zr(1-x(1))Ti(x(1)))O(3)](n)/[Pb(Zr(1-x(2))Ti(x(2)))O(3)](n) superlattices, with a 2n period, are simulated using an ab initio based approach. The x(1) and x(2) compositions are chosen to be located across the morphotropic phase boundary of the corresponding disordered alloys, while the (x(1)+x(2))/2 average composition lies inside this boundary. These superlattices exhibit an unusual thermodynamic phase transition sequence, including a triclinic ground state. They also have the kind of peculiar free-energy landscape yielding nonergodicity. The effects responsible for these anomalies are discussed.     

Influence of silica nanoparticles on dielectric spectroscopy and polarization switching responses of novel ferroelectric liquid crystals

Silica (SiO₂) nanoparticles in different wt/wt% ratios were dispersed in a novel room temperature ferroelectric liquid crystal mixture. Dielectric relaxation spectroscopy (50 Hz–1 MHz) and electro-optic responses were investigated. Dielectric permittivity gradually decreases by about 40% up to 1 kHz with increasing SiO₂ concentration in the chiral smectic C phase. However, an increase of up to 1.5% has been observed for all the silica-doped samples above 2 kHz. We noticed an increase in optical transmission with increasing SiO₂ concentration from 0.01 to 0.1 wt/wt%.     

Stability diagram for elastic domains in epitaxial ferroelectric thin films

The statics of isolated elastic domains (twins) in epitaxial thin tetragonal films grown on a cubic substrate is investigated theoretically. Different possible variants of the geometric shape of a domain are studied: plate, trapezoidal, and triangular. The nonuniform internal stresses, which also exist in polydomain epitaxial systems, are calculated by the effective-dislocation method. Hence the elastic energies stored in heterostructures with different domains are determined. The equilibrium width of a domain is calculated by minimizing the total internal energy of the heterostructure. Next, the stability diagram for isolated domains in epitaxial films is constructed from energy considerations. It is shown that in a large part of this diagram trapezoidal domains are energetically more advantageous than plate-shaped domains. The effect of an external electric field on the stability of 90° domains in epitaxial ferroelectric films is investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 127–134 (January 1997)     

Atomic-force microscopy of polarization domains in ferroelectric films

Single-crystal (001)-oriented PbZr_0.47Ti_0.53O₃ and polycrystalline (111)-oriented PbZr_0.47Ti_0.53O₃ thin ferroelectric films were studied using contact electrostatic force microscopy. Local electromechanical response measurements permitted study of the polarization vector distribution in natural and intentionally created polarization nanodomains in films. The principal components of an electromechanical response signal encountered in studies of ferroelectric films (the piezoelectric response and the additional capacitive contribution) were isolated and analyzed. The effect of tip-surface contact stiffness on the capacitive contribution to the electromechanical response signal was demonstrated experimentally and in terms of a model. It was shown that more accurate information on the distribution of the polarization vector in ferroelectric films can be gained by monitoring local variations in the tip-surface contact stiffness.     

In-plane and out-of-plane ferroelectric instabilities in epitaxial SrTiO3 films

The in-plane and out-of-plane ferroelectric instabilities in compressed (100)-epitaxial SrTiO3 films were examined by infrared reflection spectroscopy. The strongly stiffened in-plane soft mode frequency softened very slowly on cooling. On the other hand, the silent mode appeared at around 150 K, indicating an out-of-plane ferroelectric transition. This behavior points to a split of in-plane and out-of-plane ferroelectric instability temperatures due to the lowered symmetry of the SrTiO3 lattice caused by mechanical misfit strain. Infrared spectroscopy provides a possibility to detect such an effect in the strained epitaxial ferroelectric films.     

