Ferroelectric Polarization Switching Research Articles

Huge Piezoelectric Response of LaN-based Superlattices.

We construct LaN-based artificial superlattices to investigate the ferroelectricity and piezoelectricity using the volume matching conditions of the parent components that soften the elastic constant C33 and increase the piezoelectric modulus d33. The proposed superlattice consists of LaN and YN (or LaN and ScN) buckled monolayers alternately arranged along the crystallographic c-direction. The structure of polar wurtzite (w-LaYN/w-LaScN) is both mechanically and dynamically stable, and the computed energy barrier makes the ferroelectric polarization switching possible. We show that the epitaxial strain can modify the spontaneous ferroelectric polarization as well as d33. The LaN/YN superlattice exhibits a huge piezoelectric response in the unstrained state, due to their small c/a value and extremely soft C33. In addition, the epitaxial strain is revealed as effective control of the nature (indirect and direct) and value of the electronic band gap.

Key Role of Oxygen-Vacancy Electromigration in the Memristive Response of Ferroelectric Devices

Ferroelectric memristors are intensively studied due to their potential implementation in data storage and processing devices. In this work we show that the memristive behavior of metal/ferroelectric oxide/metal devices relies on the competition of two effects: the modulation of metal/ferroelectric interface barriers by the switchable ferroelectric polarization and the electromigration of oxygen vacancies, with the depolarizing field playing a fundamental role in the latter. We simulate our experimental results with a phenomenological model that includes both effects and we reproduce several non-trivial features of the electrical response, including resistance relaxations observed after external poling. Besides providing insight into the underlying physics of these complex devices, our work suggests that it is possible to combine non-volatile and volatile resistive changes in single ferroelectric memristors, an issue that could be useful for the development of neuromorphic devices.

Electrically driven transient and permanent phase transformations in highly strained epitaxial BiFeO3 thin films

Electric-field-driven phase transformation phenomena in multiferroic BiFeO3 are directly linked to the functionalities of electronic devices based on multiferroic materials. Understanding how the transformation evolves at the nanoscale under the influence of an electric field will provide fascinating insights into key parameters that utilize the transformation features. Here, we report both the electric-field-driven transient and permanent phase transformations in highly strained BiFeO3 thin films and their transformation dynamics at the nanoscale. We found that two distinct transient and permanent phase transformations were triggered below and above a coercive voltage of the polymorphic phase, indicating that ferroelectric polarization switching could promote permanent phase transformations. We also found that the transient transformations evolve via complex phase boundary motions between the coexisting phases, whereas permanent transformations occurred via nucleation of the other phases.

Current-voltage characteristics of phase boundaries PVDF-TrFE(70/30)/PANI nanocomposite

In this work, the local current-voltage characteristics of nanocomposite phase boundaries in the form of a poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) (70/30) matrix filled with conductive polyaniline (PANI) particles are studied by conductive atomic force microscopy. At negative external bias voltages, these dependencies are shown to have an interval with a negative differential resistance. This fact is caused by the switching of ferroelectric polarization of individual nanocrystallites PVDF-TrFE located near the phase boundaries in the amorphous matrix volume. The obtained results confirm assumptions about the memristive properties of the investigated nanocomposites.

Hydrothermal growth of c-axis oriented ferroelectric (Bi1/2K1/2)TiO3 films on metal substrates

Ferroelectric films fabricated on metal substrates are desirable for use in piezoelectric vibration energy harvesters. In this report, we demonstrate that dense films of lead-free ferroelectric (Bi1/2K1/2)TiO3 (BKT) with preferred c-axis orientation can be grown on nickel-based alloy substrates by utilizing the hydrothermal synthesis method. A buffer layer of LaNiO3 (LN) was formed on the substrates by the sol-gel spin coating method, and then BKT was grown by a hydrothermal reaction at a low temperature of 150 °C. The surface structure and thickness of the final BKT films were found to strongly depend on the numbers of the deposition cycles for LN and BKT, respectively. As a result, highly c-axis oriented BKT films with a smooth surface and a thickness up to 830 nm were obtained. Although the as-deposited BKT films were suggested to contain hydroxyl groups in their crystal lattice, a post-annealing process at 650 °C in air was found to be effective to remove them. The polarization response of the annealed BKT film proved the ferroelectric polarization switching in the film.

Polarization Switching in 2D Nanoscale Ferroelectrics: Computer Simulation and Experimental Data Analysis.

