Ferroelectric Polarization Switching Research Articles

Time-fractional Landau–Khalatnikov model applied to numerical simulation of polarization switching in ferroelectrics

Ferroelectrics are complex structured media that exhibit fractal properties and memory effects in terms of a number of dynamic characteristics. The most relevant applications of ferroelectrics in science and technology are associated with the primary mechanisms of polarization switching and domain structure dynamics induced by external exposure. In this study, we propose a time-fractional modification of the Landau–Ginzburg–Devonshire–Khalatnikov model to describe the dynamics of ferroelectric polarization switching. To solve the time-fractional cubic-quintic partial differential equation numerically, a computational scheme is derived. The technique combines an iterative procedure and an implicit finite-difference scheme based on an approximation of the Caputo derivative. A series of computational experiments are presented to visualize polarization switching characteristics on the example of thin films of barium titanate. A variation of the order of fractional derivative as a numerical characteristic of the memory effect allows one to “adjust” suitable regimes of simulated dynamical system. The obtained findings indicate that the generalized time-fractional model can provide better reproduction of experimental data in comparison with the classical analogue.

Sunlight-assisted ferroelectric domain switching and ionic migration in Sn-based ferroelectric

Laser-assisted ferroelectric polarization switching recently has been proved to be an effective mean to manipulate the ferroelectric domain structure, but with the possibility to damage the specimen surface due to high energy input and large thermal expansion. Compared to laser, sunlight with moderate energy is expected to be more accessible. Here, we employed a simulated sunlight illumination instead of high-energy lasers to realize the sunlight-assisted ferroelectric domain switching in Sn2P2S6 single crystals. The origin is the enhancement of localized carrier concentration due to the disproportionation reaction of Sn ions, which induces an additional internal field and assists the domain switching. The migration and accumulation of the Sn ions are also verified with scanning probe technique, which can be utilized as a resistive memory prototype. It is noteworthy that this memory effect can be significantly enhanced by sunlight illumination and, thus, make it suitable for the sunlight control of ferroelectric domain switching and ionic memory devices.

Reversible and Nonvolatile Manipulation of the Spin-Orbit Interaction in Ferroelectric Field-Effect Transistors Based on a Two-Dimensional Bismuth Oxychalcogenide

The spin-orbit interaction (SOI) offers a nonferromagnetic scheme to realize spin polarization through utilizing an electric field. Electrically tunable SOIs through electrostatic gates have been investigated; however, the relatively weak and volatile tunability limits their practical applications in spintronics. Here, we demonstrate the nonvolatile electric field control of the SOI via constructing ferroelectric Rashba architectures, i.e., two-dimensional ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}/\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$-${\mathrm{PbTiO}}_{3}$ ferroelectric field-effect transistors. The experimentally observed weak antilocalization (WAL) cusp in ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films implies the Rashba-type SOI that arises from the asymmetric confinement potential. Significantly, taking advantage of the switchable ferroelectric polarization, the WAL-to-weak-localization-transition trend reveals the competition between spin relaxation and the dephasing process, and the variation of carrier density leads to a reversible and nonvolatile modulation of the spin-relaxation time and the spin-splitting energy of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films by this ferroelectric gating. Our work provides a scheme to achieve nonvolatile control of the Rashba SOI with the utilization of ferroelectric remanent polarization.

Neurosynaptic-like behavior of Ce-doped BaTiO3 ferroelectric thin film diodes for visual recognition applications

Brain-like neuromorphic computing networks based on the human brain information processing model are gradually breaking down the memory barriers caused by traditional computing frameworks. The brain-like neural system consists of electronic synapses and neurons. The multiple ferroelectric polarization switching modulated by the external electric field is well suited to simulate artificial neural synaptic weights. Therefore, ferroelectric diodes' (FDs) synapses have great advantages in building highly reliable and energy-efficient artificial neural networks. In this paper, we demonstrate the FDs synapse, which is based on rare-earth metal-doped BaTiO3 ferroelectric dielectric layer materials. This performs short-term and long-term synaptic plasticity behaviors by modulating synaptic weights using pulsed stimuli to polarize or flip ferroelectric films. In addition, convolutional neural networks were constructed on the MNIST dataset and the Fashion-MNIST dataset to check the feasibility of the device in simulating bio-visual recognition. The results expand the application of FDs' devices in the intersection of artificial intelligence and bioscience.