Superconductor/ferroelectric epitaxial heterostructures for tunable microwave devices

The formation of epitaxial heterostructures including YBa₂Cu₃O_7−δ and SrTiO₃ thin films on R-plane sapphire substrates has been studied. The thin CeO₂ layer and the CeO₂/SrBi₂Nb₂O₉ epitaxial bilayer served as buffers. The temperature dependence of the dielectric permittivity ɛ of the SrTiO₃ film behaved in essentially different ways when incorporated in the YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ and in Ag/SrTiO₃/YBa₂Cu₃O_7−δ capacitor structures. An external bias U _b=±2.5 V applied to a plane-parallel YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ capacitor structure (T<100 K) reduced the ɛ of the SrTiO₃ film to one half its original value. Fiz. Tverd. Tela (St. Petersburg) 39, 222–227 (February 1997)     

X-ray linear dichroism dependence on ferroelectric polarization

X-ray absorption spectroscopy and photoemission electron microscopy are techniques commonly used to determine the magnetic properties of thin films, crystals, and heterostructures. Recently, these methods have been used in the study of magnetoelectrics and multiferroics. The analysis of such materials has been compromised by the presence of multiple order parameters and the lack of information on how to separate these coupled properties. In this work, we shed light on the manifestation of dichroism from ferroelectric polarization and atomic structure using photoemission electron microscopy and x-ray absorption spectroscopy. Linear dichroism arising from the ferroelectric order in the PbZr0:2Ti0:8O3 thin films was studied as a function of incident x-ray polarization and geometry to unambiguously determine the angular dependence of the ferroelectric contribution to the dichroism. These measurements allow us to examine the contribution of surface charges and ferroelectric polarization as potential mechanisms for linear dichroism. The x-ray linear dichroism from ferroelectric order revealed an angular dependence based on the angle between the ferroelectric polarization direction and the x-ray polarization axis, allowing a formula for linear dichroism in ferroelectric samples to be defined.     

Determining the influence of ferroelectric polarization on electrical characteristics in organic ferroelectric field-effect transistors

Organic ferroelectric field-effect transistors(OFeFETs) are regarded as a promising technology for low-cost flexible memories. However, the electrical instability is still a critical obstacle, which limits the commercialization process. Based on already established models for polarization in ferroelectrics and charge transport in OFeFETs, simulation work is performed to determine the influence of polarization fatigue and ferroelectric switching transient on electrical characteristics in OFeFETs. The polarization fatigue results in the decrease of the on-state drain current and the memory window width and thus degrades the memory performance. The output measurements during the ferroelectric switching process show a hysteresis due to the instable polarization. In the on/off measurements, a large writing/erasing pulse frequency weakens the polarization modulation and thus results in a small separation between on- and off-state drain currents. According to the electrical properties of the ferroelectric layer, suggestions are given to obtain optimal electrical characterization for OFeFETs.     

Flexoelectricity induced increase of critical thickness in epitaxial ferroelectric thin films

Flexoelectricity describes the coupling between polarization and strain/stress gradients in insulating crystals. In this paper, using the Landau–Ginsburg–Devonshire phenomenological approach, we found that flexoelectricity could increase the theoretical critical thickness in epitaxial BaTiO₃ thin films, below which the switchable spontaneous polarization vanishes. This increase is remarkable in tensile films while trivial in compressive films due to the electrostriction caused decrease of potential barrier, which can be easily destroyed by the flexoelectricity, between the ferroelectric state and the paraelectric state in tensile films. In addition, the films are still in a uni-polar state even below the critical thickness due to the flexoelectric effect.     

Coupled ultrafast lattice and polarization dynamics in ferroelectric nanolayers

We report the first analysis of the polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the soft mode. Optical excitation of the SrRuO(3) metal layers generates giant stress (>1 GPa) compressing the PbZr(0.2)Ti(0.8)O(3) layers by up to 2%. The resulting change of tetragonality reaches a maximum after 1.3 ps. As a result, the ferroelectric polarization P is reduced by up to 100% with a slight delay that is due to the anharmonic coupling of the two modes.