Ferroelectric-like Behavior Research Articles

Physical Origin of the Ferroelectric-Type Hysteresis in MIM Structures with Amorphous Dielectric Film.

The ferroelectric-type characteristics in metal-insulator-metal (MIM) structures with amorphous dielectrics are observed, which may open a new window for a class of new ferroelectric devices. However, the working principles of these ferroelectric devices are not well explained. It has been proposed that oxygen ion (O2-) and charged vacancy (Vo2+) migrations are associated with this ferroelectric-like behavior. In this work, a systematic experimental design is carried out to investigate the origin of O2- and Vo2+ (from the interface or bulk oxide?), the transport species (O2-, Vo2+, or both of these two species?), and the ion transport mode (long-range transport or local movement?). The experimental results answered all these questions. In addition, the role of the thickness of the dielectrics is also studied. This work sets the fundamentals of these amorphous oxide-based devices, guiding future explorations to engineer them.

Magnetic ferroelectric metal in bilayer Fe3GeTe2 under interlayer sliding

The inherent interlayer freedom in van der Waals stacked materials provides an excellent opportunity to investigate ferroelectric-like behavior through interlayer translation. Based on first-principles calculations, we find that the interlayer sliding in Fe3GeTe2 (FGT) bilayer enables the coexistence of polarization, metallicity, and ferromagnetism. We find that the polarization is induced by the uncompensated vertical interlayer charge transfer, and can be switched by an in-plane interlayer sliding. A moderate biaxial strain can reverse the polarization direction of the sliding FGT bilayer. The vertical polarization disentangles with the in-plane conductivity as was previously seen in the sliding ferroelectric WTe2 bilayer. Our work proposes an extremely rare magnetic ferroelectric metal phase that is useful for magnetoelectric and spintronic applications.

Novel 2DEG System at the HfO2/STO Interface.

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

Ferroelastic Twin-Wall-Mediated Ferroelectriclike Behavior and Bulk Photovoltaic Effect in SrTiO_{3}.

Ferroelastic twin walls in nonpolar materials can give rise to a spontaneous polarization due to symmetry breaking. Nevertheless, the bistable polarity of twin walls and its reversal have not yet been demonstrated. Here, we report that the polarity of SrTiO_{3} twin walls can be switched by an ultralow strain gradient. Using first-principles-based machine-learning potential, we demonstrate that the twin walls can be deterministically rotated and realigned in specific directions under the strain gradient, which breaks the inversion symmetry of a sequence of walls and leads to a macroscopic polarization. The system can maintain polarity even after the constraint is removed. As a result, the polarization of twin walls can exhibit a ferroelectriclike hysteresis loop upon cyclic bending, namely flexoferroelectricity. Finally, we propose a scheme to experimentally detect the polarity of the twin wall by measuring the bulk photovoltaic responses. Our findings suggest a twin-wall-mediated flexoferroelectricity in SrTiO_{3}, which could be potentially exploited as functional elements in nanoelectronic devices design.

Revisiting contrast mechanism of lateral piezoresponse force microscopy

Piezoresponse force microscopy (PFM) has been widely used for the nanoscale analysis of piezoelectric properties and ferroelectric domains. Although PFM is useful because of its simple and nondestructive features, PFM measurements can be obscured by non-piezoelectric effects that could affect the PFM signals or lead to ferroelectric-like behaviors in non-ferroelectric materials. Many research studies have addressed related technical issues, but they have primarily focused on vertical PFM. Here, we investigate significant discrepancies in lateral PFM signals between the trace and the retrace scans, which are proportional to the scan angle and the cantilever lateral tilting discrepancy. The discrepancies in PFM signals are analyzed based on intrinsic and extrinsic components, including out-of-plane piezoresponse, electrostatic force, and other factors. Our research will contribute to the accurate PFM measurements for the visualization of ferroelectric in-plane polarization distributions.

A magnetoelectric memory device based on pseudo-magnetization

We propose a new type of magnetoelectric memory device that stores magnetic easy-axis information or pseudo-magnetization, rather than a definite magnetization direction, in magnetoelectrically coupled heterostructures. Theoretically, we show how a piezoelectric/ferromagnetic (PE/FM) combination can lead to non-volatility in pseudo-magnetization exhibiting overall ferroelectric-like behavior. The pseudo-magnetization can be manipulated by extremely low voltages especially when the FM is a low-barrier nano-magnet. Using a circuit model benchmarked against experiments, we determine the switching energy, delay, switching probability and retention time of the envisioned 1T/1C memory device in terms of magnetic and circuit parameters and discuss its thermal stability in terms of a key parameter called back-voltage vm which is an electrical measure of the strain-induced magnetic field. Taking advantage of ferromagnetic resonance measurements, we experimentally extract values for vm in CoFeB films and circular nano-magnets deposited on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 which agree well with the theoretical values. Our experimental findings indeed indicate the feasibility of the proposed novel device and confirm the assumed parameters in our modeling effort.

