Ferroelectric Polarization Switching Research Articles

Ferroelectric, dielectric and electrical behavior of two-dimensional lead sulphide nanosheets

Two-dimensional pure cubic phase lead sulphide (PbS) nanosheets were synthesized using solid state reaction method at ambient pressure and low temperature ~190 °C. From 210 K–300 K, small polaron hopping conduction mechanism was found to be dominant in PbS nanosheets at frequencies 20 Hz–2 MHz. High values of dielectric constant (~200) and electrical conductivity (of the order of 10−3 S m−1 at 300 K) of PbS nanosheets were extracted suggesting that it is a proficient material for capacitive storage devices. A high value of density of states of the order of 1032 eV−1 cm−3 was obtained for PbS nanosheets. The capacitance-voltage (CV) measurements of PbS nanosheets resulted in a stable butterfly loop due to switching of ferroelectric polarization at 300 K. The permittivity calculated at 0 V capacitance was ~150 and the dielectric loss remained below ~0.50. The polarization-voltage (QV) measurements showed a remnant polarization 23 µC cm−2 in PbS nanosheets. The leakage current density was below 0.5 mA cm−2 in the range ±5 V.

Ferroelectric Heterostructures for Synapse Devices

Ferroelectric polarization related charge conduction have shown intriguing properties of ferroelectric heterostructure devices, such as ferroelectric tunnel junction, switchable diode, and ferroelectric memristor. Recently, ferroelectric heterostructure has been proposed for synapse device exhibiting a high on/off ratio, low power consumption and short process time. In ferroelectric-based synapse devices, gradual electroresistive modulation is associated with change of ferroelectric domain configuration, providing the controllability of the electric conductance for the effective operation of synapse devices. In this work, we demonstrated a ferroelectric-field-effect transistor based synapse devices with channel layer of metal to semiconducting transition (MST) oxide. Ferroelectric polarization switching can modulate the amount of applied energy to MST oxide phase transition and channel conductance at the same time. Our ferroelectric heterostructure with analog-like electroresistive modulation is highly desirable for realizing a efficient synapse device.

Development of ferroelectric domains and topological defects in vacancy doped ceramics of h-LuMnO3

Self-doping of the h-LuMnxO3±δ (0.92 ≤ x ≤ 1.12) phase and changes in the sintering time are applied to investigate the formation and annihilation of antiphase ferroelectric (FE) domains in bulk ceramics. The increase in the annealing time in sintering results in growth of FE domains, which depends on the type of vacancy, 6-fold vortices with dimensions of the order of 20 μm being observed. Interference of planar defects of the lattice with the growth of topological defects shows breaking of 6-fold symmetry in the self-doped ceramics. The role of grain boundaries in the development of topological defects has been studied. Dominance of the atypical FE domain network in very defective h-LuMnxO3±δ lattices saturated with Mn vacancies (x < 1) was also identified in the current study. After a long annealing time, scattered closed-loops of nano-dimensions are often observed isolated inside large FE domains with opposite polarization. Restoring of the polarization after alternative poling with opposite electrical fields is observed in FE domains. Stress/strain in the lattice driven by either planar defects or chemical inhomogeneity results in FE polarization switching on the nanoscale and further formation of nano-vortices, with detailed investigation being carried out by electron microscopy. Pinning of FE domains to planar defects is explored in the present microscopy analysis, and nano-scale observation of lattices is used to explain features of the ferroelectricity revealed in Piezo Force Microscopy images of the ceramics.

Ferroelectric domain structure and polarization switching speed of highly (111)-oriented polycrystalline YMnO3 thin films on glass substrates

Polycrystalline YMnO3 (YMO) thin films were deposited on (111) Pt/Ta/glass substrates via pulsed laser deposition (PLD). The low PLD deposition rate made it possible to reduce the leakage currents of the YMO thin film due to the resulting high level of crystallinity. The YMO thin film exhibited a strong ferroelectric response, including a remnant polarization of 4.2μC/cm2. The piezoresponse force microscopy study revealed that the YMO thin films had a mosaic domain structure and a ferroelectric domain wall energy lower than that of PbTiO3 thin films. It is suggested that the fast switching speed of YMO capacitors, which is below 85ns, originates from this low domain wall energy.

Asymmetry in mechanical polarization switching

Recent demonstration of a mechanical 180° switching of ferroelectric polarization has enabled an alternative polarization control mechanism based on the flexoelectric coupling between polarization and strain gradient. Mechanical switching is a highly asymmetric phenomenon associated with the inhomogeneous strain induced by an atomic force microscope (AFM) tip pressed against the ferroelectric surface. Here, we demonstrate the asymmetric domain switching behavior in the vicinity of the 180° domain wall in PbTiO3 thin films with respect to the AFM tip scanning direction. The writing-direction-dependent asymmetric domain response has been modeled by molecular dynamics simulation showing asymmetry in domain wall displacement due to the difference in the volume of mechanically switched domains. The obtained results show that the mechanically induced switching dynamics is very different from the conventional 180° switching realized by an external electric field and has to be exploited differently. In particular, nanoscale domain engineering via the tip-induced flexoelectric effect requires careful consideration of asymmetric interaction between the existing domain structures and the strain gradient.

