Inhomogeneous Field Mechanism Research Articles

Distribution of the built-in field extracted from switching dynamics in HfO2-based ferroelectric capacitor

In this work, we proposed a method to extract the distribution of the built-in field (Eb) from the switching dynamics of the TiN/HfZrO/TiN capacitor. The relationship between reversal polarization and the distribution of Eb is established based on the classic inhomogeneous field mechanism model. Both positive and negative write pulses with different amplitudes and durations are applied to obtain the distribution parameters of Eb. The distribution of Eb is fitted by a Gaussian-type distribution, and the mean value and standard deviation are about −0.02 MV/cm and 0.28 MV/cm, respectively. This work provides an effective approach to analyze Eb directly from the electrical measurement and helps optimize the device design from the polarization switching point of view.

HfO2-ZrO2 Ferroelectric Capacitors with Superlattice Structure: Improving Fatigue Stability, Fatigue Recovery, and Switching Speed.

HfO2-ZrO2 ferroelectric films have recently gained considerable attention from integrated circuit researchers due to their excellent ferroelectric properties over a wide doping range and low deposition temperature. In this work, different HfO2-ZrO2 superlattice (SL) FE films with varying periodicity of HfO2 (5 cycles)-ZrO2 (5 cycles) (SL5), HfO2 (10 cycles)-ZrO2 (10 cycles) (SL10), and HfO2 (15 cycles)-ZrO2 (15 cycles) (SL15) were studied systematically. The HfZrOx (HZO) alloy was used as a comparison device. The SL5 film demonstrated improved ferroelectric properties compared to the HZO film, with the 2 times remnant polarization (2Pr) values increasing from 41.4 to 48.6 μC/cm2 at an applied voltage of 3 V/10 kHz. Furthermore, the first-order reversal curve diagrams of different SL and HZO capacitors at different states (initial, wake-up, fatigue, and recovery) were measured. The SL capacitors were found to effectively suppress the diffusion of defects during P-V cycling, resulting in improved fatigue stability characteristics and fatigue recovery capability compared to the HZO capacitor. Moreover, an improved switching speed of the SL films compared to the HZO capacitor was concluded based on the inhomogeneous field mechanism (IFM) model. These results indicate that the SL structure has a high potential in future high-speed ferroelectric memory applications with excellent stability and recovery capability.

Evolution of the Ferroelectric Properties of AlScN Film by Electrical Cycling with an Inhomogeneous Field Distribution

AbstractThis work investigates the evolution of the ferroelectric (FE) performance of the sputtered aluminum scandium nitride (AlScN) thin film, which has a high remanent polarization (Pr, > 100 µC cm−2) and coercive field (Ec, > 6 MV cm−1), with the electric field cycling. The work aims at elucidating the underlying origin of the severe fatigue behavior, even with a relatively small number of endurance cycles (<105). When cycled with a low electric field, an internal field is created by charges trapped between the FE layer and the interfacial dielectric layer (non‐FE). On the other hand, fatigue is observed when cycled with a high electric field cycling. This work proposes a new method to simulate a switching current utilizing an inhomogeneous field mechanism and the appropriate circuit model to assess the thickness change of the non‐FE layer. It is concluded that the thickened non‐FE layer after the field cycling results in fatigue.

Dynamic scaling properties of multistep polarization response in ferroelectrics

Ferroelectrics are multifunctional smart materials finding applications in sensor technology, micromechanical actuation, digital information storage, etc. Their most fundamental property is the ability of polarization switching under an applied electric field. In particular, understanding of switching kinetics is essential for digital information storage. In this regard, scaling properties of the temporal polarization response are well-known for 180°-switching processes in ferroelectrics characterized by a unique field-dependent local switching time. Unexpectedly, these properties are now observed in multiaxial polycrystalline ferroelectrics, exhibiting a number of parallel and sequential non-180°-switching processes with distinct switching times. This behavior can be explained by a combination of the multistep stochastic mechanism and the inhomogeneous field mechanism models of polarization reversal. Scaling properties are predicted for polycrystalline ferroelectrics of tetragonal, rhombohedral, and orthorhombic symmetries and are exemplarily demonstrated by the measurements of polarization kinetics in (K,Na)NbO3-based ferroelectric ceramic over a timescale of 7 orders of magnitude. Dynamic scaling properties allow insight into the microscopic switching mechanisms, on the one hand, and into statistical material characteristics, on the other hand, thereby providing the description of temporal polarization with high accuracy. The gained deeper insight into the mechanisms of multistep polarization switching is crucial for future ultrafast and multilevel digital information storage.

