Ferroelectric Switching Behavior Research Articles

Nonvolatile ferroelectric resistive switching in α-In2Se3(2H) ferroelectric semiconductor junctions

Emerging two-dimensional van der Waals (vdW) based ferroelectric semiconductor junctions (FSJs) are promising for low-power and high-density applications. In this work, we report the vdW α-In2Se3(2H) based FSJs of different metal-In2Se3 interfaces. All vertical devices emerge with the nonvolatile ferroelectric resistive switching behavior. The Cr/α-In2Se3(2H)/Au FSJs show a typical ferroelectric resistive switching behavior under ∼1.0 V applied voltage. The switching voltage of the Ag/α-In2Se3(2H)/Au FSJs could be further decreased from 1.0 V to 0.8 V because of Ag migrations and ferroelectric reversal. Further, by optimizing the metal-In2Se3 interfaces, the Ag/α-In2Se3(2H)/Pt FSJs exhibit the ultralow switching voltage (∼0.3 V), excellent endurance (>225 cycles) and retention (>1 × 104 s). The results suggest that vdW α-In2Se3(2H) ferroelectric has great potential for low-power memristive devices.

Effect of the number and distribution of Al2O3 atomic layer deposition cycles within HfO2 layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al2O3 atomic layer deposition (ALD) cycles into a 10-nm-thick HfO2 matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (Pr) is enabled through the formation of crystalline Al-doped HfO2 (Al:HfO2) by incorporating at least two Al2O3 ALD cycles evenly into the HfO2 film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al2O3 cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 105 cycles. Notably, an improved 2 Pr of ∼9 μC/cm2 is achieved when the Al2O3 layers are concentrated near the lower region of the HfO2. On the other hand, as the Al2O3 layers are intensively located in the upper region of the HfO2, a dielectric response is observed in the polarization–voltage and current–voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO2 effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO2 layer to form the orthorhombic phase.

Polarization switching pathways of ferroelectric Zr-doped HfO2 based on the first-principles calculation

Based on the first principles calculation, the mechanisms of polarization switching behavior in ferroelectric Zr-doped HfO2 are investigated. Seven switching pathways, divided into two categories by the identified orientation of polarization switching and value, are analyzed based on atomic migration and energy barrier. The effects of Zr dopant on switching energy barrier (Eb) and spontaneous polarization (Ps) are analyzed as well. In one of the categories, two pathways with tetragonal-like transition states show low energy barriers and can be further minimized with higher Zr dopant proportion, which originates from the stabilizing effect of Zr dopant on the tetragonal phase (T, P42/nmc). Especially, in the two tetragonal-like pathways, a distorted tetragonal-like transient state (T′, Pbcn) resulting from distinct atomic displacement is transformed to a highly symmetric T-phase along with the incorporation of Zr, elucidating this pathway as energy favorable as the regular T-pathway. This work provides an atomic insight for ferroelectric switching behavior and predicts the probable ferroelectric switching pathway in Zr-doped HfO2 films.

High tunneling electroresistance in ferroelectric tunnel junctions based on two-dimensional α-In2Se3/MoTe2 van der Waals heterostructures.

Ferroelectric polarization-controlled band alignment can be realized in van der Waals heterostructures (vdWHs), which can be used to create new types of ferroelectric tunnel junctions (FTJs). In this work, we design six probable configurations of two-dimensional vdWHs based on a two-dimensional α-In2Se3 ferroelectric material which has two opposite polarization states P↑ and P↓, and the semiconductor MoTe2. First-principles calculations show robust ferroelectric polarization-controlled switching behavior between the high conductance state in configuration AA-P↓ and the low conductance state in configuration AA-P↑ in the most stable AA stacked vdWHs. Based on this vdWH, a two-dimensional transverse FTJ with AA-P↓ or AA-P↑ as the tunneling barrier and (In0.5Sn0.5)2Se3 monolayers (n-type doped) as electrodes is designed. The tunneling electroresistance ratio of the FTJs at the Fermi level reaches 1.22 × 104% when the tunneling barrier contains two repeating units N = 2 and can be greatly increased by increasing the thickness of the ferroelectric layer. Analysis of the work function, charge redistribution, and local density of states is performed to interpret the above phenomena. The findings suggest the great potential of the AA stacked α-In2Se3/MoTe2 vdWH in the design of high-performance FTJs and application in high-density non-volatile memory devices.