Inverting polar domains via electrical pulsing in metallic germanium telluride

Germanium telluride (GeTe) is both polar and metallic, an unusual combination of properties in any material system. The large concentration of free-carriers in GeTe precludes the coupling of external electric field with internal polarization, rendering it ineffective for conventional ferroelectric applications and polarization switching. Here we investigate alternate ways of coupling the polar domains in GeTe to external electrical stimuli through optical second harmonic generation polarimetry and in situ TEM electrical testing on single-crystalline GeTe nanowires. We show that anti-phase boundaries, created from current pulses (heat shocks), invert the polarization of selective domains resulting in reorganization of certain 71o domain boundaries into 109o boundaries. These boundaries subsequently interact and evolve with the partial dislocations, which migrate from domain to domain with the carrier-wind force (electrical current). This work suggests that current pulses and carrier-wind force could be external stimuli for domain engineering in ferroelectrics with significant current leakage.

Vertically arrays of Si-LaNiO3/BiFeO3/Au core-shell nano-capacitors for prominent coupled electro-optic effect

The current study delivers the first assessment of well-separated cone shaped n-type epitaxial BiFe0.9Mn0.1O3 (n-BFMO) shells integrated on LaNiO3 (LNO) buffered vertical arrays of p-type nanoporous Si nanowire cores as an absorber for high efficiency and sustainable all- oxide photovoltaic devices. The photovoltaic effect of the LNO/n-BFMO/Au radial heterojunction is investigated by registering the current-voltage curves under AM 1.5 (100 mW cm−2) illumination conditions at various ferroelectric polarization states of BFMO. The as-deposited LNO/BFMO/Au nanowire arrays fabricated by scalable Pulsed Laser deposition are completely self-polarized and produced a remarkably large open circuit photovoltage up to 0.53 V under zero poling condition. The direction of photocurrent and photovoltage is reversibly switchable in response to ferroelectric polarization switching. The short circuit photocurrent “ISC” and open circuit voltage “VOC” depict a hysteresis loop which is more or less identical to that of ferroelectric polarization and thus can be utilized as a nondestructive optical readout of the ferroelectric nonvolatile memories. By introducing special architecture for the BFO based photovoltaic device, a photoconversion efficiency ∼0.3% and External Quantum Efficiency ∼11% are achieved at zero polarization state, which can further be enhanced and modulated to ∼0.6% and 14%, respectively, at the negative poling state of BFMO.

Persistence of strong and switchable ferroelectricity despite vacancies

Vacancies play a pivotal role in affecting ferroelectric polarization and switching properties, and there is a possibility that ferroelectricity may be utterly eliminated when defects render the system metallic. However, sufficient quantitative understandings of the subject have been lacking for decades due to the fact that vacancies in ferroelectrics are often charged and polarization in charged systems is not translationally invariant. Here we perform first-principles studies to investigate the influence of vacancies on ferroelectric polarization and polarization switching in prototypical BaTiO3 of tetragonal symmetry. We demonstrate using the modern theory of polarization that, in contrast to common wisdom, defective BaTiO3 with a large concentration of vacancies (or , or ) possesses a strong nonzero electric polarization. Breaking of Ti-O bonds is found to have little effect on the magnitude of polarization, which is striking. Furthermore, a previously unrecognized microscopic mechanism, which is particularly important when vacancies are present, is proposed for polarization switching. The mechanism immediately reveals that (i) the switching barrier in the presence of is small with ΔE = 8.3 meV per bulk formula cell, and the polarization is thus switchable even when vacancies exist; (ii) The local environment of vacancy is surprisingly insignificant in polarization switching. These results provide profound new knowledge and will stimulate more theoretical and experimental interest on defect physics in FEs.

Multilevel hysteresis loop engineered with ferroelectric nano-metamaterials

Polarization switching in ferroelectric and multiferroic materials provides a means of designing a broad range of multifunctional devices. To date, such applications have typically exploited 180° ferroelectric polarization switching, where the polarization vector can be alternated between parallel or anti-parallel to the external electric field, i.e., bi-states. However, the coupling between the direction of switching and the long-range order parameter puts a limit on the well-defined polarization state, hindering the use of polarization switching and the advancement of novel applications. Using the ferroelectric nano-metamaterial concept, we demonstrate here that successive polarization switching through multiple well-defined stable states with a long-range order parameter decoupled from the direction of switching can be achieved, and consequently, controllable and multilevel hysteresis loops are obtained under a unidirectional electric field. The equilibrium polarization configuration of stable states are tailored by designing a nanoporous ferroelectric structure based on the ferroelectric nano-metamaterial concept. The orientation of the homogeneous electric field is then controlled to achieve the desired polarization switching behavior and hence the hysteresis loop level, which can pass through all stable polarization states. In addition, a general rule for the nanostructure-state-level of switching is established, from which a high level of hysteresis loop can be designed and achieved. The present study provides a foundation for novel devices based on multilevel ferroelectric switching, such as multilevel data storage memory, multi-logic gates, and electromechanical devices with multilevel control.

