Effect Of Depolarization Field Research Articles

The effects of crystal orientation on the ferroelectric and optoelectronic properties of Pt/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 heterostructures

Due to their excellent ferroelectric, semiconducting, and optoelectronic properties, perovskite ferroelectric thin films have attracted increasing attention in the photovoltaic field. Using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, flake, strip and pyramidal Na0.5Bi0.5TiO3 (NBT) films were prepared on (001), (110) and (111) SrTiO3 (STO) substrates by off-axis magnetron sputtering and pulsed laser deposition, respectively. XRD and Phi scan results show that both LSCO and NBT are perovskite structures and satisfy the epitaxial growth along the substrate STO. NBT orientation has a significant effect on the ferroelectric properties of Pt/NBT/LSCO heterostructures. The remnant polarization 2Pr values for NBT(111), NBT(110) and NBT(001) are 87.5, 62.6 and 28.5 μC/cm2, respectively, and thus 2Pr(111)>2Pr(110)>2Pr(001). NBT(111) shows the best ferroelectric properties (Pr=41.3 μC/cm2), excellent fatigue-resisting (up to 1010 cycles) and retention performance (duration 104s). The 2Pr-T association 2 Pr=41.72+16.05*T0.35 for NBT(111) was obtained according to the curve fitting, it is found that the NBT orientation has a significant effect on the PV effect for PT/NBT/LSCO photovoltaics and the open-circuit voltage ∆Voc. The short-circuit current ∆Jsc of polarization regulate intensity follows the laws of ∆Voc-111>∆Voc-110>∆Voc-001 and ∆Jsc-111>∆Jsc-110>∆Jsc-001, the Jsc for Pt/NBT(111)/LSCO heterostructures increases linearly with the illumination Ip increasing, while the Voc decreases linearly. The ferroelectro-polarized modulation PV mechanism was investigated by analyzing band structure of the Pt/NBT/LSCO heterostructures, which is attributed to the coupling effect of the ferroelectric depolarization field and the interfacial Schottky barrier.

Planar memristor and artificial synaptic simulating based on two-dimensional layered tungsten oxychloride WO2Cl2

Two-dimensional van der Waals ferroelectrics with in-plane polarization are less vulnerable to the depolarization field effect, making them practically more favorable in device applications. Herein, symmetric planar memristor based on two-dimensional van der Waals WO2Cl2 was fabricated and electrically characterized, showing significant memory effect with a current ON/OFF ratio up to 104 and stable switching cycles. Both short-term plasticity and long-term potentiation were observed in synaptic simulations by applying consecutive positive and negative pulses, respectively, which demonstrates the potential capability of this WO2Cl2 memristor in neuro-morphic computing as an artificial synapse.

Synergy of morphotropic phase boundary and internal bias fields to achieve high d33 and d33* simultaneously in lead-free BNT-based ceramics

It is difficult to obtain high d33 and d33* simultaneously in one piezoelectric ceramic system via a conventional method due to the conflicting structure–properties responding mechanism between the direct piezoelectric and converse piezoelectric effects. In this work, a synergistic approach of constructing morphotropic phase boundary and introducing the internal bias fields in (Bi0.5Na0.5)TiO3 ceramics is proposed through Sr2+ and Li+ doping for mitigating the conflict between d33 and d33*. By this strategy, both high d33 (197 pC/N) and d33* (1500 p.m./V) are realized in Srx(Bi1-xNa0.97-xLi0.03)0.5TiO3 when x = 0.24. The large d33 stems from rhombohedral and tetragonal (R3c and P4bm) phase coexistence. The internal bias field introduced by defect dipoles tends to counteract the effects of depolarization field, thus greatly aiding the external fields with higher actual excitation fields. The superposition of internal bias field, depolarization field and external fields yields strong asymmetry bipolar strain curves, thereby contributing to the large d33*. Thus, this work opens a new door for developing lead-free ceramics with excellent piezoelectric performance.