Evolution between ferroelectric photovoltaic effect and resistance switching behavior engineered by the polarization field and barrier characteristics.

The bandgap and polarization field play a key role in the ferroelectric photovoltaic effect. However, narrow bandgap induced electrical conductivity always brings out a depression of the photovoltaic performances. Based on the mechanisms of the photovoltaic effect and resistance switching behaviors in ferroelectric materials, this work realizes an evolution between the two effects by engineering the polarization field and barrier characteristics, which addresses the trade-off issues between the bandgap and polarization for ferroelectric photovoltaic effect. SrCoOx (SC, 2.5≤x≤3) with multivalent transition is introduced into Na0.5Bi0.5TiO3 (NBT) matrix material to engineered the polarization field and barrier characteristics. (1-x)NBT-xSC (x=0.03, 0.05, 0.07) solid solution films present an evolution of ferroelectric photovoltaic effect to grow out of nothing again to the disappearance of the photovoltaic effect and the appearance of resistance switching behavior. The 0.95NBT-0.05SC film achieve the open-circuit voltage of 0.81 V and the short-circuit current of 23.52 µA/cm2, and the 0.93NBT-0.07SC film obtains the resistive switching behavior with switch ratio of 100. This work provides a practicable strategy to achieve the fascinating evolution between photovoltaic effect and resistive switching.

Stability and physical properties of the K, Na and Nb doped BiFeO3 lead-free system synthesized by the solid state route

A study of the structural, dielectric, and optical properties of the lead-free ceramic compound (1-x)BiFeO3–(x)(K0.5Na0.5)NbO3 [(1-x)BFO-(x)KNN] prepared by the solid-state reaction method is presented. The structural analysis was carried out by performing Rietveld refinement on the X-ray diffraction data showing a distorted rhombohedral structure of the R3c space group for all samples. The presence of all active Raman modes confirms this result. The effect of K, Na, and Nb incorporation in the BFO structure is clearly seen in the scanning electron microscopy micrographs of the samples, and energy dispersive x-ray spectroscopy (EDS) confirms the existence of all elements. Piezoelectric force microscopy analysis reveals a well-defined domain structure for the higher concentrations of KNN as well as ferroelectric switching behavior. The room temperature dielectric response increases with KNN doping. A small hump in the dielectric permittivity vs. temperature plots is observed near 350 °C and is ascribed to a para-antiferromagnetic phase transition at the Nèel temperature. An optical absorption study revealed that tuning of the bandgap energy is possible through controlled doping, as shown in the present case, where the bandgap varies from 2.16 eV for x = 0.0 to 1.93 eV for x = 0.08. The results of this investigation demonstrate that K, Na, and Nb doping is an effective way to enhance the dielectric and optical properties of BFO.

Emergent Ferroelectric Switching Behavior from Polar Vortex Lattice.

Topologically protected polar textures have provided a rich playground for the exploration of novel, emergent phenomena. Recent discoveries indicate that ferroelectric vortices and skyrmions not only host properties markedly different from traditional ferroelectrics, but also that these properties can be harnessed for unique memory devices. Using a combination of capacitor-based capacitance measurements and computational models, it is demonstrated that polar vortices in dielectric-ferroelectric-dielectric trilayers exhibit classical ferroelectric bi-stability together with the existence of low-field metastable polarization states. This behavior is directly tied to the in-plane vortex ordering, and it is shown that it can be used as a new method of non-destructive readout-out of the poled state.

