Polarization-voltage Hysteresis Loops Research Articles

Reversible charge injection in artificially created charged domain wall region

The impact of electric field application in the charged domain wall (DW) region on the performance of DW devices remains elusive. Here, we present our observations of substantial charge injection occurring in an artificially created charged DW region. Rapid charge injection coincides with polarization switching, contributing to the transient current collected for the integration of polarization-voltage hysteresis loops. Subsequently, a gradual injection of charges persists under externally applied voltages, diffusing into the thin film from the DW region to screen the space defect dipoles and effectively suppressing the imprint field. The process of charge injection proves reversible and is notably more pronounced in aged ferroelectric thin films, as evidenced in planar BiFeO3 nanodevices where the charge injection amount is five orders of magnitude higher than charges generated from polarization switching. This work constitutes a significant contribution to understanding the intricate process of charge injection and its influence on charged DW.

Achieving excellent ferroelectric and dielectric performance of HfO2/ZrO2/HfO2 thin films under low operating voltage

Although the HfO2-based ferroelectric thin films are compatible with the complementary metal-oxide-semiconductor (CMOS) technology and exhibit excellent scalability, they still suffer from low dielectric tunability, metastable ferroelectric orthorhombic phase and poor endurance. Here, we fabricated the HfO2/ZrO2/HfO2 sandwich structured thin films via atomic layer deposition process as well as the influence of voltage cycling, Hf/Zr ratios and annealing temperature on ferroelectricity and dielectric tunability of them was investigated systematically. At pristine states, the antiferroelectric-like behavior of polarization-voltage hysteresis loops become more and more obvious with the decrement of ZrO2 content in the middle layers. After waking-up, the large remnant polarization (2Pr, ∼43 μC/cm2) and high dielectric tunability (∼51 %) can be achieved under low operating voltage of 2.5 V. Noting that the 2Pr is still higher than 40 μC/cm2 and the dielectric tunability remains about 50 % after 109 cycles. In addition, the HfO2/ZrO2/HfO2 thin films also exhibited high energy storage properties with the total energy storage density of ∼62 J/cm3 and the recoverable energy storage density of ∼32 J/cm3. These results indicated that the strategy of sandwich structure design is beneficial to promoting the development of electronic devices towards miniaturization, integrated, and multifunction along with high performance and reliability.

Fluorination Enables Dual Ferroelectricity in Both Solid- and Liquid-Crystal Phases.

Ferroelectric materials are a special type of polar substances, including solids or liquid crystals. However, obtaining a material to be ferroelectric in both its solid crystal (SC) and liquid crystal (LC) phases is a great challenge. Moreover, although cholesteric LCs inherently possess the advantage of high fluidity, their ferroelectricity remains unknown. Here, through the reasonable H/F substitution on the fourth position of the phenyl group of the parent nonferroelectric dihydrocholesteryl benzoate, we designed ferroelectric dihydrocholesteryl 4-fluorobenzoate (4-F-BDC), which shows ferroelectricity in both SC and cholesteric LC phases. The fluorination induces a lower symmetric polar P1 space group and a new solid-to-solid phase transition in 4-F-BDC. Beneficial from fluorination, the SC and cholesteric LC phases of 4-F-BDC show clear ferroelectricity, as confirmed by well-shaped polarization-voltage hysteresis loops. The dual ferroelectricity in both SC and cholesteric LC phases of a single material was rarely found. This work offers a viable case for the exploration of the interplay between ferroelectric SC and LC phases and provides an efficient approach for designing ferroelectrics with dual ferroelectricity and cholesteric ferroelectric liquid crystals.

Learning the right channel in multimodal imaging: automated experiment in piezoresponse force microscopy

We report the development and experimental implementation of the automated experiment workflows for the identification of the best predictive channel for a phenomenon of interest in spectroscopic measurements. The approach is based on the combination of ensembled deep kernel learning for probabilistic predictions and a basic reinforcement learning policy for channel selection. It allows the identification of which of the available observational channels, sampled sequentially, are most predictive of selected behaviors, and hence have the strongest correlations. We implement this approach for multimodal imaging in piezoresponse force microscopy (PFM), with the behaviors of interest manifesting in piezoresponse spectroscopy. We illustrate the best predictive channel for polarization-voltage hysteresis loop and frequency-voltage hysteresis loop areas is amplitude in the model samples. The same workflow and code are applicable for any multimodal imaging and local characterization methods.