Self-powered ultraviolet/visible photodetector based on CuBi2O4/PbZr0.52Ti0.48O3 heterostructure

Ferroelectric materials with significant remnant electric polarization have recently gained research focus for the fabrication of self-power photodetectors due to their photovoltaic effect. However, the photodetectors based on ferroelectric materials display poor performance owing to their weak photovoltaic effect of the ferroelectric depolarization field (Ed) on photo-generated charge carrier separation. In this work, we fabricate self-powered photodetectors based on CuBi2O4/PZT heterostructure. The heterojunction reveals a photoresponsivity of 0.24 mA/W and a detectivity of 2.40 × 1010 Jones under 0 V bias with a maximum 7.51 μW/cm2 illumination intensity at 425 nm, which is substantially improved compared to PZT-only device. The observed rise and fall time of the composite CuBi2O4/PZT thin film is around 24 and 46 ms, respectively, which is faster than that of PZT photodetector (67 and 106 ms). Moreover, the photocurrent CuBi2O4/PZT of device is observed to be 12.7 × 10−10 A in the downpoling state and 6.28 × 10−10 A in upward poling state, revealing that the direction of polarization field can modulate the photocurrent. This work provides a promising strategy for fabricating ferroelectric heterojunction photodetectors with enhanced performance.

Origin of the Critical Thickness in Improper Ferroelectric Thin Films

Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.

Multifunctional graphene metadevice using dual metamaterial induced transparency in terahertz regime

The dual metamaterial induced transparency (dual-MIT) phenomenon, which can be significantly tailored via the depolarization field effect of the graphene overlayer, is demonstrated in our fabricated multifunctional terahertz metastructures composed of unequal-lengthed 3-bar arrays. Those metasurfaces could actively manipulate the phase and group delay at various frequencies, as well as the different polarization conversion capability. The introduction of graphene reinforces the amplitude modulation of dual MITs and further reduces the quenched power by 4 times with a rotated angle. Combined with carrier dynamics, the dual-MIT transient regulation can realize an ultrafast all-optical switch with 2-time enhancement in modulation speed. Based on the advantage of the dual-MIT windows with different frequencies and magnitudes, we have illustrated the encrypted imaging on the erasure and reappearance of the target pattern. Our work provides a lower power consumption option for ultrafast all-optical switch and opens up the opportunities for future communication and information security in terahertz regime.

Publisher’s Note: “Reduction of depolarization field effect on ferroelectric switching process in semiconductor—relaxor ferroelectric composite” [J. Appl. Phys. 131, 154102 (2022)

Publisher’s Note: “Reduction of depolarization field effect on ferroelectric switching process in semiconductor—relaxor ferroelectric composite” [J. Appl. Phys. 131, 154102 (2022)

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.

The effects of Eu3+ doping on the epitaxial growth and photovoltaic properties of BiFeO3 thin films

1 Eu 3+ doping induces a phase transition from tetragonal to rhombohedral for BiFeO 3 film; 2 A decrease in coercivity and a sharp change in band gap upon Eu 3+ doping have been observed; 3 Eu 3+ can be used to tailor the photovoltaic effect of ITO/Eu x Bi 1- x FeO 3 / Ca 0.96 Ce 0.04 MnO 3 ; Nowadays, photovoltaic effect has been widely studied in various ferroelectric materials due to its applications as optoelectronic devices. In this work, with BiFeO 3 (BFO) films as the photovoltaic materials, we report the effects of Eu 3+ doping content on the phase structure, ferroelectric and optical properties of BFO films grown on Ca 0.96 Ce 0.04 MnO 3 /YAlO 3 (001) substrate. We found that a small doping level of 0.05 could induce a phase change of BFO from tetragonal to rhombohedral, due to the shrinking of the lattice upon Eu 3+ doping and the breaking of surface terrace structure induced by Ca 0.96 Ce 0.04 MnO 3 layer. This results in a sharp band gap reduction from 3.30 eV to 2.60 eV, and a decrease in the coercivity of ferroelectric polarization switching. Based on these findings, we investigate the photovoltaic effects of ITO/Eu x Bi 1- x FeO 3 /Ca 0.96 Ce 0.04 MnO 3 vertical capacitors. It is found that the short-circuit current density ( J sc ) decreases with increasing Eu 3+ doping, whereas the open-circuit voltage ( V oc ) first increases to a level of 0.1 V and then decreases with further Eu 3+ doping. This could be explained by the combined effect of Schottky-junction and depolarization field on the photovoltaic process. Our research suggests that a moderate Eu 3+ doping is helpful for enhancing the photovoltaic effect of BFO thin film devices. .