Impedance switching for neural recording with ferroelectric microelectrodes

We demonstrate ferroelectric and bipolar impedance switching behavior in 18-nm-thick epitaxial BaTiO3 (BTO) films in an electrolyte–ferroelectric–semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. Cyclic voltammetry measurements in EFS configuration, with a phosphate-buffered saline solution acting as the liquid electrolyte top contact, indicate characteristic ferroelectric switching peaks in the bipolar current–voltage loop. Moreover, small-signal electrochemical impedance spectroscopy measurements (applied root mean square voltage VRMS = 10 mV) on pre-poled EFS devices indicate bipolar impedance switching behavior. Also, a maximum ratio of the two different impedance magnitudes of ∼1.5 was observed at frequency f = 100 Hz. The observed impedance switching corresponds to a resistive switching effect, which could be explained by the modulation of the space charge region at the BTO/electrolyte interface via fixed ferroelectric polarization charges. Our approach represents a key step toward neural recordings with ferroelectric microelectrodes.

Competition between Stepwise Polarization Switching and Chirality Coupling in Ferroelectric GeS Nanotubes

Ferroelectricity of group-IV chalcogenides MX (M = Ge, Sn; X = Se, S) monolayers has been extensively investigated. However, how the ferroelectricity evolves in their one-dimensional nanotubes remains largely unclear. Employing an accurate deep-learning interatomic potential of first-principles precision, we uncover a general stepwise mechanism for polarization switching in zigzag and chiral GeS nanotubes, which has an energy barrier that is substantially lower than the one associated with the conventional one-step switching mechanism. The switching barrier (per atom) gradually decreases with increasing the number of intermediate steps and converges to a value that is almost independent of the tube diameter. In the chiral GeS nanotubes, the switching path of polarization with chirality coupling is preferred at less intermediate steps. This study unveils novel ferroelectric switching behaviors in one-dimensional nanotubes, which is critical to coupling ferroelectricity and chirality.

In-plane charged domain walls with memristive behaviour in a ferroelectric film.

Domain-wall nanoelectronics is considered to be a new paradigm for non-volatile memory and logic technologies in which domain walls, rather than domains, serve as an active element. Especially interesting are charged domain walls in ferroelectric structures, which have subnanometre thicknesses and exhibit non-trivial electronic and transport properties that are useful for various nanoelectronics applications1-3. The ability to deterministically create and manipulate charged domain walls is essential to realize their functional properties in electronic devices. Here we report a strategy for the controllable creation and manipulation of in-plane charged domain walls in BiFeO3 ferroelectric films a few nanometres thick. By using an in situ biasing technique within a scanning transmission electron microscope, an unconventional layer-by-layer switching mechanism is detected in which ferroelectric domain growth occurs in the direction parallel to an applied electric field. Based on atomically resolved electron energy-loss spectroscopy, in situ charge mapping by in-line electron holography and theoretical calculations, we show that oxygen vacancies accumulating at the charged domain walls are responsible for the domain-wall stability and motion. Voltage control of the in-plane domain-wall position within a BiFeO3 film gives rise to multiple non-volatile resistance states, thus demonstrating the key functional property of being a memristor a few unit cells thick. These results promote a better understanding of ferroelectric switching behaviour and provide a new strategy for creating unit-cell-scale devices.

Anomalous Phase Transition, Polarization Switching, and Relaxation in a Novel Composite Consisting of Dimethylammonium Aluminum Sulfate Hexahydrate and Oxidized Multiwalled Carbon Nanotubes

A novel ferroelectric composite consisting of dimethylammonium aluminum sulfate hexahydrate (CH3)2NH2Al(SO4)2·6H2O and oxidized multiwalled carbon nanotubes was developed to investigate its dielectric relaxation, phase transition, and ferroelectric switching behaviors. Experimental methods combined with theoretical analyses were employed. The results showed that the filler of oxidized multiwalled carbon nanotubes caused the freezing of domain-wall motion in (CH3)2NH2Al(SO4)2·6H2O at 98.2 K, leading to a decrease in dielectric constant, dielectric loss, and the polarization. In addition, an increase in dielectric permittivity and dielectric loss was found at higher temperatures. The role of hydroxyl groups incorporated in the carbon nanotubes after its oxidation was responsible for the obtained anomalies.