Effect of thickness scaling on the switching dynamics of ferroelectric HfO2–ZrO2 capacitors

HfO2-based ferroelectric (FE) films have attracted immense attention in the research of ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistors (FeFETs) due to their excellent ferroelectric performance and high compatibility with CMOS integration. The study of switching dynamics is essential in understanding and optimizing HfO2-based FE films for nonvolatile memory applications. To explore the effect of thickness scaling on the polarization switching speed of the HfO2-ZrO2 (HZO) FE film, we demonstrate TaN/HZO/TaN capacitors fabricated with 6, 10, and 20 nm-thick HZO films. The inhomogeneous field mechanism (IFM) model was used to characterize the polarization switching of polycrystalline HZO films. The results showed that the characteristic time constants for the 6, 10, and 20-nm-thick HZO samples were 283, 43.7, and 20.5 ps, respectively, which is inconsistent with the variation in coercive electric field (EC) with sample thickness. These results demonstrate the distribution of the local fields in capacitors with various HZO thicknesses and contribute to the design and optimization of HZO films for advanced nanonode applications.

Effect of high pressure anneal on switching dynamics of ferroelectric hafnium zirconium oxide capacitors

Investigation of the polarization switching mechanism in ferroelectric hafnium zirconium oxide (HZO) film is of great importance for developing high-quality ferroelectric memory devices. Recently, several works have been reported to describe the switching process of polycrystalline HZO film using the inhomogeneous field mechanism (IFM) model. However, no report has recorded the effect of high pressure annealing (HPA) on the polarization switching process. In this paper, we have carried out a careful investigation on the switching properties of HZO capacitors annealed at 600 °C with various high pressure conditions (1, 50, and 200 atm) using the IFM model. As pressure increases to 200 atm, the ferroelectric properties were enhanced in the HZO films, and, as a result, highest remanent polarization (Pr of 24.5 μC/cm2) was observed when compared with 1 and 50 atm. Similarly, as HPA increases, the HZO capacitors showed a decrement of the coercive field, which significantly improved the switching properties. The time consumed for reversing 80% polarization was 113.1, 105.7, and 66.5 ns for the sample annealed at 1, 50, and 200 atm, respectively. From the IFM model, the smallest active field (2.997 MV/cm) and a uniform distribution of the local electric field (0.304) were observed at 200 atm. Furthermore, the characteristic time constant (τ0) showed a decreasing trend (34.7, 18.1, and 11.7 ps) with increasing HPA. The improved switching properties and detailed findings recorded in this study may be helpful for developing the ferroelectric hafnia based non-volatile memory applications.

Polarization switching in thin doped HfO2 ferroelectric layers

The deployment of ferroelectrics in device concepts such as Ferroelectric Random Access Memory and Ferroelectric Field Effect Transistors requires a good understanding of the polarization switching mechanisms. While several reports already exist involving classical perovskite ferroelectrics, only recently has the switching dynamics in HfO2-based layers started to be addressed. In this work, the Kolmogorov–Avrami–Ishibashi (KAI), the Nucleation Limited Switching (NLS), the Landau–Khalatnikov (LK), and the Inhomogeneous Field Mechanism (IFM) models for polarization switching are surveyed and evaluated with the existing body of literature. Data concerning NLS and IFM are compared to experiments undertaken in this study. After excluding the KAI model because of considerations dealing with film morphology and domain wall energy, we conclude that the NLS, the LK, and the IFM models do not necessarily mutually exclude each other, but rather give a diverse perspective on the switching phenomenon based on thermodynamic, kinetic, statistic, microscopic, and/or macroscopic points of view.

Stochastic model of dispersive multi-step polarization switching in ferroelectrics due to spatial electric field distribution

A stochastic model for polarization switching in tetragonal ferroelectric ceramics is introduced, which includes sequential 90°- and parallel 180°-switching processes and accounts for the dispersion of characteristic switching times due to a nonuniform spatial distribution of the applied field. It presents merging of the recent multistep stochastic mechanism with the earlier nucleation limited switching and inhomogeneous field mechanism models. The model provides a much better description of simultaneous polarization and strain responses over a wide time window and a deeper insight into the microscopic switching mechanisms, as is exemplarily shown by comparison with measurements on lead zirconate titanate.

