Defects In Ferroelectrics Research Articles

Determination of the Activation Energy of Defects in Ferroelectrics by the Method of Temperature Activation–Relaxation of the Dielectric Permittivity

The article proposes a method of temperature activation–relaxation of the permittivity for determining the activation energy of defects in ferroelectrics using lead zirconate–titanate Pb(Zr,Ti)O3 samples as an example. This method is based on the analysis of relaxation of the permittivity after thermal annealing and the analysis of the temperature activation of the permittivity of the Pb(Zr,Ti)O3 ferroelectric. The equality of the activation energy corresponding to the process of migration of oxygen vacancies and the thermal energy of the decay of the domain structure was established, which was confirmed by studying the surface of the samples by scanning electron microscopy. When this temperature was reached, the surface of the domain walls was detached from oxygen vacancies, which are pinning centers. This manifested itself in photographs of the microstructure as a change in the ordering of the domains emerging on the surface of the sample, which led to an irreversible decrease in the permittivity of the sample. For the obtained activation energies, the physical process of domain wall motion activation is established, which is determined by their pinning on structural defects (oxygen vacancies). It is assumed that the irreversible decay of the domain structure occurs when the domain walls are displaced by distances exceeding the elementary lattice parameter of the ferroelectric. The proposed method can be part of a comprehensive study that includes electrophysical, microscopic and X-ray methods.

Determination of the Activation Energy of Defects in Ferroelectrics by the Method of Temperature Activation–Relaxation of the Dielectric Permittivity

Determination of the Activation Energy of Defects in Ferroelectrics by the Method of Temperature Activation–Relaxation of the Dielectric Permittivity

Observation of dislocation-controlled domain nucleation and domain-wall pinning in single-crystal BaTiO3

Electro-mechanical interactions between topological defects and domain walls play a key role in the macroscopic response of bulk and thin-film ferroelectrics. The applications of ferroelectrics are derived from their inherent ability to nucleate new domains and to move the domain walls that separate adjacent domains. Here, we report dislocation-mediated domain nucleation in single-crystal BaTiO3, achieved by dislocations generated via high-temperature uniaxial compression on a notched sample. We also present a direct observation of domain-wall pinning of 90° ferroelastic domain walls by dislocations using in situ transmission electron microscopy. Dense and well-aligned “forest” dislocations, featuring {100}⟨100⟩ slip systems oriented in the out-of-plane [001] direction, exclusively nucleate in-plane domain variants. We reveal that the 90° domain walls are strongly pinned by imprinted dislocations due to the presence of their associated stress fields. Our findings may advance our understanding of the control of defects in ferroelectrics and propose a strategy applicable to both emerging nanoelectronic and bulk applications.

Electrical topological defects induced by terahertz laser pulses

Recent experimental evidence and theoretical simulations have shown that electrical polarization can be tuned and even deterministically controlled in ferroelectrics by applying laser pulses having a frequency that is higher than that of the main polarization (soft) mode. Here, by using ab initio computations, we further discover that such pulses can also create nontrivial topological polar phases in bulk ferroelectric materials. Such a phenomenon is found to originate from nonthermal quenching process in which the applied intense ultrashort laser pulse first squeezes the ferroelectric phase into a cubic-like paraelectric phase at a moderate temperature and then to a renucleation process akin to the Kibble-Zurek mechanism creating nontrivial topological polar defects in the bulk ferroelectric ground-state phase. Our work therefore presents a novel and optical mean of creating topological defects in ferroelectrics.

Intrinsic Mechanical Compliance of 90° Domain Walls in PbTiO3

AbstractFunctionality of domain walls and other topological defects in ferroelectrics is being widely investigated for applications in electronic devices. While the intrinsic electronic properties of a wall have been considered, its inherent mechanical properties remain explored very little, despite the fact that coupling between strain and polarization is prevalent in many of these materials. Herein, an in‐depth study of variations in nanomechanical properties at 90o domain walls and their adjacent domains in single‐crystalline lead titanate (PbTiO3) is presented as a prototypical ferroelectric material using a combination of various atomic force microscopy (AFM)‐based methods. Considerable variations of elastic moduli are found at 90o domain walls extending up to ~100 nm into the domain areas. AFM nanoindentation also allows to extract local domain wall hardness and plastic and elastic deformation energies. These findings have implications for the design of ferroelectric domain wall functionality that incorporates the intrinsic elastic compliance of a domain wall.