Switching Behavior of Dip‐Coated Organic Ferroelectric Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

AbstractSupramolecular organic ferroelectrics have unique properties related to plasticity and freedom of design but the extensive post‐deposition treatments that are typically required to achieve saturation polarization limit their applicability. A possible solution is to pre‐align the molecules. Here, the effect of dip‐coating on the pre‐alignment of the prototypical supramolecular ferroelectric trialkylbenzene‐1,3,5‐tricarboxamide (BTA) and the resulting changes in the ferroelectric switching behavior of the resulting thin films are analyzed. Dip‐coated films are characterized by different BTA concentrations and dip‐coating velocities. Landau–Levich and evaporation regimes are identified. The corresponding ferroelectric switching behavior is investigated and reveals that dip‐coating in general enhances the ferroelectric switching of pristine films. Dip‐coating perpendicular to the electrodes (parallel to the electric field direction) has the most significant effect, and without further alignment steps, leads to equal switching behavior as films that are aligned by post‐deposition treatment, that is, field annealing. The results are expected to be transferable to other small molecular ferroelectrics.

High Rate Deposition of Piezoelectric AlScN Films by Reactive Magnetron Sputtering from AlSc Alloy Targets on Large Area.

This paper reports on the deposition and characterization of piezoelectric AlXSc1-XN (further: AlScN) films on Si substrates using AlSc alloy targets with 30 at.% Sc. Films were deposited on a Ø200 mm area with deposition rates of 200 nm/min using a reactive magnetron sputtering process with a unipolar–bipolar hybrid pulse mode of FEP. The homogeneity of film composition, structural properties and piezoelectric properties were investigated depending on process parameters, especially the pulse mode of powering in unipolar–bipolar hybrid pulse mode operation. Characterization methods include energy-dispersive spectrometry of X-ray (EDS), X-ray diffraction (XRD), piezoresponse force microscopy (PFM) and double-beam laser interferometry (DBLI). The film composition was Al0.695Sc0.295N. The films showed good homogeneity of film structure with full width at half maximum (FWHM) of AlScN(002) rocking curves at 2.2 ± 0.1° over the whole coating area when deposited with higher share of unipolar pulse mode during film growth. For a higher share of bipolar pulse mode, the films showed a much larger c-lattice parameter in the center of the coating area, indicating high in-plane compressive stress in the films. Rocking curve FWHM also showed similar values of 1.5° at the center to 3° at outer edge. The piezoelectric characterization method revealed homogenous d33,f of 11–12 pm/V for films deposited at a high share of unipolar pulse mode and distribution of 7–10 pm/V for a lower share of unipolar pulse mode. The films exhibited ferroelectric switching behavior with coercive fields of around 3–3.5 MV/cm and polarization of 80–120 µC/cm².

Reduction of depolarization field effect on ferroelectric switching process in semiconductor–relaxor ferroelectric composite