Reversible transition between the polar and antipolar phases and its implications for wake-up and fatigue in HfO2-based ferroelectric thin film

Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (OI and OII) within the unit cells of a ferroelectric (Hf0.5Zr0.5)O2 thin film. A reversible transition between the polar Pbc21 and antipolar Pbca phases, where the crystal structures of the 180° domain wall of the Pbc21 phase and the unit cell structure of the Pbca phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar P42/nmc phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.

Tunable terahertz transmission behaviors and coupling mechanism in hybrid MoS2 metamaterials

A hybrid metamaterial with the integration of molybdenum disulfide (MoS2) overlayer is proposed to manipulate the terahertz (THz) wave. The simulated results indicate that the introduction of MoS2 layer could significantly modify the resonant responses with large resonance red-shift and bandwidth broadening due to the depolarization field effect, especially for the structure on the small permitivity substrate. Additionally, the wide-band modulator in off-resonant region and a switch effect at resonance can be achieved by varying the conductivity of MoS2 layer. Further theoretical calculations based on the Lorentz coupling model are consistent with the simulated results, explicating the response behaviors originate from the coupling between MoS2 overlayer and the metastructure. Our results could provide a possibility for active control THz modulator and switchable device based on the MoS2 overlayer and advance the understanding of the coupling mechanism in hybrid structures.

Depolarization field effect on elasticity of unpoled piezoelectric ceramics

Multiple poling steps and complex electrode configurations are indispensable for high-performance state-of-the-art piezoelectric devices, yet those create partially poled or utterly unpoled regions that significantly change behaviors of piezoelectric devices. Several studies have dealt with method to obtain elastic compliance, yet little attention has been paid to method to obtain elastic loss. Furthermore, physical mechanism to explain the behavior of unpoled piezoelectric ceramic is missing. In this study, we experimentally obtain both effective elastic compliance and elastic loss of unpoled piezoelectric ceramics, using recently developed method called partial electrode (PE), and propose physical mechanism to explain the elasticity. The derivation process for analytical admittance equation for PE is explained and verified with finite element analysis (FEA) simulation. The complex elastic compliance was determined by fitting experimental admittance curve with derived analytical solutions. With the aid of intensive and extensive elastic compliances and elastic losses, we concluded that elasticity of unpoled piezoelectric material lies in between intensive-ness and extensive-ness. We explained this “elastic intermediacy” with depolarization field effect, which is one of the most important phenomena in piezoelectricity.

Estimate the electrical activity in a neuron under depolarization field

Abstract The physical electric variable is included into the known Hindmarsh-Rose (HR) model for estimating the depolarization field effect and then external current forcing is applied to detect the firing responses. Based on the proposed model, the effects of the amplitude and frequency of the sinusoidal current on the firing mode of the neuron are studied by using bifurcation analysis. It is found that there is a peak of firing interval of neurons with the increasing of stimulation current intensity. In the presence of electric field, the firing pattern of neuron is transformed from single busting to intermittent multimodal busting with the increasing of frequency and amplitude of electric field. The largest Lyapunov exponent is drawn for verification. In the absence of electric field, the two neurons coupled in the first variable achieve synchronization in busting mode. If there is an external electric field, the two neurons can achieve intermittent multimodal busting firing synchronization even without direct variable couple since the energy is injected into the coupled system by the external electric field. Our results show that the periodic external electric field and external current stimulation play an important role in the neuronal firing pattern.

Highly Sensitive and Tunable Self-Powered UV Photodetectors Driven Jointly by p-n Junction and Ferroelectric Polarization.