Exploring Charged Defects in Ferroelectrics by the Switching Spectroscopy Piezoresponse Force Microscopy.

Monitoring the charged defect concentration at the nanoscale is of critical importance for both the fundamental science and applications of ferroelectrics. However, up-to-date, high-resolution study methods for the investigation of structural defects, such as transmission electron microscopy, X-ray tomography, etc., are expensive and demand complicated sample preparation. With an example of the lanthanum-doped bismuth ferrite ceramics, a novel method is proposed based on the switching spectroscopy piezoresponse force microscopy (SSPFM) that allows probing the electric potential from buried subsurface charged defects in the ferroelectric materials with a nanometer-scale spatial resolution. When compared with the composition-sensitive methods, such as neutron diffraction, X-ray photoelectron spectroscopy, and local time-of-flight secondary ion mass spectrometry, the SSPFM sensitivity to the variation of the electric potential from the charged defects is shown to be equivalent to less than 0.3 at% of the defect concentration. Additionally, the possibility to locally evaluate dynamics of the polarization screening caused by the charged defects is demonstrated, which is of significant interest for further understanding defect-mediated processes in ferroelectrics.

Defects in ferroelectric HfO2.

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Амплитудные зависимости диэлектрических потерь в тонкопленочном наногранулированном композите (x)Ni-(1-x)PZT

At temperatures Т below the ferroelectric Curie point and various strengths of a constant electric field E=, the dependences of dielectric losses tgδ in a thin-film nanogranular composite (x)Ni – (1 –x)[Pb0.81Sr0.04(Na0.5Bi0.5)0.15][(Zr0.575Ti0.425)]O3 on the strength of an alternating electric field E~ were studied. It was found that tgδ almost does not vary with increasing E~ up to the strength of some threshold field Et and smoothly increases at E~ > Et. The value of Et decreases as Т increases while x and E= decreases. The observed dependences of tgδ(E~) and Et on Т, x, and E= are explained in terms of the model of interaction of domain boundaries with pinning point defects in ferroelectrics

Unveiling the pinning behavior of charged domain walls in BiFeO3 thin films via vacancy defects

Manipulation of electronic states in functional ferroelectrics is promising for next generation electronics devices. The charged domain walls in ferroelectric materials especially facilitate the electronic state modulation and are promising for developing interface-based devices. However, the major challenges impeding the application are their intentional manipulation and the elusive pinning behavior. Here, results that charged domain walls in BiFeO3 films can be pinned and regulated by oxygen vacancy planar distributions controlled by oxygen pressure during film growth are reported. Using aberration-corrected scanning transmission electron microscopy complemented by theoretical simulations, rich pinning behavior of tail-to-tail charged domain walls by oxygen vacancy plates is revealed. At high annealing oxygen pressure, 71° charged domain walls are stabilized by narrow vacancy plates. Decreasing the oxygen pressure, the transformation from 71° to 109° charged domain walls happens by expanding the vacancy plates, as collaborated by phase field simulations. Besides, the 71°-109° charged domain wall pairs are stabilized due to further interaction between two neighboring vacancy plates. These results provide the active modulation of the electronic states and illuminate the rich pinning behavior of domain walls by vacancy defects in ferroelectrics, which in turn could provide implications for designing potential electronics devices.

Artificial creation and separation of a single vortex\u2013antivortex pair in a ferroelectric flatland

Topological defects have received much attention due to their stability against perturbations and potential applications in nonvolatile high-density memory. Topologically non-trivial textures can be compelled by constraints on boundary condition, geometrical structure, and curved space. Ferroelectric vortices have been realized in various finite-sized nanostructures that allow such constraints to be produced. However, manipulation of topological excitations in otherwise topologically trivial flat ferroelectrics remains a tantalizing challenge. Here we show that a vortex–antivortex pair can be produced by a momentary electric pulse using a tip in a usual Kittel’s stripe domain of a BiFeO3 thin film. Moreover, we demonstrate that the distance between the paired vortex and antivortex can be controlled by dragging the biased tip. The spatial distribution of the local piezoresponse vectors is directly mapped using angle-resolved piezoresponse force microscopy and analyzed by local winding number calculation. Our findings offer a useful concept for the control of topological defects in ferroelectrics.