Tunable polarization-drived superior energy storage performance in PbZrO 3 thin films

Antiferroelectric PbZrO₃ films have great potential to be used as the energy storage dielectrics due to the unique electric field-induced phase transition character. But the phase transition process always accompanies polarization hysteresis effect that induces large energy loss and lowers the breakdown strength, leading to inferior energy storage density as well as low efficiency. In this work, the synergistical strategies by doping smaller ions of Li⁺-Al³⁺ to substitute Pb²⁺ and lowering the annealing temperature from 700℃ to 550℃ are proposed to change the microstructure and tune the polarization character of PbZrO₃ films, excepting to dramatically improve the energy storage performances. The prepared Pb_(1-x)(Li_0.5Al_0.5)_xZrO₃(abbreviated as P_(1-x)(L_0.5A_0.5)_xZO) films exhibit ferroelectric-like (<em>FE</em>) rather than antiferroelectric (<em>AFE</em>) character once the doping content of Li⁺-Al³⁺ ions reaches 6 mol%, accompanying a significant improvement of energy storage density of 49.09 J/cm³,but energy storage efficiency is only 47.94% due to the long-correlation of ferroelectric domains. Accordingly, the low-temperature annealing is carried out to reduce the crystalline degree and the polarization loss. P_0.94(L_0.5A_0.5)_0.06ZO films annealed at 550℃ deliver a linear-like polarization rather than ferroelectric-like behavior annealed at 700℃, the lowered remanent polarization as well as improved breakdown strength (4814 kV/cm) result in the superior energy storage density of 58.7 J/cm³ and efficiency of 79.16%, simultaneously possessing excellent frequency and temperature stability, and good electric fatigue tolerance.

Room-temperature ferroelectric-like behavior in anatase TiO2 epitaxial films prepared by chemical solution deposition

Ferroelectrics of simple oxides have been of great interest due to they are easy to integrate with silicon semiconductors. The FE hysteresis loop was measured for the first time in pure anatase-phase titanium dioxide is reported, demonstrating that the ferroelectric-like behavior in a-TiO2 epitaxial thin films (7.5 nm to 228.8 nm) prepared by chemical solution deposition method on (001) SrTiO3 based substrates. Synchrotron based X-ray diffraction and reciprocal space mapping analysis prove that we have prepared high-quality epitaxial films. According to the results of the FE hysteresis loop, Pr can reach approximately 0.42 μC/cm2 in a-TiO2 film with a thickness of 14.5 nm. Furthermore, the PFM shows that the ferroelectric-like behavior is exhibited in a-TiO2 film of 7.5 nm and that the PFM polarization switch can be maintained for more than 100 min. All of these seem to indicate the presence of ferroelectric behavior in a-TiO2 films. We recognize that the origin of ferroelectric-like behavior is due to the distortion induced by strain and vacancy. However, since the polarization value is not very large and the PFM decays, the electrochemical effect has a non-negligible influence on the intrinsic ferroelectric performance.

Modeling and Characterization of Pre-Charged Collapse-Mode CMUTs

Recently, the applications of ultrasound transducers expanded from high-end diagnostic tools to point of care diagnostic devices and wireless power receivers for implantable devices. These new applications additionally require that the transducer technology must comply to biocompatibility and manufacturing scalability. In this respect, Capacitive Micromachined Ultrasound Transducers (CMUTs) have a strong advantage compared to the conventional PZT based transducers. However, current CMUTs require a large DC bias voltage for their operation, which limits the miniaturizability of these devices. In this study, we propose a pre-charged collapse-mode CMUT for immersive applications that can operate without an external bias by means of a charge trapping Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer embedded in the dielectrics between the top and bottom electrodes. The built-in charge layer was analytically modeled and four layer stack combinations were investigated and characterized. The measurement results of the CMUTs were then used to fit the model and to quantify the amount and type of trapped charge. It was found that these devices polarize due to the ferroelectric-like behavior of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and the amount of charge stored in the charge-trapping layer was estimated to be approximately 0.02C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Their acoustic performance shows a transmit and receive sensitivity of 8.8 kPa/V and 13.1V/MPa respectively. In addition, we show that increasing the charging temperature, the charging duration, and the charging voltage results in a higher amount of stored charge. Finally, results of ALT tests showed that these devices have a lifetime of more than 2.5 years at body temperature.