Temperature-dependent dynamic ferroelectric hysteresis of semiconductor–relaxor ferroelectric (0–3) type composite {0.30(ZnO)–0.70[(Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBTO)]} has been investigated using polarization–electric field (P–E) loops, current density–electric field (J–E) curves, and temperature-dependent dielectric permittivity. It is well known that the polarization reversal mechanism can be explained by the concept of ferroelectric domain switching kinetics, which depends strongly on the temperature. The present work ascribes the role of polar nanoregion induced thermal depolarization field on the temperature-dependent ferroelectric hysteresis loop along with polarization reversal mechanism. The present composite exhibits unique ferroelectric switching behavior above the thermal depolarization temperature (∼100 °C), which is observed in P–E and J–E loops. The depolarization field-induced pinched P–E loops of a BNBTO solid solution above Td (∼100 °C) have been significantly overcome by the incorporation of semiconductor (ZnO) particles, which extensively described the underlying mechanism in the present context. In addition, the temperature-dependent polarization reversal mechanism displays unique two-stage processes [low-T (<100 °C) and high-T (>100 °C)] for the minor loops (∼30 and 40 kV) and saturated loops (∼45 kV) as described by the electric field–temperature phase diagram. The present results may provide a distinct way to Bi0.5Na0.5TiO3-based solid solutions for high-temperature piezoelectric applications.

Role of Defects and Power Dissipation on Ferroelectric Memristive Switching

AbstractAdvancement of information technology requires low power, high speed, and large capacity non‐volatile memory. Memristors have potential applications for not only information storage but also neuromorphic computation. Memristive devices are mostly focused on the use of binary oxides as the resistive switching materials. On the other hand, polarization assisted memristive devices based on ternary ferroelectric oxides are attracting more attention due to their unique switching properties. However, the underlying switching mechanisms and the current–voltage rotation direction are still not fully understood yet. By comparing stoichiometric BaTiO3, BiFeO3, and Bi1‐xFeO3‐δ ferroelectric memristors with different cation stoichiometry, it is found that off‐stoichiometry‐induced traps can play a critical role in controlling the ferroelectric memristive switching behavior. Ferroelectrics with slight off‐stoichiometry show greatly enhanced switching properties, and the switching on/off ratio is mainly determined by the trap energy levels and concentrations. The rotation direction of current–voltage hysteresis loop is affected by the defects, which can be controlled by synthesis and power dissipation. These findings provide insight in understanding the role of defects in ferroelectric memristors and offer guidance to design ferroelectric memristors with enhanced performance.

Domain Dynamics and Local Level Switching Behaviors of Sol–Gel Nanopowder-Derived Lead-Free Piezoelectric Bi0.5(Na0.78K0.22)0.5TiO3 Ceramics

The material properties of sol–gel nanopowder-derived lead-free Bi0.5(Na0.78K0.22)0.5TiO3 (BNKT) ceramics are systematically investigated for high-response piezoelectric device applications. To elucidate the principle of the domain dynamics and local level ferroelectric switching behaviors of environmentally friendly BNKT ceramics, we systematically analyze the Rayleigh parameters and the first-order reversal curve (FORC) of the BNKT ceramics and compare the results with those of the standard measurement method. Based on the characterization of Rayleigh parameters, it is clarified that the contribution of the domain-wall motion of the sol–gel-derived BNKT ceramics is higher than that of solid-state reaction-derived ceramics. We also analyze the FORC distribution diagram-associated hysteron density functions for visualizing reversible and irreversible contributions of domain walls and switching behaviors above the Rayleigh regime. The switching behaviors are explained by considering the defects and microstructural differences of the BNKT ceramics, which are affected by both reversible and irreversible contributions. The sol–gel-derived BNKT ceramics show highly localized and well-defined FORC distribution diagrams with the sharp maximum of the irreversible component and the low reversible contribution, which clearly indicate more homogeneous switching properties and less defects than solid-state reaction-derived ceramics because each switching unit gives almost identical contributions to the total polarization. In this study, the developed technique and gained information are used to explore a fundamental understanding of the domain dynamics and local level switching behaviors of lead-free piezoelectric ceramics. We believe that our investigation certainly positions itself in various multifunctional electronic applications without using toxic lead-based piezoelectrics.