Ferroelectric (FE) materials are thought to be promising materials for self-powered ultraviolet (UV) photodetector applications because of their photovoltaic effects. However, FE-based photodetectors exhibited poor performance because of the weak photovoltaic effect of FE depolarization field (Edp) on the separation of photo-generated carriers. In this work, self-powered photodetectors based on both Edp and built-in electric field at the p-n junction (Ep-n) were designed to obtain enhanced device performance. A NiO/Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) heterojunction-based device is constructed to take advantage of energy level alignments that favor electron extraction. The device exhibits a tunable performance upon varying the polarization direction of PLZT. The NiO/PLZT heterojunction-based device with the PLZT layer in the poling down state shows a higher responsivity [R = (1.8 ± 0.12) × 10-4 A/W] and detectivity [D* = (3.69 ± 0.2) × 109 Jones], a faster response speed (τr = 0.34 ± 0.03 s, τd = 0.36 ± 0.02 s), and a lower dark current [Idark = (1.3 ± 0.19) × 10-12 A] under zero bias than the PLZT-based device because of the synergistic effects of Edp and Ep-n. Moreover, under weak-light illumination (0.1 mW/cm2), it exhibits even higher R [(6.3 ± 1.2) × 10-4 A/W] and D* [(1.29 ± 0.26) × 1010 Jones] values, which surpass those of most previously reported FE-based self-powered photodetectors. Our work emphasizes the role of the coupling effect between Ep-n and Edp in the photovoltaic process of NiO/PLZT heterojunction-based devices and provides an effective way to promote the self-powered UV photodetector applications.

Enhanced photovoltaic effect in Ca and Mn co-doped BiFeO3 epitaxial thin films

In this work, we proposed modification of BiFeO3 (BFO) by Mn and Ca co-doping, and realized the epitaxial growth of Bi0.98Ca0.02Fe0.95Mn0.05O3 (BCFMO) thin films on (100)-oriented SrTiO3 (STO) substrates with a SrRuO3 (SRO) buffer layer via pulsed laser deposition (PLD). A decreased bandgap (Eg = 2.41 eV) and enhanced remanent polarization (Pr = 92.5 μC/cm2) were simultaneously achieved in the BCFMO thin film. A tunable photovoltaic (PV) effect by changing the direction of the poling electric field was observed in the BCFMO-based device. We investigated the PV mechanism for the device by analyzing its band structure and attribute it to the coupling effects of the ferroelectric depolarization field and the interface Schottky barriers. The BCFMO-based device under favorable upward poling shows the best PV performance with an open-circuit voltage (Voc) of −0.29 V and short-circuit current density (Jsc) of 0.26 mA/cm2, which are much higher than those for the BFO-based device (Voc = -0.11 V, Jsc = 0.08 mA/cm2), resulting from the narrower bandgap and larger Pr in the BCFMO film. The approach of simultaneously introducing Ca and Mn into the BFO, enabling both a higher polarization and narrower bandgap, opens a new way for improving the performance of BFO-based PV devices.

Effect of depolarization field on steep switching characteristics in negative capacitance field effect transistors

A theoretical model for describing the effect of depolarization field (Ed) in the negative capacitance (NC) field effect transistor (FET) was derived. Based on this model, the electrical properties of the metal-ferroelectric-insulator-semiconductor (MFIS) field effect transistor including the relationship between the depolarizing field and the gate voltage (Vg), Ed and silicon surface potential, and the drain-source current of NC-FET and Vg (transfer characteristics) were theoretically investigated. The computing results demonstrated that the surface potential amplification and the steep subthreshold slope characteristics happened at the gate voltage of about 0.4 V, results from the steep increase of Ed dependent on Vg in the NC-FETs. It is expected that the derived results may provide some insight into the design and performance improvement for the NC-MFIS-FET.

Heterogeneous ferromagnetic state in small particles and its connection with ferroelectricity

PTs in spherical particles of a cubic ferromagnetic was considered in the framework of the Ginzburg-Landau phenomenological theory. Concentrating on depolarizing field effects, we study the competition between states with homogeneous magnetization and vortex structures. For large sphere radii (R>Rc), a PT to a vortex state is possible, while for R<Rc it can be a homogeneous state. We obtain the inhomogeneous distribution of the ferromagnetic order parameter in the form of 2D and 3D vortices, for which the absolute value of the local magnetization depends on the distance from the centre of the vortex. This approach can be used for nano-sized particles. In our work, we compute the polarization caused by ferromagnetic 2D and 3D vortices within the phenomenological approach.