Polarization-Dominated Internal Timing Mechanism in a Ferroelectric Second-Order Memristor

Second-order memristors are considered as ideal synaptic emulators for their capability of exhibiting ${\mathrm{Ca}}^{2+}$-like dynamics. Recently, ferroelectric second-order memristors were developed, but whether their temporal conductance evolution is related to polarization dynamics remains unclear owing to the difficulty in directly measuring polarization in these devices. This issue is addressed here by using a ferroelectric diode (FD) that shows both second-order memristive behavior and well-shaped polarization-voltage hysteresis loops. It is demonstrated that the resistance-state change in the FD is triggered by polarization switching, arising from polarization-controlled Schottky emission. Moreover, concurrent conductance decay and polarization relaxation are observed, and their correlation is quantitatively evidenced, suggesting that conductance decay is caused by the polarization-relaxation-induced increase in the Schottky barrier height. Using polarization relaxation as an internal timing mechanism, our FD-based second-order memristor faithfully emulates various synaptic functions, where short-term plasticity is indispensable, including excitatory postsynaptic current, paired-pulse facilitation, the transition from short-term plasticity to long-term plasticity, learning experience, and associative learning. Our study not only reveals a polarization-dominated internal timing mechanism in the FD-based second-order memristor, but also demonstrates that such a device is a promising building block for biorealistic neuromorphic systems.

Biferroelectricity of a homochiral organic molecule in both solid crystal and liquid crystal phases

Ferroelectricity, existing in either solid crystals or liquid crystals, gained widespread attention from science and industry for over a century. However, ferroelectricity has never been observed in both solid and liquid crystal phases of a material simultaneously. Inorganic ferroelectrics that dominate the market do not have liquid crystal phases because of their completely rigid structure caused by intrinsic chemical bonds. We report a ferroelectric homochiral cholesterol derivative, β-sitosteryl 4-iodocinnamate, where both solid and liquid crystal phases can exhibit the behavior of polarization switching as determined by polarization–voltage hysteresis loops and piezoresponse force microscopy measurements. The unique long molecular chain, sterol structure, and homochirality of β-sitosteryl 4-iodocinnamate molecules enable the formation of polar crystal structures with point group 2 in solid crystal phases, and promote the layered and helical structure in the liquid crystal phase with vertical polarization. Our findings demonstrate a compound that can show the biferroelectricity in both solid and liquid crystal phases, which would inspire further exploration of the interplay between solid and liquid crystal ferroelectric phases.

Axial-Chiral BINOL Multiferroic Crystals with Coexistence of Ferroelectricity and Ferroelasticity.

Chirality exists everywhere from natural amino acids to particle physics. The introduction of point chirality has recently been shown to be an efficient strategy for the construction of molecular ferroelectrics. In contrast to point chirality, however, axial chirality is rarely used to design ferroelectrics so far. Here, based on optically active 1,1'-bi-2-naphthol (BINOL), which has been applied extensively as a versatile chiral reagent in asymmetric catalysis, chiral recognition, and optics, we successfully design a pair of axial-chiral BINOL multiferroics, (R)-BINOL-DIPASi and (S)-BINOL-DIPASi. They experience a 2F1-type full ferroelectric/ferroelastic phase transition at a high temperature of 362 and 363 K, respectively. Piezoelectric force microscopy and polarization-voltage hysteresis loops demonstrate their ferroelectric domains and domain switching, and polarized light microscopy visualizes the evolution of stripe-shaped ferroelastic domains. The axial-chiral BINOL building block promotes the generation of the polar structure and ferroelectricity, and the organosilicon component increases the rotational energy barrier and thus the phase transition temperature. This work presents the first axial-chiral high-temperature multiferroic crystals, offering an efficient path for designing molecular multiferroics through the introduction of axial chirality.

Effect of thickness and frequency of applied field on the switching dynamics of multiferroic bismuth ferrite thin films

Epitaxial films of multiferroic $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ (BFO) with different thicknesses are grown on $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$ buffered $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (001) substrates using the pulsed laser deposition technique. The room-temperature polarization-voltage (P-V) hysteresis loop, leakage current and average domain size of films with varying thicknesses have been investigated. The frequency dependence of the coercive voltage and variation of the coercive voltage as a function of film thickness are ascertained to examine the validity of the Ishibashi-Orihara model and the semi-empirical Kay-Dunn scaling relationship, respectively. The results demonstrate that at room temperature the BFO films indeed follow the Ishibashi-Orihara (I-O) model and Kay-Dunn scaling law. The dimensionality of the domains acquired by piezoresponse force microscopy imaging further validates that found from the I-O model scaling exponent and is consistent with Kittel's law regarding dependence of domain width on film thickness. These combined results elucidate the fundamental properties that govern the switching kinetics of BFO and are relevant for BFO-based multiferroic materials in next generation logic and memory devices operating over a wide range of frequencies.

Quantitative Local Probing of Polarization with Application on HfO2 -Based Thin Films.