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply sub-picosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO$_3$ ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond timescales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables direct comparison with first-principles-based molecular dynamics simulations, with good agreement obtained.

Multiferroic Transitions and Misfit Phase Diagram in Oxygen‐Deficient Epitaxial (111) PbTiO3

Unusual multiferroic phase transitions in intrinsically nonmagnetic ferroelectric PbTiO3 are demonstrated. Misfit strain activates local (anti)ferromagnetism at oxygen vacancies in PbTiO3 via the orbital symmetry breaking and splitting of the defect electronic states. A nonlinear magnetoelectric effect is present due to magnetic phase transitions coincident with ferroelectric polarization switching, providing a new route to magnetoelectric and multiferroic oxides. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Fabrication of high-density BiFeO3 nanodot and anti-nanodot arrays by anodic alumina template-assisted ion beam etching

Efficient and cost-competitive fabrication of high-quality ferroelectric and multiferroic nanostructures is of general interest. In this work, a top-down nano-patterning technique is developed by the Ar+ ion beam etching in combination with the sacrificed ultrathin anodic alumina (AAO) mask. This technique is demonstrated by preparation of the epitaxial BiFeO3 (BFO) nanostructures of various geometries, including nanodot and anti-nanodot arrays. The lateral dot size is as small as ∼60 nm and an ultrahigh dot density of ∼60 Gbit/inch2 is achieved. It is revealed that the etching process involves sequential shape evolution of both the AAO mask and the underlying BFO film, resulting in the nanodots and anti-nanodots arrays of various geometries. The as-etched BFO nanodots array exhibits well-established ferroelectric domain structures and reversible polarization switching, as examined by piezoresponse force microscopy (PFM). It is suggested that this technique is extendable to fabrication of a wide range of functional oxide nanostructures for potential nanoelectronic applications.

Reproducible resistive switching in the super-thin Bi2FeCrO6 epitaxial film with SrRuO3 bottom electrode

The reproducible and reliable resistive switching is observed in the ultrathin Bi2FeCrO6 (BFCO) epitaxial film on (001) SrTiO3 substrate with SrRuO3 as the bottom electrode. The as-grown BFCO film allows its ferroelectric polarization switching under external electric field. With a 100-nm-radius tip contacting film surface, a stable bipolar resistive switching was observed through the conductive atomic force microscope. Furthermore, the resistive switching at negative bias was observed and its high/low current ratio is above 15 among a thousand of current versus voltage curves measured by the scanning tunneling microscope with a non-contacting nm-scale tip. It is argued that this transport mechanism is due to quantum tunneling, and the resistive switching in these junctions is because of ferroelectric switching.

Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P63mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P63/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (Ec) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in lowering Ec during polarization, with a 5% biaxial expansion resulting in a decrease of Ec to 3.5 MV/cm. Comparison with calculated values for conventional ferroelectric materials suggests that the ferroelectric polarization switching of wurtzite-structured ZnO may be achievable by preparing high-quality ZnO thin films with suitable strain levels and low defect concentrations.

Charge-Trapping Phenomena in HfO2-Based FeFET-Type Nonvolatile Memories

Ferroelectric field effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO2) thin films show high potential for future embedded nonvolatile memory applications. However, HfO2 films besides their recently discovered ferroelectric behavior are also prone to undesired charge trapping effects. Therefore, the scope of this paper is to verify the possibility of the charge trapping during standard operation of the HfO2-based FeFET memories. The kinetics of the charge trapping and its interplay with the ferroelectric polarization switching are analyzed in detail using the single-pulse $I_{D}$ – $V_{G}$ technique. Furthermore, the impact of the charge trapping on the important memory characteristics such as retention and endurance is investigated.

Designing room-temperature multiferroic materials in a single-phase solid-solution film

The search for multiferroic materials with simultaneous ferroelectric and ferromagnetic properties in a single phase at room temperature continues to be fuelled from the perspective of developing multifunctional devices. Here we design a single-phase multiferroic La0.67Sr0.33MnO3-BaTiO3 film, which possesses epitaxial single-crystal and solid-solution structure, high magnetic Curie temperature (~640 K) as well as switchable ferroelectric polarization. Moreover, a notable strain-mediated magnetoelectric coupling at room temperature in the way of modulating the magnetism with an external applied voltage is also observed. The synthetic solid-solution multiferroic film may open an extraordinary avenue for exploring a series of room-temperature multiferroic materials.