Ferroelectric polarization controlled surface potential and charge transport across ITO-Nd-doped BiFeO3 polycrystalline ceramic bulk for photoelectrochemical solar water splitting

The spontaneous polarization in ferroelectrics provides an effective way to engineer interfacial charge distribution at the ferroelectric heterojunctions, thereby depletion width in p-n junction and energy band structures. Here, we integrated p-type multiferroic 7% neodymium-doped bismuth ferrite Bi0.93Nd0.07FeO3 (BFO7Nd) polycrystalline ceramic bulk with n-type transparent conductive indium tin oxide (ITO) to demonstrate the influence of ferroelectric polarization on surface potential and its effect on photoelectrochemical (PEC) water reduction. Kelvin probe force microscopy (KPFM) results reveal the surface potential change during ferroelectric polarization switching, suggesting that the polarization bound charges may have an effect on space charge region between ITO-BFO7Nd, thus dominating surface potential at the electrode/electrolyte interface. This implies that the interface barrier between ITO and BFO7Nd can be easily modulated by polarization bound charges. The electron lifetime of upward-polarization (P-up) ITO-BFO7Nd is estimated 29.30 ms, about 15 times longer than unpoled ITO-BFO7Nd. The photocurrent density of P-up ITO-BFO7Nd is ∼1.2 × 10−6 A/cm2 (at 0 V vs RHE), which is one-order higher compared to that of the unpoled and P-down states under illumination with an intensity of 100 mWcm−2. This work shows that ferroelectric polarization combined with photogenerated carriers is an effective approach for effective charge transfer in multiferroic ceramic to boost PEC solar water splitting.

Revealing photovoltaic behavior in 2D hybrid perovskite ferroelectric single-crystalline microwire arrays for self-powered photodetectors

Hybrid perovskite ferroelectrics produce a revival of interest in ferroelectric photovoltaics because of the robust ferroelectricity coexisting with superior semiconducting properties. An electric field via perovskite ferroelectrics can affect their photovoltaic behavior; however, the fundamental understanding of the electric-field-induced effects remains comparatively elusive. Herein, (EA)2(MA)2Pb3Br10 single-crystalline microwire arrays (MWs) are synthesized for the fabrication of self-powered photodetectors and characterized with evident in-plane multiaxial ferroelectricity by piezoresponse force microscopy (PFM) measurements. Upon systematic investigations via a dynamic poling process, including electrical-poling-dependent photocurrent, Kelvin probe force microscopy (KPFM) mapping and ferroelectric polarization switching, we reveal that the coupling of ion migration and ferroelectric photovoltaic effect dominate the photovoltaic behavior within (EA)2(MA)2Pb3Br10 MWs. Such electric-field-induced effects are responsible for the self-powered ability in (EA)2(MA)2Pb3Br10 MWs-based photodetectors with accelerated response time, switchable photoelectric responses and large short-circuit current density. Our findings provide fundamental insight into the photovoltaic behavior under an electric field in perovskite ferroelectrics, and the electrical-poling-manipulated dynamics pave the way for innovative self-powered optoelectronic devices.