Switching dynamics of ferroelectric HfO2-ZrO2 with various ZrO2 contents

To explore the effect of the ZrO2 content on the switching speed of ferroelectric HfO2-ZrO2 (FE-HZO), we demonstrate 10 nm FE-HZO capacitors fabricated with 5:5, 6:4, and 7:3 HfO2 and ZrO2 atomic-layer-deposition-cycling ratios. The FE-HZO devices show high remanent polarization (Pr) of 26, 20, and 11 μC/cm2 for the 5:5, 6:4, and 7:3 samples, respectively. The FE-HZO capacitors with lower ZrO2 contents show increasing coercive fields, which intuitively seem to increase switching difficulty. However, the FE-HZO devices with 50 mol. %, 40 mol. %, and 30 mol. % ZrO2 contents show decreasing switching times for 80% polarization, namely, 1.2, 0.9, and 0.7 μs, respectively. Because of the polycrystalline nature of FE-HZO, the distribution of local fields in the film is analyzed based on the inhomogeneous field mechanism model. The results show that the FE-HZO devices with lower ZrO2 contents have higher active fields and less uniform distribution of local fields. However, time constants for the 5:5, 6:4, and 7:3 samples decrease dramatically, being 137, 98, and 14 ps, respectively. These results unveil the distribution of the local fields in FE-HZO with varying ZrO2 contents and are helpful for understanding and optimizing the switching dynamics of FE-HZO for non-volatile memory applications.

Polarization-switching dynamics in bulk ferroelectrics with isometric and oriented anisometric pores

Highly porous ferroelectric ceramics possess remarkably less polarizability than dense ceramics; instead they display high tunability of various physical properties. Particularly, the shape and orientation of pores as well as the total porosity exhibit a great effect on the polarization-switching dynamics. In the present work, finite-element simulations of the electric-field distributions and related statistical distributions of local switching times are analysed and compared with the switching characteristics of porous lead zirconate titanate ceramics, extracted from the experiment by means of the inhomogeneous field mechanism model of polarization switching. Surprisingly, the simulated statistical field-distributions turn out to be virtually independent of the pore-size distribution; however, they are sensitive to the anisometric shape and orientation of the pores. Additionally, they exhibit notable broadening with increasing porosity; an effect confirmed by experimental observations.

Fatigue effect on polarization switching dynamics in polycrystalline bulk ferroelectrics

Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM-analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.

Pulse-modulated multilevel data storage in an organic ferroelectric resistive memory diode.

We demonstrate multilevel data storage in organic ferroelectric resistive memory diodes consisting of a phase-separated blend of P(VDF-TrFE) and a semiconducting polymer. The dynamic behaviour of the organic ferroelectric memory diode can be described in terms of the inhomogeneous field mechanism (IFM) model where the ferroelectric components are regarded as an assembly of randomly distributed regions with independent polarisation kinetics governed by a time-dependent local field. This allows us to write and non-destructively read stable multilevel polarisation states in the organic memory diode using controlled programming pulses. The resulting 2-bit data storage per memory element doubles the storage density of the organic ferroelectric resistive memory diode without increasing its technological complexity, thus reducing the cost per bit.

Effect of texturing on polarization switching dynamics in ferroelectric ceramics

Highly (100),(001)-oriented (Ba0.85Ca0.15)TiO3 (BCT) lead-free piezoelectric ceramics were fabricated by the reactive templated grain growth method using a mixture of plate-like CaTiO3 and BaTiO3 particles. Piezoelectric properties of the ceramics with a high degree of texture were found to be considerably enhanced compared with the BCT ceramics with a low degree of texture. With increasing the Lotgering factor from 26% up to 94%, the piezoelectric properties develop towards the properties of a single crystal. The dynamics of polarization switching was studied over a broad time domain of 8 orders of magnitude and was found to strongly depend on the degree of orientation of the ceramics. Samples with a high degree of texture exhibited 2–3 orders of magnitude faster polarization switching, as compared with the ones with a low degree of texture. This was rationalized by means of the Inhomogeneous Field Mechanism model as a result of the narrower statistical distribution of the local electric field values in textured media, which promotes a more coherent switching process. The extracted microscopic parameters of switching revealed a decrease of the critical nucleus energy in systems with a high degree of texture providing more favorable switching conditions related to the enhanced ferroelectric properties of the textured material.