Understanding and modulation of resistive switching behaviors in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 multilayer junctions

The insertion layer in the ferroelectric multilayer junctions plays a key role in regulating the energy band structure of interface and their resistive switching behavior. Here, PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 (PZT/LSMO/NSTO) heterostructures with different LSMO thicknesses were prepared by pulsed laser deposition and their ferroelectric properties and resistive switching behaviors were investigated. It is found that the ferroelectric properties of the heterostructures almost do not change with the increasing in the thickness of LSMO, while the resistive switching behaviors are closely related to the LSMO thickness, and the maximum switching ratio of about 103 can be achieved in the PZT/LSMO/NSTO heterostructure with the LSMO thickness of 18 nm. By analyzing the I-V curves and energy band structures, it can be concluded that the resistive switching behaviors depend on the competition between the tunability of depletion layer width and the ability of ferroelectric filed effect. These results provide insights to understand ferroelectric resistive switching in ferroelectric heterostructures, and demonstrate an effective way to improve resistive switching performance by adjusting the band structure through the appropriate thickness of the insertion layer.

Ferroelectric switching behavior of nanoscale Hf0.5Zr0.5O2 grains

Grain size has shown significant effect on the ferroelectricity of HfO2-based thin films. However, its microscopic mechanism remains unclear. For this purpose, Hf0.5Zr0.5O2 film with different sized grains was prepared by radio frequency magnetron sputtering. The piezoresponse force microscope characterization of the domain switching properties of nanograins shows that domain switching ability of the grain becomes better as its size decreases within the size range we studied. Then, density functional theory (DFT) calculation of grains’ energy proves that the lower surface energy of ferroelectric phase is the key factor to make the ferroelectric phase more stable and larger proportion in smaller sized grain. Finally, a multi-phase coexistence phase field model is developed based on the DFT calculation. It is found out that if the contribution of surface energy to the total energy of nano-grain increases, the volume fraction of ferroelectric phase will increase. And (001) ferroelectric phase with the lowest surface energy density will be the most stable and account for the largest volume fraction. Our finding provides a microscopic insight into the “grain size effect” and also some guidelines for high-performance ferroelectric thin films preparation through “grain size engineering”.

Fully epitaxial ferroelectric ScGaN grown on GaN by molecular beam epitaxy

We report on the ferroelectric properties of single-phase wurtzite ScGaN grown on GaN by plasma-assisted molecular beam epitaxy. Distinct ferroelectric switching behavior was confirmed by detailed electrical characterization. Coercive fields in the range of 2.0–3.0 MV/cm and large, retainable remnant polarization in the range of 60–120 μC/cm2 are unambiguously demonstrated for ScGaN epilayers with Sc contents of 0.31–0.41. Taking advantage of the widely tunable energy bandgap of III-nitride semiconductors, the demonstration of ferroelectricity in ScGaN, together with the recently reported ferroelectric ScAlN, will enable a broad range of emerging applications with combined functionality in ferroelectric, electronic, optoelectronic, photovoltaic, and/or photonic devices and systems.

Ferroelectric switching in GeTe through rotation of lone-pair electrons by Electric field-driven phase transition

Germanium telluride (GeTe), which has long been used in phase-change devices, has very unique structural characteristics. While atomic movement and related bond-order switching have been believed to determine the ferroelectric characteristics of this compound, structural changes accompanying ferroelectric switching have not been examined. In this study, using transmission electron microscopy and Raman spectroscopy, we directly observed the atomic movement and the related change in the local structure of GeTe during ferroelectric switching under an applied E-field. The crystal symmetry of GeTe could be alternated between two phases (R3m (rhombohedral) and Cm (monoclinic)) by changing the compound's polarization direction, indicating that the ferroelectric switching behavior of GeTe can be controlled. This observation suggests that ferroelectric switching of GeTe may originate from the phase transition between R3m and Cm through the formation of a morphotropic phase boundary (MPB). Since these two phases can generate MPBs with similar energies, the rotation of lone-pair direction during the phase transition can effectively change the polarization direction. The exotic phenomenon of ferroelectric switching, which involves changes in the chemical bonds, can be predicted on the basis of the role of several dopants through DFT calculations.