The polarization switching kinetics of nanosized ferroelectric crystals and the transition between homogeneous and domain switching in nanoscale ferroelectric films are considered. Homogeneous switching according to the Ginzburg-Landau-Devonshire (LGD) theory is possible only in two-dimensional (2D) ferroelectrics. The main condition for the applicability of the LGD theory in such systems is its homogeneity along the polarization switching direction. A review is given of the experimental results for two-dimensional (2D) films of a ferroelectric polymer, nanosized barium titanate nanofilms, and hafnium oxide-based films. For ultrathin 2D ferroelectric polymer films, the results are confirmed by first-principle calculations. Fitting of the transition region from homogeneous to domain switching by sigmoidal Boltzmann functions was carried out. Boltzmann function fitting data enabled us to correctly estimate the region sizes of the homogeneous switching in which the LGD theory is valid. These sizes contain several lattice constants or monolayers of a nanosized ferroelectrics.

Theoretical and numerical analysis of the Landau–Khalatnikov model of ferroelectric hysteresis

An initial-boundary value problem for the reaction-diffusion Landau–Khalatnikov equation applied to describe polarization switching in ferroelectrics is studied from both theoretical and computational points of view. The existence and uniqueness of a weak solution of the initial-boundary value problem are proved. An absolutely stable monotonic numerical scheme of the second-order accuracy combined with an iterative procedure is constructed. Numerical simulation of the polarization hysteresis in ferroelectrics with the first-order phase transition is performed. The results of the computations obtained on the base of different modifications of the model are compared with experimental data. An important role of the diffusion term and its scaling effect for an adequate description of the polarization switching in ferroelectrics are shown.

Toward Intrinsic Ferroelectric Switching in Multiferroic BiFeO_{3}.

Using pulsed ferroelectric measurements, we probe switching dynamics in multiferroic BiFeO_{3}, revealing low-ns switching times and a clear pathway to sub-ns switching. Our data is well described by a nucleation and growth model, which accounts for the various timescales in the switching process, namely (1)the ferroelectric polarization switching (bound-charge) dynamics and (2)the RC-limited movement of free charge in the circuit. Our model shows good agreement with observed data and begins to bridge the gap between experiment and theory, indicating pathways to study ferroelectric switching on intrinsic timescales.

Tip-force-induced ultrafast polarization switching in ferroelectric thin film: A dynamical phase field simulation

Dynamical phase field simulation is performed to reveal the dynamic characteristics of the tip-force-induced polarization switching in ferroelectric thin films. We demonstrate nontrivial influences of kinetic coefficient μ related to the second-order time derivative term in the dynamic equation of polarization on the mechanical switching behavior. It is found that such a term causes an oscillation feature of the switching process. Two characteristic switching times, i.e., the time when the inversed polarization begins to appear (denoted as τS1) and the time when the fraction of switched (c−) domain is largest during the loading process (denoted as τS2), can be defined to describe the tip-force-induced switching behavior. Both τS1 and τS2 are found to be affected by factors like misfit strain, temperature, and film thickness. Remarkably, the mechanical switching of polarization can be rather fast, with the switching time comparable to that of electrical switching. Due to the nontrivial dynamical effects, other important phenomena are observed: (a) the size and the pattern of switched domain (i.e., cylinder vs ring) in a single-point switching event strongly depend on the loading time, (b) the critical force of mechanical switching may be largely decreased by choosing a proper loading time, and (c) a large and stable domain pattern can still be written by a sweeping tip despite that the switched domain is not stable in the single-point switching event. Our study should provide new insights into the ultrafast phenomena in ferroelectric polarization switching under mechanical stimuli.

A Theoretical Study of Multidomain Ferroelectric Switching Dynamics With a Physics-Based SPICE Circuit Model for Phase-Field Simulations

In this paper, the multi-domain nature of ferroelectric (FE) polarization switching dynamics in a metal-ferroelectric-metal (MFM) capacitor is explored through a physics-based phase field approach, where the three-dimensional time-dependent Ginzburg-Landau (TDGL) equation and Poisson's equation are self-consistently solved with the SPICE simulator. Systematically calibrated based on the experimental measurements, the model well captures transient negative capacitance in pulse switching dynamics, with domain interaction and viscosity being the key parameters. It is found that the influence of pulse amplitudes on voltage transient behaviors can be attributed to the fact that the FE free energy profile strongly depends on how the domains are interacted. This finding has an important implication on the charge-boost induced by stabilization of negative capacitance in an FE + dielectric (DE) stack since the so-called capacitance matching needs to be designed at a specific operation voltage or frequency. In addition, we extract the domain viscosity dynamics during polarization switching according to the experimental measurements. For the first time, a physics-based circuit-compatible SPICE model for multi-domain phase field simulations is established to reveal the effect of domain interaction on the FE energy profile and microscopic domain evolution.