High piezoelectricity in epitaxial BiFeO3 microcantilevers

The large switchable ferroelectric polarization and lead-free composition of BiFeO3 make it a promising candidate as an active material in numerous applications, in particular, in micro-electro-mechanical systems (MEMS) when BiFeO3 is integrated in a thin film form on a silicon substrate. Here, 200-nm-thick Mn-doped BiFeO3 thin films have been epitaxially grown on a SrRuO3/SrTiO3/Si substrate and patterned into microcantilevers as prototype device structures for piezoelectric actuation. The devices demonstrate excellent ferroelectric response with a remanent polarization of 55 μC/cm2. The epitaxial BiFeO3 MEMS exhibit very high piezoelectric response with transverse piezoelectric coefficient d31 reaching 83 pm/V. The BiFeO3 cantilevers show larger electromechanical performance (the ratio of curvature/electric field) than that of state-of-art piezoelectric cantilevers, including well-known PZT (Pb(Zr,Ti)O3) and the hyper-active PMN–PT (Pb(Mg1/3Nb2/3)O3-PbTiO3). In addition, the piezoelectricity in BiFeO3 MEMS is found to depend on the ferroelectric polarization direction, which could originate from the flexoelectric effect and be exploited to further enhance the electromechanical performance of the devices. These results could potentially lead to a replacement of lead-based piezoelectrics by BiFeO3 in many microdevices.

Ion Drift and Polarization in Thin SiO2 and HfO2 Layers Inserted in Silicon on Sapphire.

To reduce the built-in positive charge value at the silicon-on-sapphire (SOS) phase border obtained by bonding and a hydrogen transfer, thermal silicon oxide (SiO2) layers with a thickness of 50–310 nm and HfO2 layers with a thickness of 20 nm were inserted between silicon and sapphire by plasma-enhanced atomic layer deposition (PEALD). After high-temperature annealing at 1100 °C, these layers led to a hysteresis in the drain current–gate voltage curves and a field-induced switching of threshold voltage in the SOS pseudo-MOSFET. For the inserted SiO2 with a thickness of 310 nm, the transfer transistor characteristics measured in the temperature ranging from 25 to 300 °C demonstrated a triple increase in the hysteresis window with the increasing temperature. It was associated with the ion drift and the formation of electric dipoles at the silicon dioxide boundaries. A much slower increase in the window with temperature for the inserted HfO2 layer was explained by the dominant ferroelectric polarization switching in the inserted HfO2 layer. Thus, the experiments allowed for a separation of the effects of mobile ions and ferroelectric polarization on the observed transfer characteristics of hysteresis in structures of Si/HfO2/sapphire and Si/SiO2/sapphire.

Formation of ferroelectric multi-domains and electrical conduction in epitaxial BiFeO3 nanodots fabricated with AAO nanotemplates

The electrical conduction property of ferroelectric domains formed in nanoscale dots is an important issue for miniaturized non-volatile random access memory applications. Herein, the conduction characteristics of ferroelectric domain structures formed in epitaxial BiFeO3 (BFO) nanodots are evaluated. Nanoscale BFO dots are fabricated on Nb-doped SrTiO3 (Nb:STO) substrates using an anodic aluminum oxide (AAO) nanotemplate, spin-coating, and etching processes. Using various sizes, such as 65 nm, 50 nm, and 35 nm, of the BFO nanodots, the domain wall current and the current that is dependent on the direction of polarization are measured. A piezoresponse force microscope study reveals that multiple ferroelectric domains coexist in one BFO nanodot. A large current is measured using a conducting atomic force microscope at the domain wall location in the ferroelectric domains. After ferroelectric polarization switching of the domains, the current is slightly dependent on the polarization direction. The electrical resistance state is dependent on the presence or absence of domain walls within the ferroelectric domains of the BFO nanodots.

Remote Oxygen Scavenging of the Interfacial Oxide Layer in Ferroelectric Hafnium-Zirconium Oxide-Based Metal-Oxide-Semiconductor Structures.

Discovery of ferroelectricity in HfO2 has sparked a lot of interest in its use in memory and logic due to its CMOS compatibility and scalability. Devices that use ferroelectric HfO2 are being investigated; for example, the ferroelectric field-effect transistor (FEFET) is one of the leading candidates for next generation memory technology, due to its area, energy efficiency and fast operation. In an FEFET, a ferroelectric layer is deposited on Si, with an SiO2 layer of ∼1 nm thickness inevitably forming at the interface. This interfacial layer (IL) increases the gate voltage required to switch the polarization and write into the memory device, thereby increasing the energy required to operate FEFETs, and makes the technology incompatible with logic circuits. In this work, it is shown that a Pt/Ti/thin TiN gate electrode in a ferroelectric Hf0.5Zr0.5O2 based metal-oxide-semiconductor (MOS) structure can remotely scavenge oxygen from the IL, thinning it down to ∼0.5 nm. This IL reduction significantly reduces the ferroelectric polarization switching voltage with a ∼2× concomitant increase in the remnant polarization and a ∼3× increase in the abruptness of polarization switching consistent with density functional theory (DFT) calculations modeling the role of the IL layer in the gate stack electrostatics. The large increase in remnant polarization and abruptness of polarization switching are consistent with the oxygen diffusion in the scavenging process reducing oxygen vacancies in the HZO layer, thereby depinning the polarization of some of the HZO grains.