Ferroelectric-like behavior in HfO2/Al2O3/AlN metal-insulator-semiconductor capacitor through AlN thermal stress

By modulating the thermal stress during film growth, the strained aluminum nitride (AlN) thin films with ferroelectric-like behavior were successfully grown by metal organic chemical vapor phase deposition (MOCVD) on silicon (Si) (111) substrate. The capacitors with strained AlN were fabricated and experimentally demonstrated, the linear to ferroelectric-like behavior on the fabricated AlN capacitors was observed, and the behavior was explained in terms of residual strains in the film. This work reports the ferroelectric-like properties of AlN film grown under specific deposition conditions for the first time.

Artificially induced ferroelectric-like behavior in an antiferroelectric sandwich structure by interface engineering

Interface engineering is essential for achieving fascinating interfacial functionalities in a single all-oxide-interface-based device. In the present work, a sandwich structure (Pb0.94La0.06(Zr0.95Ti0.05)O3 (PLZT)/HfO2/ Pb0.94La0.06(Zr0.95Ti0.05)O3) was fabricated via a chemical solution approach. A distinct “ferroelectricity-like” behavior with high Pmax (∼ 80 μC/cm2) and Pr (∼ 36 μC/cm2) is demonstrated. The dielectric HfO2 thin layer presents a tetragonal symmetry structure, which stabilizes a slight distorted structure of the upper PLZT layer (PLZT(T)) with a= 4.19(9) Å, b= 4.10(6) Å, β ∼ 91.04˚. In PLZT(T), the ferroelectric (FE) phase is identified as the matrix embedded with a small amount of AFE nanodomains, while the bottom PLZT layer (PLZT(B)) exhibits typical AFE incommensurately modulated structures. The near-interface structures in both PLZT layers are characterized by ferroelectric polarizations with head-to-tail configuration across the heterointerface. Such discontinuous, downward polarizations support the accumulation of oxygen vacancies at the heterointerface that facilitate the local polarization enhancement. It is the combination effect of stable ferroelectric polarization in the PLZT(T) layer, interfacial oxygen vacancies and large surface to volume ratio that leads to the superior polarization performance of the antiferroelectric sandwich structure. It indicates that interface engineering is a feasible approach to manipulate the ferroic behavior.

Switchable Electric Dipole from Polaron Localization in Dielectric Crystals.

Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping (D^{+}) and the corresponding bound polaron (e^{-}). A dipole of 6.15Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of Sr_{3}Ti_{2}O_{7}. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard U interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectriclike behavior reported here is localized and distinct from conventional spontaneous lattice polarization.

Exploring the energy storage density in 60Bi2O3–10SrO-30Fe2O3 lead-free relaxor glass for designing energy storage devices

Strontium doped glass was synthesized by the conventional melt-quenching technique. Systematically measured dielectric properties revealed relaxor ferroelectric-like behavior in a broad range of frequency and temperature. The glassy nature of the present sample was investigated by X-ray diffraction (XRD) measurements. glass transition temperature, crystallization temperatures and melting temperature are determined by differential thermal analysis (DTA). The low frequency Raman spectra displays a Boson peak at 72 cm−1, which confirms polar clusters (PCs) formation. The present sample depicts a wide as well as diffuse peak of dielectric permittivity ε'(ω) and tangent loss tan δ. wide as well as diffuse peak shifted to the higher temperatures (from 538 K to 553 K), a typical relaxor behavior is indicated by this type of diffuse transition. The presence of polar clusters (PCs) imbedded in the glass matrix indicates the presence of relaxor ferroelectric (RFE) behavior in this glass. The measured energy storage density of the strontium doped glass was 4 mJ.cm−3 with efficiency of 70% under an applied electric field of 17 kV.cm−1 at room temperature. Magnetic properties via vibrating-sample magnetometer (VSM) show a typical anti-ferromagnetic behavior. These results depict that the present glass can be regarded a promising candidate for designing energy storage devices.

A Physics-Based Model for Mobile-Ionic Field-Effect Transistors With Steep Subthreshold Swing

A physics-based model and the corresponding simulation framework for the mobile-ionic field-effect transistor (MIFET) exhibiting the ferroelectric-like behaviors are innovatively proposed based on two-dimensional (2D) Poisson&#x2019;s equation and non-equilibrium Green&#x2019;s function (NEGF), coupling with ion drift-diffusion equations. The simulation framework captures the dynamic distribution of mobile ions&#x2019; concentrations within dielectric along the external electric field. TaN/amorphous-ZrO2/TaN capacitors are experimentally characterized for the model calibration. It is proved that the mobile ions dominate the ferroelectriclike behaviors in MIFETs. Sub-60 mV/decade can be achieved in MIFETs based on the proposed model, which is consistent with the experimental results.