Stress-induced reversible and irreversible ferroelectric domain switching

Ferroelectric materials have been extensively explored for applications in electronic devices because of their ferroelectric/ferroelastic domain switching behaviour under electric bias or mechanical stress. Recent findings on applying mechanical loading to manipulate reversible logical signals in non-volatile ferroelectric memory devices make ferroelectric materials more attractive to scientists and engineers. However, the dynamical microscopic structural behaviour of ferroelectric domains under stress is not well understood, which limits the applications of ferroelectric/ferroelastic switching in memory devices. Here, the kinetics of reversible and irreversible ferroelectric domain switching induced by mechanical stress in relaxor-based ferroelectrics was explored. In-situ transmission electron microscopy investigation revealed that 90° ferroelastic and 180° ferroelectric domain switching can be induced by low and high mechanical stresses. The nucleation and growth of nanoscale domains overwhelm the defect-induced pinning effect on the stable micro-domain walls. This study provides deep insights for exploring the mechanical kinetics for ferroelectric/ferroelastic domains and a clear pathway to overcome the domain pinning effect of defects in ferroelectrics.

Comparative study of impurities in lithium tantalite and lithium niobate

S 38677 The point defects in ferroelectrics may influence the physical properties of phononic crystals LiTaO3 (LT) and LiNbO3 (LN). The initial wafers, periodically poled (PP) structures of LT with 1-mm-domains, and PP-LN with 0.45-mm-domains are investigated. Photoluminescence (PL) is taken at room temperature 310-nm -excitation. The slit width in optical spectrometer is set to 1 nm. The PL spectra of LT reveal multiple impurities including Fe + ,W, Th, Cs, Kr, Cu, Xe. Unlike PL from LN, there are no lines of the F-center, Ar and some other elements. The intensities of PL-lines in both LT and LN demonstrate a saw-type periodicity along the direction normal to the Z-axis. For instance, the PL line of Fe + defect in LT varies ± 15% with periodicity about 200 ± 40 nm. In the case of PP LT phononic crystals, the difference in Fe + line amplitude remains about the same; however, the periodicity becomes equal to a ferroelectric domain length of 1 mm. There is a difference between the two periodicities of charged defects in PP-LT and PP-LN; the positions of maxima in PL-lines are close to the interdomain walls in PP-LN, but it is not the case for PP-LT. Possible practical applications are discussed.

Stable charged antiparallel domain walls in hyperferroelectrics

Charge-neutral 180° domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180° domain walls running perpendicular to the polarization directions are highly energetically unfavorable because of the depolarization field and are difficult to stabilize. We explore both neutral and charged 180° domain walls in hyperferroelectrics, a class of proper ferroelectrics with persistent polarization in the presence of a depolarization field, using density functional theory. We obtain zero temperature equilibrium structures of head-to-head and tail-to-tail walls in recently discovered ABC-type hexagonal hyperferroelectrics. Charged domain walls can also be stabilized in canonical ferroelectrics represented by LiNbO3 without any dopants, defects or mechanical clamping. First-principles electronic structure calculations show that charged domain walls can reduce and even close the band gap of host materials and support quasi-two-dimensional electron(hole) gas with enhanced electrical conductivity.

Frustration and Self-Ordering of Topological Defects in Ferroelectrics.

A first-principles-based effective Hamiltonian technique is used to investigate the interplay between geometrical frustration and the ordering of topological defects in a ferroelectric nanocomposite consisting of a square array of BaTiO_{3} nanowires embedded in a Ba_{0.15}Sr_{0.85}TiO_{3} matrix. Different arrangements of the wires' chiralities geometrically frustrate the matrix, which in response exhibits point topological defects featuring self-assembled ordered structures spatially fluctuating down to the lowest temperatures. These fluctuations thereby endow the system with residual configurational entropy from which many properties characteristic of geometric frustration, such as the ground state degeneracy and the broadness of the dielectric response, are further found to originate.