Blocking of Conducting Channels Widens Window for Ferroelectric Resistive Switching in Interface‐Engineered Hf0.5Zr0.5O2 Tunnel Devices

AbstractFilms of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.

MoS2-based ferroelectric field-effect transistor with atomic layer deposited Hf0.5Zr0.5O2 films toward memory applications

Recently, hafnium oxide (HfO2)-based ferroelectric materials have achieved phenomenal success in next-generation nonvolatile memory applications. In this study, we fabricated Hf0.5Zr0.5O2 (HZO) ferroelectric capacitors and back-gate field-effect transistors (FETs) with few-layered molybdenum disulfide (MoS2) nanosheets as the channel and a ferroelectric Hf0.5Zr0.5O2 film as the gate dielectric. Good dielectric and ferroelectric properties have been observed from the HZO film fabricated with atomic layer deposition-based techniques. The MoS2–HZO ferroelectric FETs (FeFETs) have exhibited excellent performance including ferroelectric polarization switching with a high on/off ratio and negligible degradation in endurance and retention properties. Our results shown here suggest that MoS2–HZO FeFETs can be a promising alternative for next-generation nonvolatile memories.

Hybrid Stochastic Fractal-Based Approach to Modeling the Switching Kinetics of Ferroelectrics in the Injection Mode

The paper is devoted to the development and implementation of a hybrid stochastic fractal-based approach for modeling electron-induced kinetics of polarization switching in ferroelectrics as the self-similar memory physical systems. The mathematical model of fractal dynamic system includes the initial value problem for the fractional order differential equation. Computational schemes for solving fractional differential problem are constructed using the Adams–Bashforth–Moulton type predictor-corrector methods. The stochastic algorithm based on the Monte Carlo method is proposed to simulate the domain nucleation process during restructuring the domain structure in ferroelectrics. The polarization switching current of ferroelectrics in the electron injection mode is evaluated to demonstrate the computational experiment results with a comparison of the experimental data.

Reply to “Comment on ‘Ultrafast polarization switching in ferroelectrics' ”

Recently, a method of ultrafast polarization switching in ferroelectrics has been suggested. The basic idea of the method is to employ the effect of self-acceleration of polarization dynamics due to a resonator feedback field. This is the idea of principle whose efficiency is demonstrated by an Ising-type model in a transverse field. Of course, the practical realization of the method requires the choice of appropriate materials, which is a separate problem. For example, the standard order-disorder ferroelectrics with spatially symmetric double-well potentials cannot be used for this purpose since, because of the symmetry, they lack the transverse polarization. However, ferroelectrics with asymmetric potentials, possessing this polarization, can be used. Moreover, if the potential asymmetry, hence, the transverse polarization, could be regulated, e.g., by shear stress or shear strain, this could provide a tool for governing the process of polarization switching.

Comment on “Ultrafast polarization switching in ferroelectrics”

The mechanisms of ultrafast switching of ferroelectric polarization are of great interest and importance currently both from the fundamental and from the practical points of view. One such mechanism, harnessing feedback from the resonator field, was recently proposed by Yukalov and Yukalova [Phys. Rev. Research 1, 033136 (2019)]. The Ising model in a transverse field that is used there is the standard to order-disorder ferroelectrics. However, I note that the assumption of a nonzero transverse electric dipole moment associated with a transverse pseudospin, essential for the proposed mechanism, does not hold for the most common order-disorder ferroelectrics mentioned there for reasons of symmetry. Therefore, care should be taken when choosing the material to which the mechanism can be applied.

Enhanced Photoelectrochemical Performance by Interface Engineering in Ternary g‐C3N4/TiO2/PbTiO3 Films

AbstractPolycrystalline ferroelectric PbTiO3 (PTO) films deposited on transparent fluorine‐doped tin oxide (FTO) glass have been proved to be a photocathode for photoelectrochemical (PEC) water splitting. However, the hitherto reported PEC performances remain inferior to meet the requirements for practical applications. Herein, it is reported that a compact TiO2 (TO) buffer layer, inserted between PTO and g‐C3N4 (CN), improves significantly its PEC performance. The optimal CN/TO/PTO film shows a photocathodic current density of −68.5 µA cm−2 at 0 V versus Ag/AgCl electrode under 100 mW cm−2 Xe‐lamp illumination, which is 4.5 and 2 times higher than that of pristine PTO and CN/PTO films. The performance improvement can be attributed to the following reasons: one is the improved light harvesting ability owing to TO and CN, and the other is that TO functions as a buffer layer to build a type II and a Z‐scheme interfaces between PTO, TO, and CN to promote charge carrier transfer and separation. Last but not the least, the ternary photoelectrode also shows tunable PEC performance by ferroelectric polarization switching.