Relaxor ferroelectric-like behavior in 10PbTiO3–10Fe2O3–30V2O5–50B2O3 glass for energy storage applications

Relaxor ferroelectric-like behavior in 10PbTiO3–10Fe2O3–30V2O5–50B2O3 glass for energy storage applications

Amorphous ZrO2 Tunnel Junction Memristor With a Tunneling Electroresistance Ratio Above 400

An amorphous ZrO2 based tunneling junction memristor (TJM) with a tunneling electroresistance (TER) ratio above 400 is demonstrated. It is attributed to the modulation of tunneling barrier width induced by the accumulation of oxygen vacancies ( $V_{O}^{+}$ ) and negative charges near the ZrO2/semiconductor interface. The ferroelectric-like behaviors attributed to the voltage-modulated switching of the dipoles consisting of $V_{O}^{+}$ and negative charges in ZrO2 are characterized by a polarization-voltage test. The ZrO2 TJM achieves a TER ratio above 400 under 2.5/–1.5 V at 100 ns write/erase pulse condition, over 104 cycles program/erase endurance, and >104 s data retention.

Non-linearity in engineered lead magnesium niobate (PbMg1/3Nb2/3O3) thin films

The local compositional heterogeneity associated with the lack of long-range ordering of Mg2+ and Nb5+ in PbMg1/3Nb2/3O3 (PMN) is correlated with its characteristic “relaxor” ferroelectric behavior. Earlier work [Shetty et al., Adv. Funct. Mater. 29, 1804258 (2019)] examined the relaxor behavior in PMN thin films grown at temperatures below 1073 K by artificially reducing the degree of disorder via synthesis of heterostructures with alternate layers of Pb(Mg2/3Nb1/3)O3 and PbNbO3, as suggested by the “random-site model.” This work confirmed the development of ferroelectric domains below 150 K in long-range-ordered films, epitaxially grown on (111) SrTiO3 substrates using alternate target timed pulsed-laser deposition of Pb(Mg2/3Nb1/3)O3 and PbNbO3 targets with 20% excess Pb. In this work, the first through third-harmonic dielectric charge displacement densities and complex dielectric susceptibilities were analyzed as a function of temperature and frequency in zero-field-cooled PMN films with short- and long-range ordering. The long-range ordering decreased the dispersion in the first- and third-harmonic dielectric charge displacement densities relative to short-range-ordered films. It was found that the one-dimensional ordering achieved in the long-range-ordered film is insufficient to achieve a fully normal ferroelectric state. In the presence of quenched random electric fields, these films require a small ac field to facilitate percolation of the polar nano-regions, enabling normal ferroelectric-like behavior at lower temperature (T &amp;lt; 240 K). The films behave like a typical relaxor near room temperature. With reduced ordering, the short-range films exhibit greater dispersion in linear and higher order harmonic dielectric charge displacement density.

Оn the problem of the 240 K singularity in water

Water’s properties are exceptional at low temperatures. Dielectric constant demonstrates ferroelectric-like behavior. We treat an idea to interpret the fundamental feature of water, 240 K singularity, through the Maxwell speed distribution of freely moving water molecules. We use the ion-molecular model recently proposed by us, in which water molecules make the translation-oscillator movement with rapid potential/kinetic energy transformation. The model provides direct access to the velocity distribution through the kinetic energy of free thermal motion.

Memory Behavior of an Al2O3 Gate Dielectric Non-Volatile Field-Effect Transistor

Non-volatile field-effect transistor (FET) with amorphous Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dielectric is demonstrated on Si substrate, which is enabled by the voltage modulation of the oxygen vacancy and negative charge dipoles in gate insulator. Ferroelectric-like behavior in TaN/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.70</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.30</sub> stacks with different thicknesses of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is proved by polarization-voltage tests, positive-up and negative-down tests, piezoresponse force microscopy, and electrical measurements. The Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> capacitors attain over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> cycles' endurance of polarization versus voltage measurement. A 6.5 nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> non-volatile FET achieves a memory window above 0.6 V under ±2 V at 100 ns program/erase (P/E) condition, over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cycles P/E endurance, and >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s data retention at room temperature.

Electrical Characterization of Sputter Deposited AlxSc1-XN Thin Films

Nitride-based materials including AlxSc1-xN (ASN) films have also been proven to show a ferroelectricity with a box-like characteristic recently. In this paper, ASN thin films were deposited by RF sputtering with different metal electrode materials, Al, W, TiN and Ni. Leakage current measurements hardly showed difference among the electrode materials. A Schottky barrier height (SBH) at metal/ASN interface was as small as 0.86 eV, irrespective to the electrode material, suggesting a strong Fermi level pinning (FLP). Capacitance-voltage (CV) measurements revealed ferroelectric-like hysteresis behavior for all the electrode materials.