Owing to their switchable spontaneous polarization, ferroelectric materials have been applied in various fields, such as information technologies, actuators, and sensors. In the last decade, as the characteristic sizes of both devices and materials have decreased significantly below the nanoscale, the development of appropriate characterization tools became essential. Recently, a technique based on conductive atomic force microscopy (AFM), called AFM-positive-up-negative-down (PUND), is employed for the direct measurement of ferroelectric polarization under the AFM tip. However, the main limitation of AFM-PUND is the low frequency (i.e., on the order of a few hertz) that is used to initiate ferroelectric hysteresis. A significantly higher frequency is required to increase the signal-to-noise ratio and the measurement efficiency. In this study, a novel method based on high-frequency AFM-PUND using continuous waveform and simultaneous signal acquisition of the switching current is presented, in which polarization-voltage hysteresis loops are obtained on a high-polarization BiFeO3 nanocapacitor at frequencies up to 100kHz. The proposed method is comprehensively evaluated by measuring nanoscale polarization values of the emerging ferroelectric Hf0.5 Zr0.5 O2 under the AFM tip.

Effect of Hydrogen on Hafnium Zirconium Oxide Fabricated by Atomic Layer Deposition Using H2O2 Oxidant

Hafnium zirconium oxide (HZO) has been studied intensively due to its potential application to memory and logic devices based on its outstanding dielectric and ferroelectric properties. Because many process factors can significantly impact the properties of HZO thin film, an optimized process or post‐treatment is necessary to ensure its performance. Herein, the effect of hydrogen on HZO thin film can make the film degraded. Analysis of the polarization–voltage hysteresis loops and I–V measurements indicate a leaky behavior in the film. To address this issue, post‐oxygen‐plasma treatment is performed on the film, which results in the decrease in hydrogen ions inside the film and consequently reduces the leakage current density. This finding can provide new insight into the effect of hydrogen incorporation for HZO thin films and methodology to remove leaky components inside ferroelectric films using O2 plasma treatment.

Size and shape effects on polarization reversal of nanoscale BaTiO3

We report piezo force microscopy (PFM) investigations on the ferroelectric switching characteristics of spherical and rod shaped non-interacting nanocrystalline BaTiO3 (BTO). Polarization-voltage hysteresis loop (P-E hysteresis loop) measurement shows that the strong polarization reversal observed for the nanorod (NR) shaped BTO than the spherical nanoparticle (NP). Asymmetric P-E loop is observed for all kind of BTO NRs. From the PFM spectroscopy and ferroelectric switching studies reveal that the defect dipoles present in the BTO NR at below 60 nm have no significant effect on the asymmetric P-E hysteresis loop. It is strongly depends on the strain between the substrate and BTO NRs.

Ferroelectricity of as-deposited HZO fabricated by plasma-enhanced atomic layer deposition at 300 °C by inserting TiO2 interlayers

We report the observation of ferroelectricity in hafnium-zirconium-oxide thin films in the as-deposited state, namely, after deposition at a low temperature of 300 °C without post-metallization annealing. The Hf0.5Zr0.5O2 (HZO) thin film was interposed between two TiO2 interlayers, and all films were produced by plasma enhanced atomic layer deposition and integrated into a TiN-based metal-insulator-metal capacitor. The ferroelectric nature of the as-deposited HZO film was evaluated by a polarization-voltage hysteresis loop, and a 2Pr value of ∼7.4 μC/cm2 was achieved. Grazing incidence x-ray diffraction measurements and atomic-resolution scanning transmission electron microscopy characterization revealed the co-existence of fully crystallized polar orthorhombic and monoclinic phases of the dielectric in the as-deposited sample. We concluded that the nucleation and growth of the crystalline polar non-centrosymmetric orthorhombic phases in the 10 nm HZO thin film were prompted by the available energy from the plasma and the tensile lattice mismatch strain provided by the TiO2 interlayer.

Enhanced Ferroelectric Properties and Insulator-Metal Transition-Induced Shift of Polarization-Voltage Hysteresis Loop in VOx-Capped Hf0.5Zr0.5O2 Thin Films.

