Switching In Ferroelectric Thin Films Research Articles

On the mechanisms of tip-force induced switching in ferroelectric thin films: the crossover of depolarization, shear strain and flexoelectricity

The recent observation of mechanical switching of ferroelectric polarization has placed the mechanical manipulation of ferroelectrics on an equal footing with the conventional electrical manipulation. However, discussions on the exact switching mechanisms due to mechanical loads are ongoing for the complexity in experimental situations. In this work, based on continuum mechanical and thermodynamic modeling and simulation, we analyze the mechanisms of tip-force induced switching in ferroelectric thin films. The roles of depolarization, shear strain and flexoelectricity in mechanical switching, both in normal and sliding loading modes, are separated out and the switching characteristics are analyzed. The depolarization field in the film is demonstrated to enable bidirectional switching. The coupling between shear strain and polarization components is shown to be important in the sliding loading mode. A great influence of flexoelectricity-modified polarization boundary condition on the switching process is revealed. The previous speculation that the switching process experiences an intermediate paraelectric phase is proved. The regulation of loading force, misfit strain, temperature and film thickness on the switching are further given for each mechanism. Taking all of the three mechanisms into account, we present the phase diagrams of mechanical switching for films in an initial upward or downward polarization state. The revealed characteristics of various switching mechanisms should provide useful guidelines for their verification in experiments, and the tunability of the switching by various influencing factors is instructive for the design and optimization of ferroelectric devices via mechanical engineering.

Pressure-induced switching in ferroelectrics: Phase-field modeling, electrochemistry, flexoelectric effect, and bulk vacancy dynamics

Pressure-induced polarization switching in ferroelectric thin films has emerged as a powerful method for domain patterning, allowing to create predefined domain patterns on free surfaces and under thin conductive top electrodes. However, the mechanisms for pressure induced polarization switching in ferroelectrics remain highly controversial, with flexoelectricity, polarization rotation and suppression, and bulk and surface electrochemical processes all being potentially relevant. Here we classify possible pressure induced switching mechanisms, perform elementary estimates, and study in depth using phase-field modelling. We show that magnitudes of these effects are remarkably close, and give rise to complex switching diagrams as a function of pressure and film thickness with non-trivial topology or switchable and non-switchable regions.

Atomic Scale Structure Changes Induced by Charged Domain Walls in Ferroelectric Materials

Charged domain walls (CDWs) are of significant scientific and technological importance as they have been shown to play a critical role in controlling the switching mechanism and electric, photoelectric, and piezoelectric properties of ferroelectric materials. The atomic scale structure and properties of CDWs, which are critical for understanding the emergent properties, have, however, been rarely explored. In this work, using a spherical-aberration-corrected transmission electron microscope with subangstrom resolution, we have found that the polarization bound charge of the CDW in rhombohedral-like BiFeO3 thin films not only induces the formation of a tetragonal-like crystal structure at the CDW but also stabilizes unexpected nanosized domains with new polarization states and unconventional domain walls. These findings provide new insights on the effects of bound charge on ferroelectric domain structures and are critical for understanding the electrical switching in ferroelectric thin films as well as in memory devices.

Strain-controlled switching kinetics of epitaxial PbZr0.52Ti0.48O3 films

We investigate the effect of biaxial strain on the switching of ferroelectric thin films. The strain state of epitaxial PbZr0.52Ti0.48O3 films is controlled directly and reversibly by the use of piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) substrates. At small external electric fields, the films show switching characteristics consistent with a creep-like domain wall motion. In this regime, we find a huge decrease of the switching time under compressive strain. For larger external electric fields, the domain wall motion is in a depinning regime. The effect of compressive strain is more moderate in this region and shows a reduction in the switching kinetics.

Switching behavior of ferroelectric thin films with deadlayers

Domain switching in ferroelectric thin films with deadlayers subjected to external electric fields is investigated by a phase field model. The simulation results show that the deadlayers of ferroelectric thin films induce a strong inhomogeneous depolarization field which makes polarizations deflect from the normal direction of thin films. The locations with larger depolarization fields are nucleation centers for new domains during the polarization switching and the switching process includes two sequential 90° polarization switchings, which decrease the coercive field significantly. It is found that the thickness of the deadlayer has a significant influence on the switching behavior of the ferroelectric thin films. Both the coercive field and remanent polarization are dependent on the thickness of the deadlayer.

Polarization Switching in Ferroelectric Thin Films Undergoing First-Order Phase Transitions

The main switching properties in ferroelectrics undergoing first-order phase transitions are simulated within the framework of the extended Ishibashi dipole-lattice model including the dipole-dipole interaction in a two-dimensional case for ferroelectric nanoscale objects. The peculiarities of the temperature dependence of the switching rate and the pyroelectric coefficient are discussed in the range of coexistence of the metastable states. The used coefficients of the long-range and short-range interactions between the dipoles are taken from the dielectric and structure measurements inBaTiO3.