Flexoelectricity-Driven Mechanical Switching of Polarization in Metastable Ferroelectrics.

Flexoelectricity-based mechanical switching of ferroelectric polarization has recently emerged as a fascinating alternative to conventional polarization switching using electric fields. Here, we demonstrate hyperefficient mechanical switching of polarization exploiting metastable ferroelectricity that inherently holds a unique mechanical response. We theoretically predict that mechanical forces markedly reduce the coercivity of metastable ferroelectricity, thus greatly bolstering flexoelectricity-driven mechanical polarization switching. As predicted, we experimentally confirm the mechanical polarization switching via an unusually low mechanical force (100nN) in metastable ferroelectric CaTiO_{3}. Furthermore, the use of low mechanical forces narrows the width of mechanically writable nanodomains to sub-10nm, suggesting an ultrahigh data storage density of ≥1 Tbit cm^{-2}. This Letter sheds light on the mechanical switching of ferroelectric polarization as a viable key element for next-generation efficient nanoelectronics and nanoelectromechanics.

Intrinsically Switchable Dual‐Band Scandium‐Aluminum Nitride Lamb‐Wave Filter

A dual‐band scandium‐aluminum nitride (Sc0.28Al0.72N) Lamb‐wave bandpass filter is implemented with intrinsically configurable center frequencies at 628 MHz and 1.07 GHz. The filter is created from electrical coupling of dual‐mode resonators with complementary‐switchable operation states defined by spatial polarization tuning in ferroelectric Sc0.28Al0.72N film. The resonators are created with patterning interdigitated transducers (IDT) on the top surface and exploit the floating bottom electrode for application of low‐frequency polarization switching pulses. Switching polarization direction under consecutive electrode fingers in unified and alternating directions results in excitation of Lamb modes with different wavelengths equal and twice the IDT pitch size, with a frequency ratio of ≈1.70. The intrinsically configurable dual‐band filter shows bandwidths of 11.9 MHz and 10.8 MHz, insertion losses of 2.6 and 2.8 dB, and on‐off switching isolations of 12 and 9 dB, when operating at 628 MHz and 1.07 GHz, respectively. The intrinsic configuration of center frequency, enabled by ferroelectric polarization switching, highlights the potential of dual‐band scandium‐aluminum nitride Lamb‐wave filters for creation of compact multifrequency spectral processors.

Tunable magnetic scattering and ferroelectric switching at the LaAlO3/EuTiO3/Sr0.99Ca0.01TiO3 interface

Ferroelectric and ferromagnetic orders rarely coexist, and magnetoelectric coupling is even more scarce. A possible avenue for combining these orders is by interface design, where orders formed at the constituent materials can overlap and interact. Using a combination of magnetotransport and scanning superconducting quantum interference device measurements, we explore the interactions between ferroelectricity, magnetism, and the two-dimensional electron system (2DES) formed at the novel ${\mathrm{LaAlO}}_{3}/{\mathrm{EuTiO}}_{3}/{\mathrm{Sr}}_{0.99}{\mathrm{Ca}}_{0.01}{\mathrm{TiO}}_{3}$(001) heterostructure. We find that the electrons at the interface experience magnetic scattering appearing along with a diverging Curie-Weiss-type behavior in the ${\mathrm{EuTiO}}_{3}$ layer. The 2DES is also affected by the switchable ferroelectric polarization at the ${\mathrm{Sr}}_{0.99}{\mathrm{Ca}}_{0.01}{\mathrm{TiO}}_{3}$ bulk. While the 2DES interacts with both magnetism and ferroelectricity, we show that the presence of the conducting electrons has no effect on magnetization in the ${\mathrm{EuTiO}}_{3}$ layer. Our results provide a first step towards realizing a new multiferroic system where magnetism and ferroelectricity can interact via an intermediate conducting layer.