Polar metals as electrodes to suppress the critical-thickness limit in ferroelectric nanocapacitors

Enhancing the performance of nanoscale ferroelectric (FE) field-effect transistors and FE capacitors for memory devices and logic relies on miniaturizing the metal electrode/ferroelectric area and reducing the thickness of the insulator. Although size reductions improve data retention, deliver lower voltage threshold switching, and increase areal density, they also degrade the functional electric polarization. There is a critical nanometer length tFE∗ below which the polarization disappears owing to depolarizing field effects. Here, we show how to overcome the critical thickness limit imposed on ferroelectricity by utilizing electrodes formed from a novel class of materials known as polar metals. Electronic structure calculations on symmetric polar-metal electrode/FE capacitor structures demonstrate that electric polarizations persist to the sub-nanometer scale with tFE∗→0 when a component of the polar axis in the electrode is perpendicular to the electrode/insulator interface, i.e., aligned along the direction of the polar displacements in the ferroelectric. Our results reveal the importance of interfacial dipolar coherency in sustaining the polarization, which provides a platform for atomic structure-based design of functions that deteriorate in reduced dimensions.

Dielectric properties of ferroelectric nanocomposites: effects of thermal stresses and filler shape anisotropy

Ferroelectric nanocomposites are intriguing nonhomogeneous materials, which may have unusual phase states and specific physical properties useful for practical applications. Here we theoretically describe dielectric properties of nanocomposites comprising single-domain ferroelectric nanocrystals embedded into a linear dielectric medium. First, small-signal intrinsic permittivities of spheroidal PbTiO3 and BaTiO3 crystallites surrounded by an isotropic matrix with linear elastic properties are calculated with the aid of a nonlinear thermodynamic theory. It is shown that thermal stresses caused by differences in thermal expansion between the inclusions and the matrix may strongly influence the intrinsic permittivities of ferroelectric nanocrystals. Second, macroscopic dielectric responses of ferroelectric–dielectric composites are evaluated in the Maxwell Garnett approximation. For effective permittivities of such composites, generalized relations are derived, which allow for both the shape and dielectric anisotropies of ferroelectric nanocrystals. Numerical calculations of the effective permittivities are performed for composites comprising PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. For nanocomposites involving randomly oriented and similarly aligned spheroidal inclusions, temperature dependences of the effective permittivities are determined with the account of phase transitions occurring in strained ferroelectric nanocrystals and suppression of their dielectric responses due to the depolarizing-field effect created by low-permittivity matrix. Our theoretical calculations show that effective permittivities of composites comprising needle-like or disc-shaped ferroelectric inclusions could strongly exceed the permittivity of the matrix. Most importantly, we predict that BaTiO3–K2O-SiO2 composites with such inclusions should exhibit a broad dielectric peak around the room temperature, which is associated with shifted structural transitions in strained BaTiO3 nanocrystals. The presented theory provides guidelines for the development of ferroelectric nanocomposites with enhanced dielectric responses, which could be suitable for applications in supercapacitors and other advanced electronic devices.

Influence of Depolarizing Fields and Screening Effects on Phase Transitions in Ferroelectric Composites.

The temperature of the transition to the polar state in ferroelectric composites, representing spherical ferroelectric inclusions embedded in a dielectric matrix, under a depolarizing field effect is investigated. This temperature is determined both in the absence and presence of screening effects of the depolarizing field of the bound charges of spontaneous polarization at the inclusions surface. The absence case shows that the Curie point shift is determined by the ratio of the Curie constant of the ferroelectric inclusion to the permittivity of the matrix. Screening effects show that the transition temperature shift decreases through multiplying the value by a decreasing factor equal to the ratio of the screening length to the radius of the ferroelectric inclusion. Examples of the materials for the position of the Curie point on the temperature scale, largely determined by the tilting action of the depolarizing field and the compensating shielding effects, are given.