Stability of the unswitched polarization state of ultrathin epitaxialPb(Zr,Ti)O3in large electric fields

The initial stage of polarization switching in ferroelectric thin films depends on phenomena that occur at characteristic time scales of tens to hundreds of nanoseconds, including the nucleation polarization domains and the propagation of domain walls. These long intrinsic times allow short-duration electric fields with magnitudes far above the low-frequency coercive electric field to be applied across capacitor devices without inducing switching. Using time-resolved x-ray microdiffraction, we have found that a series of 50 ns duration electric field pulses switches the polarization of a 35-nm-thick ferroelectric $\text{Pb}(\text{Zr},\text{Ti}){\text{O}}_{3}$ film only at electric fields greater than 1.5 MV/cm, a factor of three higher than the low-frequency coercive field. There is no switching in response to a large number of short pulses with amplitudes lower than 1.5 MV/cm, even when the total duration reaches several milliseconds. In comparison, a series of microsecond-duration pulses causes cumulative changes in the area of switched polarization and eventually switches the entire capacitor. The difference between long- and short-duration electric field pulses arises from effects linked to domain nucleation and charge transport in the ferroelectric film. A phase-field model shows that the shrinking of the switched domain in the interval between pulses is a less important effect. This opportunity to apply large fields for short times without inducing switching by domain-wall motion raises the possibility that future experiments could reach the intrinsic coercive field of ferroelectric layers and provides a way to study the properties of materials under high electric fields.

Formulization of long-time domain switching around the coercive field from imprint measurements on ferroelectric thin films

The time interval for the completion of slow polarization reversal in ferroelectric thin films measured in the order of magnitude of 10−9–100s is broadened by more than two decades as the applied field approaches the coercive field of domain switching. The domain-switching kinetics is conjectured to change from the classical Kolmogorov–Avrami–Ishibashi (KAI) model at high fields to nucleation-limited-switching (NLS) model at low fields. However, the true voltage drop across the film from the real-time measurement of the oscilloscope remains constant at a coercive voltage Vc during polarization reversal irrespective of the applied voltage. Therefore, the above high-field assumption for KAI does not exist. From our measurements, a long-time effect of imprint can occur for any slow processes of physical phenomena, even if the imprint field is very weak. The imprinted Vc can be estimated either from the voltage shift in the curve of switched polarization versus the applied voltage or directly from the height variation in domain-switching current plateau after an imprint voltage Vimp for time timp (1μs<timp<10ms). Vc reduces continually with timp under Vimp>0 to exhibit a linear time dependence in a logarithmic scale above an initial time on the order of 1.0μs, even if under Vimp<Vc, which foretells the possibility of the initial unswitched domains with negative polarizations to switch into positive states with the prolonged imprint time under a positive voltage of Vimp. After formulation of the Vc-timp dependence under Vimp according to imprint principles, we derive the time evolution of slow polarization reversal without evoking KAI and NLS approaches once the Vc distribution is obtained for a genuine thin film. The spectrum derivation is completely comparable to experimental measurements, which suggests the common physics between imprint and long-time domain switching of ferroelectric thin films.

On the Electric Fields Contributions to Ferroelectric Thin Films Switching Properties

We have studied the polarization switching in ferroelectric thin films by using a time dependent Ginzburg Landau model. On the contrary to what is described for the case of intrinsic switching which is homogeneous, we have found that by considering the surface effect, this process becomes inhomogeneous. Moreover additional fields contributions induced by the bound charges and the space charges, change the kinetic and, can depending on their origin, modify or not the nature of the switching process.

POLARIZATION SWITCHING IN FERROELECTRIC THIN FILMS STUDIED BY OPTICAL SECOND HARMONIC GENERATION

ABSTRACT Domain configuration during polarization switching in ferroelectric thin films was studied by optical second harmonic generation on different time scales: quasistatic (0.1 Hz), low-frequency (0.1–1 kHz) and fast (0.1 GHz). It is shown that within the electric pulses of several tenth of nanoseconds domain configuration may be different from the equilibrium.

Domain Switching of Ferroelectric Thin Films by Using a Scanning Probe Microscope

Domain Switching of Ferroelectric Thin Films by Using a Scanning Probe Microscope

Electromagnetically induced switching of ferroelectric thin films

We analyze the interaction of an electromagnetic spike (one cycle) with a thin layer of ferroelectric medium with two equilibrium states. The model is the set of Maxwell equations coupled to the undamped Landau-Khalatnikov equation, where we do not assume slowly varying envelopes. From linear-scattering theory, we show that low-amplitude pulses can be completely reflected by the medium. Large-amplitude pulses can switch the ferroelectric. Using numerical simulations and analysis, we study this switching for long and short pulses, estimate the switching times, and provide useful information for experiments.