Residual tensile stresses and piezoelectric properties in BiFeO3-Bi(Zn1/2Ti1/2)O3-PbTiO3 ternary solid solution perovskite ceramics

For low dielectric loss perovskite-structured (1-x-y)BiFeO3-xBi(Zn1/2Ti1/2)O3-yPbTiO3 (BF-BZT-PT) (x = 0.04-0.15 and y = 0.15-0.26) ceramics in rhombohedral/tetragonal coexistent phase, structural phase transitions were studied using differential thermal analyzer combined with temperature-dependent dielectric measurement. Two lattice structural phase transitions are disclosed in various BF-BZT-PT perovskites, which is different from its membership of BiFeO3 exhibiting just one lattice structural phase transition at Curie temperature TC= 830oC. Consequently, residual internal tensile stresses were revealed experimentally through XRD measurements on ceramic pellets and counterpart powders, which are reasonably attributed to special structural phase transition sequence of BF-BZT-PT solid solution perovskites. Low piezoresponse was observed and argued extrinsically resulting from residual tensile stresses pinning ferroelectric polarization switching. Post-annealing and subsequent quenching was found effective for eliminating residual internal stresses in those BZT-less ceramics, and good piezoelectric property of d33 ≥ 28 pC/N obtained for 0.70BF-0.08BZT-0.22PT and 0.05 wt% MnO2-doped 0.70BF-0.04BZT-0.26PT ceramics with TC ≥ 640oC, while it seemed no effective for those BZT-rich BF-BZT-PT ceramics with x = 0.14 and 0.15 studied here.

Rashba-Dresselhaus spin-splitting in the bulk ferroelectric oxideBiAlO3

It has been recently suggested that the coexistence of ferroelectricity and Rashba-like spin-splitting effects due to spin-orbit coupling in a single material may allow for a non-volatile electric control of spin degrees of freedom. In the present work, we compared the structural and ferroelectric properties of tetragonal and rhombohedral phases of ferroelectric BiAlO$_3$ by means of density-functional calculations. In both phases, we carefully investigated Rashba and Dresselhaus effects, giving rise to spin-splitting in their bulk electronic structure, particularly near the conduction band minimum, supplementing our first-principles results with an effective $ k\cdot p$ model analysis. The full reversal of the spin texture with ferroelectric polarization switching was also predicted. BiAlO$_3$ can therefore be considered as the first known oxide to exhibit a coexistence of ferroelectricity and Rashba-Dresselhaus effects.

Time-Dependent Negative Capacitance Effects in Al2O3/BaTiO3 Bilayers.

The negative capacitance (NC) effects in ferroelectric materials have emerged as the possible solution to low-power transistor devices and high-charge-density capacitors. Although the steep switching characteristic (subthreshold swing < sub-60 mV/dec) has been demonstrated in various devices combining the conventional transistors with ferroelectric gates, the actual applications of the NC effects are still some way off owing to the inherent hysteresis problem. This work reinterpreted the hysteretic properties of the NC effects within the time domain and demonstrated that capacitance (charge) boosting could be achieved without the hysteresis from the Al2O3/BaTiO3 bilayer capacitors through short-pulse charging. This work revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was demonstrated that a nonhysteretic NC effect can be achieved only within certain limited time and voltage ranges, but that these are sufficient for critical device applications.

Enhancing micro-electromechanical and tuning optical properties in Nd-doped BIT thin film

Bi4Ti3O[Formula: see text] (BIT) and Bi[Formula: see text]Nd[Formula: see text]Ti3O[Formula: see text] (BNT) thin films were deposited on Pt/Ti/SiO2/Si (100) substrates using pulsed laser deposition. The surface morphologies, ferroelectric domain structures and polarization switching were investigated by atomic force microscopy (AFM) and piezoelectric force microscopy (PFM). The phase and amplitude images of PFM show that the BIT and BNT thin films have clear domain structures. Comparison of the surface morphologies and domain structures indicates that the grain boundaries limit the shape of domain and affect the domain structure. The micro-electromechanical performance was characterized by the effective piezoelectric coefficient [Formula: see text] of the thin films. The result shows that the maximum effective [Formula: see text] value (100 pm/V) of BNT thin film is larger than that of BIT thin film (30 pm/V). This can be ascribed to BNT thin film with a preferred growth direction of [Formula: see text]-axis, resulting in effective enhancement of [Formula: see text]. Besides, all the thin films exhibit good optical transmittance in the range of 500–800 nm and the optical band gaps increase from 3.43 eV to 3.52 eV due to Nd doping.