Ferroelectric diode characteristic and tri-state memory in self-assembled BiFeO3 nanoislands with cross-shaped domain structure

Exotic polarization domain configurations in BiFeO3 nanoislands have recently been achieved, promising for exploring next-generation nanoelectronics. Here, different from the earlier reported BiFeO3 nanoislands with a very thin cross-shaped domain wall on LaAlO3 substrates, we observed the cross-shaped domains with a downward polarization separating quad-domains with an upward polarization, which is confirmed by spherical aberration corrected scanning transmission electron microscopy and piezoresponse force microscopy. Interestingly, the cross- and quad-domains show diode-like transport behaviors but with different rectification directions owing to their different polarization orientations. Specifically, an intriguing two-step ferroelectric polarization switching can be realized, which locally results in a tri-state nonvolatile memory. These results broaden the understanding of the interesting polarization configurations in BiFeO3 nanoislands and highlight their potential as high-density information storage.

High-quality ferroelectric Bi0.5K0.5TiO3‒BiFeO3 solid-solution single crystals grown under high-pressure oxygen atmosphere

Ferroelectric (Bi0.5K0.5)TiO3–BiFeO3 solid solutions are promising materials for high-temperature piezoelectric applications because of their high depolarization temperature. In this study, we demonstrate the successful growth of high-quality (Bi0.5K0.5)TiO3–BiFeO3 single crystals near the morphotropic phase boundary by a flux method. Crystal growth at a high oxygen pressure of 0.9 MPa drastically reduces leakage currents, which enables ferroelectric polarization switching and a piezoelectric response associated with a rotation of polar nanosized domains. This study contributes to further detailed analyses of the relationship between crystal structures and ferroelectric properties of the (Bi0.5K0.5)TiO3–BiFeO3 solid-solution system.

Quasi-analytical model of surface potential and drain current for trigate negative capacitance FinFET: a superposition approach

Negative capacitance (NC) obtained from the ferroelectric polarization switching is a widely adopted approach for the realization of low power, high-performance devices. In this paper, for the first time, we have developed a 3D quasi-analytical model for the surface potential and drain current of the trigate NC-FinFET using the superposition approach. Till date, only double gate (DG) uniformly doped NC-FinFET structures have been explored, which does not reveal the practicality of the device. Therefore, we perform an extensive device evaluation: (a) by solving the Poisson’s equation separately for the side gates (DG) and the top gate to acquire a complete model for trigate FinFET using the superposition principle; (b) to mimic the actual source/drain (S/D) doping, we included Gaussian doping in our proposed model; (c) with the incorporation of the laterally extended gate and S/D underlap. The model data are found in good agreement with the well-calibrated simulation data. We have taken the parabolic approximation method and appropriate boundary conditions to solve the Poisson’s equation.

Effect of ferroelectric polarization switching on the magnetization and exchange bias in BiFeO3/La2/3Sr1/3MnO3

The magnetic properties and exchange bias of the heterostructures of a ferroelectric-antiferromagnetic room-temperature multiferroic BiFeO3 (BFO) and a ferromagnetic La2/3Sr1/3MnO3 (LSMO), grown on SrTiO3 (STO) substrate using pulsed laser (PLD) deposition technique, as a function of temperature are explored. The irreversible behaviour of temperature-dependent field cooled (FC) and zero-field cooled (ZFC) magnetization curves, with blocking temperature (TB) ~250 K, and exponential decay of coercive field (HC) below TB, reveal a spin-glass like state in BFO/LSMO heterostructure. The exchange bias is observed in as-grown as well as electrically poled BFO in BFO/LSMO which diminishes above the blocking temperature. A considerable impact of electrical poling on the magnetic properties and exchange bias is noticed in the BFO/LSMO heterostructure. In the proposed model, the coupled ferroelectric/antiferromagnetic orders in BFO along with the inflection of exchange interactions between Fe3+ and Mn3+/4+ in the vicinity of the interface due to the movement of Fe3+ in BFO relative to Mn3+/4+ in LSMO during the ferroelectric polarization switching is the key factor that strongly influences the magnetic properties and exchange bias BFO/LSMO heterostructure.