Nanosecond Domain Wall Dynamics in FerroelectricPb(Zr,Ti)O3Thin Films

Domain wall motion during polarization switching in ferroelectric thin films is fundamentally important and poses challenges for both experiments and modeling. We have visualized the switching of a Pb(Zr, Ti)O(3) capacitor using time-resolved x-ray microdiffraction. The structural signatures of switching include a reversal in the sign of the piezoelectric coefficient and a change in the intensity of x-ray reflections. The propagation of polarization domain walls is highly reproducible from cycle to cycle of the electric field. Domain wall velocities of 40 m s(-1) are consistent with the results of other methods, but are far less than saturation values expected at high electric fields.

ANALYSIS OF THE DIELECTRIC CONTRIBUTION TO THE FERROELECTRIC POLARIZATION INVERSION CURRENT

ABSTRACT We have studied pulse switching in ferroelectric thin films and analyze the dielectric contribution to the polarization inversion current, the so-called non-switching current, in order to describe the kinetics of the switching process. A real time analysis of the voltage wave form shows that the dielectric contribution does not consist of one single component but has to be decomposed into two parts. This decomposition, resulting from the material's dynamic response, has a direct consequence on the wave form of the non-switching current and hence on the net polarization inversion current and on the ferroelectric switching time.

Effect of built-in bias fields on the nanoscale switching in ferroelectric thin films

Polarization switching in Pb(Zr0.3Ti0.7)O3 thin films has been studied in nanoscale level using piezoresponse force microscopy. It is found that the switching in ferroelectric thin films is significantly influenced by the built-in bias field. It has two different origins: one time-independent, and the other time-dependent, which can be reasonably attributed to the effect of the film/electrode interface and the space charges, respectively. The redistribution of the unstable field can be triggered by a long pulse, and is shown to follow an exponential law.

Dynamic process of domain switching in ferroelectric films

In this article, a general variational formulation is established for the dynamics of domain switching in ferroelectric thin films. By incorporating the kinetic energy in the Landau free energy expansion, we obtain a second-order time-dependent Ginzburg–Landau evolution equation, which can be used to analyze the frequency-dependent dielectric and ferroelectric properties of the materials. A numerical model is developed to calculate the time-dependent polarization and electric-field distribution during the domain switching process. As an example of practical importance, a ferroelectric film with 180° domain boundaries perpendicular to the film surface is studied. Domain switching is found to proceed in two stages, the initial nucleation and growth of the ferroelectric domain, followed by the expansion of the domain through the sidewise motion of the 180° domain walls, until the spontaneous polarization in the entire electrified area is reversed.

Prediction of domain nucleation and growth during polarization switching in ferroelectric thin films

A three-dimensional constitutive model for polarization switching in ferroelectric ceramics is presented and implemented into a finite element code. The developed code is applied to investigate the nucleation and growth of domains in a ferroelectric thin film. Initially the thin film is completely poled upward and its bottom electrode is grounded. Then a point on the top surface of the film is subjected to a constant positive electric potential for a certain period of time, leading to a polarization switching downward. The distribution of electric field is not homogeneous within the film and so neither the distribution of polarization. It is found that initially the switched zone is of a dagger shape but after the switched zone penetrates the film thickness it changes to the shape of a reversed cup with the lower part wider that the upper part. It is also observed that the width of switched zone increases with the period and magnitude of applied potential. The predictions are qualitatively compared with experimental observations.

Depolarizing-field-mediated 180° switching in ferroelectric thin films with 90° domains

Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr0.2Ti0.8O3 thin films is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that, under an electric field induced between the PFM tip and the bottom electrode, the 180° switching occurs in both c and a domains. After the removal of this field, the spontaneous reversal of the out-of-plane and in-plane polarizations back to the initial orientations takes place, evolving via heterogeneous development of antiparallel 180° domains. The switching of in-plane polarization inside a domains and the preferential formation of reversed 180° domains at 90° domain walls are explained by the effects of the depolarizing fields caused by transient polarization charges appearing on these domain walls.

Effect of Domain Nucleation on Switching of Ferroelectric Thin Films

Domain nucleation close to a domain wall and within an unswitched domain of a ferroelectric thin film is analyzed theoretically in terms of the local field, from which a model of ferroelectric polarization switching is developed. The coercive field is calculated as a function of film thickness, and the prediction is found to be in good agreement with experiments on PbZr x Ti 1 m x O 3 thin films.

Switching Dynamics in Ferroelectric Thin Films: An Experimental Survey

We have conducted a broad survey of switching behavior in thin films of a range of ferroelectric materials, including some materials that are not typically considered for FeRAM applications, and are hence less studied. The materials studied include: strontium bismuth tantalate (SBT), barium strontium titanate (BST), lead zicronate titanate (PZT), and potassium nitrate (KNO 3 ) Switching in ferroelectric thin films is typically considered to occur by domain nucleation and growth. We discuss two models of frequency dependence of coercive field, the Ishisbashi-Orihara theory where the limiting step is domain growth and the model of Du and Chen where the limiting step is nucleation. While both models fit the data fairly well the temperature dependence of our results on PZT and BST suggest that the nucleation model of Du and Chen is more appropriate for the experimental results that we have obtained.