Mesoscopic mechanism of the domain wall interaction with elastic defects in uniaxial ferroelectrics

The role of elastic defects on the kinetics of 180° uncharged ferroelectric domain wall motion is explored using continuum time-dependent Landau-Ginzburg-Devonshire equation with elastic dipole coupling. In one dimensional case, ripples, steps, and oscillations of the domain wall velocity appear due to the wall-defect interactions. While the defects do not affect the limiting-wall velocity vs. field dependence, they result in the minimal threshold field required to activate the wall motions. The analytical expressions for the threshold field are derived and the latter is shown to be much smaller than the thermodynamic coercive field. The threshold field is linearly proportional to the concentration of defects and non-monotonically depends on the average distance between them. The obtained results provide the insight into the mesoscopic mechanism of the domain wall pinning by elastic defects in ferroelectrics.

Quantum pinning-transition due to charge defects in ferroelectrics

We investigate the pinning of domain walls in ferroelectrics on the basis of the two phonon bound state (TPBS) or discrete breather state due to impurity energy levels above the phonon continua in ferroelectrics such as LiNbO3 in order to show the pinning transition, which indicates the point of easiest switching. We predict, with the help of our quantum calculations, that every ferroelectric material has such a critical point of easy switching. Here we describe the quantum origin of pinning through the findings of analytical and numerical calculations, as interpreted by a TPBS concept by such impurity or nonlinearity induced modes, by plotting various TPBS parameters against the Landau coefficient and percent impurity content. This new approach might lead to many interesting applications for device manufacturing.

Point defects in ferroelectrics with perovskite structure

First-principle calculations of the geometry and electronic structure of lattice defects (substituents, vacancies, and binary complexes) in perovskites are considered. A supercell method and modifications of a cluster method for simulating point lattice defects in insulators are discussed. Experimental data on dielectric relaxation in magnesium- and manganese-doped strontium titanate and possible mechanisms of the formation of reorientable polar defects are considered.

Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO3 and KTaO3 are performed in the framework of the density functional theory. The results are presented for the Li+ impurity ion at the A site in the KTaO3 and SrTiO3 ferroelectrics; the Mn2+, Cd2+, Ca2+, Mg2+, and Zn2+ ions at the A site and the Mn4+ and Mg2+ ions at the B site in the SrTiO3 compound; and the MNTi2+-VO and MgTi2+-VO complexes in the SrTiO3 ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the CaSr2+, CdSr2+, MnTi4+, and MgTi2+ ions have a stable central position, whereas the LiK+ ion in the KTaO3 compound and the LiSr+, MnSr2+, and ZnSr2+ defects in the SrTiO3 ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the LiSr+ and MgTi2+ neutral defects.

Interaction of point defects in ferroelectrics ‐parameter identification and simulation

AbstractFerroelectric materials are used in a wide field of applications, where they are exposed to a high number of mechanical and electrical load cycles. This involves degradation of the material and a decrease of the electromechanical coupling capability, which is usually called electric fatigue. The causes are assumed to be ionic and electronic charge carriers, which interact with each other, with microstructural elements in the bulk and with interfaces. Accumulation of defects can lead to degradation, mechanical damage and dissociation reactions, for more details see e.g. [3].In order to get a better understanding of the defect accumulation processes, a model based on material forces is used in [6] to simulate the interaction of defects in periodic and in infinite cells. Applying thermodynamically reasonable kinetic laws, defect migration is simulated in a deterministic way in order to understand the general tendency of defect formations. The transversally isotropic material is modelled with linear electromechanical coupling. Here, the defect parameters used in the continuum model are obtained by fitting the results of molecular dynamics (MD) simulations to the continuous spatial fields. Transferring data from the atomic to the continuum level is a field of active research and no unique solution can be presented. On the atomic level, Coulomb–interaction causes a displacement field incompatible to an elastic solution. To address this difficulty, the volume change of a domain around the defect is used to determine defect parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Specific features of light scattering from charged defects in ferroelectrics and pyroelectrics

Abstract A short review is given on theoretical results on the influence of point charged defects on phase transition anomalies of the elastic and inelastic light scattering in polar crystals. The theory explains some experimental data but the majority of theoretical predictions are still to be substantiated by the experiments.