Statistical electric field and switching time distributions in PZT 1Nb2Sr ceramics: Crystal- and microstructure effects

Dispersive polarization response of ferroelectric PZT ceramics is analyzed assuming the inhomogeneous field mechanism of polarization switching. In terms of this model, the local polarization switching proceeds according to the Kolmogorov-Avrami-Ishibashi scenario with the switching time determined by the local electric field. As a result, the total polarization reversal is dominated by the statistical distribution of the local field magnitudes. Microscopic parameters of this model (the high-field switching time and the activation field) as well as the statistical field and consequent switching time distributions due to disorder at a mesoscopic scale can be directly determined from a set of experiments measuring the time dependence of the total polarization switching, when applying electric fields of different magnitudes. PZT 1Nb2Sr ceramics with Zr/Ti ratios 51.5/48.5, 52.25/47.75, and 60/40 with four different grain sizes each were analyzed following this approach. Pronounced differences of field and switching time distributions were found depending on the Zr/Ti ratios. Varying grain size also affects polarization reversal parameters, but in another way. The field distributions remain almost constant with grain size whereas switching times and activation field tend to decrease with increasing grain size. The quantitative changes of the latter parameters with grain size are very different depending on composition. The origin of the effects on the field and switching time distributions are related to differences in structural and microstructural characteristics of the materials and are discussed with respect to the hysteresis loops observed under bipolar electrical cycling.

Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ferroelectrics

Analysis of polarization switching dynamics by means of the inhomogeneous field mechanism model allows insight into the microscopic mechanism of reversed polarization domain nucleation. For all chemical compositions studied, two distinct field regions of nucleation are established. In the high-field region, the activation energy barrier is found to be inversely proportional to the local field according to the Merz law. In contrast, the barriers in the low-field region exhibit a linear field dependence with a minimum in the compositional region of phase instability, which can explain the corresponding peak ferroelectric properties.

Polarization switching dynamics by inhomogeneous field mechanism in ferroelectric polymers

The understanding of polarization switching dynamics of ferroelectrics is of great importance for practical applications and has been steadily advanced for ferroelectric ceramics and polymers for more than half a century. The temporal behaviour of polarization reversal in ferroelectric copolymers such as P(VDF-TrFE) cannot be satisfactorily explained by simple models such as the classical Kolmogorov–Avrami–Ishibashi nucleation and growth theory.In this paper the inhomogeneous field mechanism (IFM) model recently proposed for PZT ceramics has been applied to polymer ferroelectrics for the first time. The model is based on the assumption that the switching volume is divided into many spatial regions with independent dynamics, only determined by the local electric field. The local field values are randomly distributed over the ensemble of regions due to intrinsic inhomogeneities of the material. Therefore an inhomogeneous switching behaviour is induced by the varying local fields of each region. The statistical distribution of local field values can be directly extracted from the experimental data. The model satisfactorily describes virgin P(VDF-TrFE) samples over a broad time-field domain covering eight orders of magnitude of poling time and electric field values from 30–150 kV mm−1. In the same way we can conclude that the IFM model is adaptive to both ferroelectric ceramics and semi-crystalline polymers.

Universal Polarization Switching Behavior of Disordered Ferroelectrics

AbstractUniversal scaling features of polarization switching are established experimentally in rather different classes of disordered ferroelectrics: in well‐studied lead‐zirconate titanate (PZT) ferroelectrics, in recently synthesized Cu‐stabilized 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 (BNT‐BT) relaxor ferroelectrics, and in classical organic ferroelectrics P(VDF‐TrFE). These scaling properties are explained by an extended concept of an inhomogeneous field mechanism (IFM) of polarization dynamics in ferroelectrics. Accordingly, disordered ferroelectrics exhibit a wide spectrum of characteristic switching times due to a statistical distribution of values of the local electric field. How this distribution can be extracted from polarization measurements is demonstrated. Generally, it is shown that the polarization response is primarily controlled by the statistical characteristics of disorder rather than by a temporal law of the local polarization switching.

Dynamics of polarization reversal in virgin and fatigued ferroelectric ceramics by inhomogeneous field mechanism

Temporal behavior of ferroelectric ceramics during the polarization switching cannot be satisfactorily explained by simple Debye or even stretched exponential laws. These materials exhibit rather a wide spectrum of characteristic times interpreted by different authors as switching or nucleation waiting times, the physical reasons for a wide time distribution still remaining unclear. A new model of polarization switching presented here suggests that the characteristic time variance in the ferroelectrics originates from the random distribution of the local electric fields due to intrinsic randomness of the material. The presented theory allows a direct extraction of the distribution of field values from the experiment. Systematic studies of polarization switching in fatigued lead zirconate titanate demonstrate the evolution of the field distribution with increasing level of fatigue. Plausible cause of the formation of regions subject to different field strengths is the generation of defects such as microcracks, pores, or voids in the course of fatigue. Suitability of the proposed model is demonstrated by an excellent correlation between experimental and calculated data for virgin and differently fatigued samples in a broad time-field region covering the electric field values of 0.5--2.5 kV/mm and nine orders of the magnitude of poling time.