A Vis‐SWIR Photonic Synapse with Low Power Consumption Based on WSe2/In2Se3 Ferroelectric Heterostructure

AbstractNeuromorphic visual sensory and memory systems that can sense, learn and memorize optical information have great potential in many areas such as image recognition and autonomous driving. However, most current artificial neuromorphic vision technology is suffering from large power consumptions (>100 pJ per switching), high circuitry complexity, and difficulty in miniaturization due to the physical separation of the optic sensing, processing, and memory units. Here, a photonic neuromorphic device based on WSe2/In2Se3 van der Waals (vdW) heterostructure is developed to meet the requirements of high‐performance photonic synapse devices. Owing to the optically controlled ferroelectric polarization switching, the device shows light‐tunable synaptic functions. For the first time, the light‐stimulated synaptic behavior extends from visible light to short‐wavelength infrared region (up to 1800 nm). Taking advantage of the ultra‐low dark current of the heterojunction, the power consumption of the photonic synapse can be as low as 258 fJ per switching under a bias of −0.1 V, comparable to biological synapses. These findings demonstrate a novel photonic synapse, deepen the understanding of light‐controlled polarization switching in semiconducting ferroelectric materials, and open up new application fields in multifunctional electronic and photonic devices based on 2D ferroelectric vdW heterostructures.

Impact of Al x Ga1−x barrier thickness and Al composition on electrical properties of ferroelectric HfZrO/Al2O3/AlGaN/GaN MFSHEMTs

Ferroelectric (FE) HfZrO/Al2O3 gate stack AlGaN/GaN metal-FE-semiconductor heterostructure field-effect transistors (MFSHEMTs) with varying Al x Ga1−x N barrier thickness and Al composition are investigated and compared by TCAD simulation with non-FE HfO2/Al2O3 gate stack metal–insulator–semiconductor heterostructure field-effect transistors (MISHEMTs). Results show that the decrease of the two-dimensional electron gas (2DEG) density with decreasing AlGaN barrier thickness is more effectively suppressed in MFSHEMTs than that in MISHEMTs due to the enhanced FE polarization switching efficiency. The electrical characteristics of MFSHEMTs, including transconductance, subthreshold swing, and on-state current, effectively improve with decreasing AlGaN thickness in MFSHEMTs. High Al composition in AlGaN barrier layers that are under 3-nm thickness plays a great role in enhancing the 2DEG density and FE polarization in MFSHEMTs, improving the transconductance and the on-state current. The subthreshold swing and threshold voltage can be reduced by decreasing the AlGaN thickness and Al composition in MFSHEMTs, affording favorable conditions for further enhancing the device.

Ferroelectric switching and current characteristics dependence on ferroelectric polarization direction of microwave synthesized BiFeO3 nanocubes

Herein, we studied the ferroelectric switching and current characteristics of BiFeO3 (BFO) nanocubes dispersed on the surface of a Nb-doped SrTiO3 (Nb:STO) substrate based on the ferroelectric polarization orientation. The microwave synthesis method afforded BFO nanocubes with an average size of ∼50 nm, which were dispersed on the Nb:STO substrate surface and the substrate was subsequently subjected to heat treatment at 500 °C for 1 h. The piezoelectric d33 hysteresis loop, ferroelectric domain structure, and ferroelectric polarization switching characteristics of the 50-nm-sized BFO nanocubes were examined using piezoresponse force microscopy. Finally, atomic force microscopy confirmed the dependency of current characteristics on the ferroelectric polarization orientation of the BFO nanocubes, verifying the applicability of BFO nanocubes as storage media for ferroelectric polarization information.