Thin films of vinylidene fluoride copolymer with tetrafluoroethylene: dynamics of ferroelectric polarization switching

The vinylidene fluoride copolymer with tetrafluoroethylene P(VDF-TFE) is a typical ferroelectric polymer that has not been sufficiently studied so far in comparison with pure polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene. The purpose of this work was to reveal the features of forming ferroelectric polarization and its switching in thin films of P(VDF-TFE) under various conditions, such as the level of the applied electric field and the duration of its action within 8 orders of magnitude from 10 μs to 100 s. The sequence of voltage pulses applied during poling and polarization reversal allowed to reveal the characteristics of the polarization switching dynamics. To increase the resolution of the measurements, two complementary methods of recording electrical induction were used. The features of polarization and space charge relaxation over time have been studied by thermally stimulated depolarization method. A new phenomenon of gradual separation of the relaxation processes associated with polarization and space charge was observed. Both components were accompanied by trapped charges that compensated the depolarizing field. Several practical recommendations have been formulated regarding the desirable values of poling parameters and additional processing to increase the ferroelectric polarization stability.

Configurable Resistive Response in BaTiO3 Ferroelectric Memristors via Electron Beam Radiation.

Ferroelectric oxide memristors are currently in the highlights of a thriving area of research aiming at the development of nonvolatile, adaptive memories for applications in neuromorphic computing. However, to date a precise control of synapse-like functionalities by adjusting the interplay between ferroelectric polarization and resistive switching processes is still an ongoing challenge. Here, it is shown that by means of controlled electron beam radiation, a prototypical ferroelectric film of BaTiO3 can be turned into a memristor with multiple configurable resistance states. Ex situ and in situ analyses of current/voltage characteristics upon electron beam exposure confirm the quasi-continuous variation of BaTiO3 resistance up to two orders of magnitude under the typical experimental conditions employed in electron beam patterning and characterization techniques. These results demonstrate an unprecedented effective route to locally and scalably engineering multilevel ferroelectric memristors via application of moderate electron beam radiation.

Ferroelectric polarization switching induced from water adsorption in BaTiO3 ultrathin films

The influence of water on the out-of-plane polarization of ${\mathrm{BaTiO}}_{3}$ (BTO) ultrathin films is studied by means of density functional theory calculations. The adsorption is investigated for different coverages on both terminations of BTO with, for each case, all possible states of polarization, namely, paraelectric, polarized upward, and polarized downward. We thus demonstrate different behavior as a function of the termination. While ${\mathrm{H}}_{2}\mathrm{O}$ adsorbs only dissociatively on the BaO termination, with a reinforced interaction compared to BTO without out-of-plane polarization, only molecular adsorption is observed on the ${\mathrm{TiO}}_{2}$ termination. In addition, the presence of water is able to switch the polarization. Whatever the initial state of polarization is, water induces a downward state on the BaO termination and an upward polarization on the ${\mathrm{TiO}}_{2}$ one. A detailed analysis of this phenomenon is given.

Switching On/Off Negative Capacitance in Ultrathin Ferroelectric/Dielectric Capacitors.

Ferroelectric (FE) and dielectric (DE) insulator bilayer stacks provide a promising gate for low-power microelectronic devices. To fully realize the FE polarization switching, the DE layer must be ultrathin in the FE/DE bilayer stack. Motivated by this, this work presents the first successful fabrication and characterization of Fe/FeOx/Al2O3/Fe FE/DE bilayer capacitors using in vacuo atomic layer deposition (ALD) with a total FE/DE stack thickness <3-4 nm. A key tuning parameter in generating the FE/DE bilayer capacitors is the thickness of an Al wetting layer between the bottom Fe electrode and the ALD-Al2O3 DE layer. At a large thickness in exceeding 1.0 nm, high-quality conventional DE capacitors of 2.2 nm thick ALD-Al2O3 were obtained with dielectric constant (εr) ∼8.0 that is close to εr ∼ 9.2 for the Al2O3 bulk single crystal with an effective oxide thickness of 1.0 nm. By reducing the Al wetting layer thickness to below 1.0 nm, a thin ferroelectric FeOx interfacial layer of a thickness of 1-2 nm forms, enabling the achievement of a FeOx/Al2O3 FE/DE bilayer capacitor with static negative capacitance. Since all ferroelectric materials are piezoelectric, we show that a dynamic switching on/off of the negative capacitance can be achieved under the application of an external force on the ultrathin FE/DE capacitors through manipulation of the electric dipoles. This result not only provides a viable approach for generating ultrathin FE/DE bilayer capacitors but also offers a promising solution to low-power consumption microelectronics.