A capping layer is known to be critical for stabilizing the ferroelectric (FE) orthorhombic phase (o-phase) in a HfO2-based thin film. Here, vanadium oxide (VOx), a functional oxide exhibiting the insulator-metal transition, is used as a novel type of a capping layer for the Hf0.5Zr0.5O2 (HZO) thin film. It is demonstrated that the VOx capping layer (VCL) can enhance the FE properties of the HZO thin film comprehensively. Specifically, the HZO thin film with a VCL shows large remanent polarization (2Pr ≈36.9 μC/cm2), relatively small coercive field (Ec ≈1.09 MV/cm), high endurance (up to 109 cycles), and long retention (>105 seconds). The enhanced FE properties may be attributed to the VCL-induced stabilization of the FE o-phase and suppression of oxygen vacancies at the interface. Furthermore, the HZO thin film with a VCL exhibits a successive rightward shift of polarization-voltage (P-V) hysteresis loop as the temperature increases. This is well correlated with the insulator-metal transition in a VCL, which can modulate the interfacial built-in field and thus cause the P-V loop shift. It is therefore demonstrated that a VCL not only enhances the FE properties of HZO thin films but also provides a temperature degree of freedom to modulate the FE properties, which may open up a new pathway to develop HfO2-based FE memories with high performance and novel functionalities.

Effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with imprint

We investigated the effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with an imprint. From the polarization-voltage hysteresis loops, wake-up and imprint effects were observed. We measured the polarization switched by the voltage pulse at various durations and voltages. The switching behaviors could be explained by the nucleation-limited-switching model with a Lorentzian distribution of the characteristic switching time. Due to the wake-up phenomena, the center of the distribution increased, and the width became narrower, which could be related to improvement of the uniformity of the film. The polarization switching data from the positive and negative bias pulses was analyzed by using the same empirical equation with the effective electric-field corrected for the imprint effect. The increase of the characteristic time due to wake-up was carried out by an increase in the field-independent coefficient, τ1, without a change in the activation field, α.

Effects of Pb content and electrode materials on the ferroelectric properties of Pb(Zr0.52Ti0.48)O3 thin films

We investigated the effects of Pb content and electrode materials on the ferroelectric properties of metal/Pb(Zr0.52Ti0.48)O3/metal thin film capacitors for non-volatile memory device applications. The increase of Pb content resulted in a change from the non-ferroelectric pyrochlore phase to the ferroelectric perovskite phase, and consequently, the enhancement of polarization-voltage hysteresis loops. Non-noble metal/PZT/noble metal asymmetric electrode structures were fabricated by using non-noble metals and a noble metal with different work functions. The asymmetric electrode and the resultant internal field, not only made a shift in the initial hysteresis loops, but also caused continuous degradation of the reliability characteristics of ferroelectric capacitors.

Low-frequency charge trapping and bistable domain switching in Mg-doped LiNbO3 single crystal films

Lithium niobate single-crystal films are interesting for the integration of optoelectronic and surface acoustic wave devices. However, the domains have the preferred orientations, which limit their application in non-volatile memory devices. Here, we fabricated 5% Mg-doped LiNbO3 single-crystal films by ion-slicing of surface layers of bulk lithium niobate single-crystals. The Mg doping can improve the electrical performance of the films with the appearance of a definitive coercive voltage irrespective of applied voltages in polarization-voltage (P-V) hysteresis loops, unlike the undoped films where the apparent coercive voltages in P-V hysteresis loops equal the maximum applied voltages. Subsequent unipolar pulse stressing shows comprehensive charge injection into the films to occur at repetitive periodicities below 0.5 s that can symmetrize P-V hysteresis loops along the voltage axis. The injected charge in contradiction to an internal imprint field can change domain switching dynamics that can be described by the model of a non-ferroelectric passive layer in series with an ideal ferroelectric layer. The dielectric permittivities at low frequencies show the Maxwell-Wager relaxation of the injected charge with the broad distribution of relaxation times. As the periodicity is above 2 s, the injected charge within each cycle can be driven out of the film during intermittent time by the imprint field. This study shows the possibility of charge injection in stabilization of bipolar domain orientations in LiNbO3 single-crystal films.

Chemical solution deposition of ferroelectric Sr:HfO2 film from inorganic salt precursors

Strontium doped hafnium oxide (Sr:HfO2) ferroelectric thin films with strontium concentrations ranging from 0 to 20mol% were processed with inorganic hafnium source and strontium source using a chemical solution deposition technique. The co-existence of monoclinic phase and ferroelectric orthorhombic phase in the Sr doped HfO2 thin film was confirmed by x-ray diffraction and high-resolution transmission electron microscopy results. The atomic force microscope measurements were adopted and the thin films showed crack-free surface. The intrinsic ferroelectricity of the doped HfO2 thin films could be demonstrated by polarization–voltage hysteresis loops together with piezoelectric force microscope. The maximum twofold remnant polarization value of 3.02 μC/cm2 with a coercive field of 2.0 MV/cm was achieved when the Sr content was 7.5 mol%. Meanwhile, the polarization didn't show obvious degradation until 107 electric cycles, indicating the good fatigue performance of the Sr:HfO2 film. These findings indicate that it is a feasible way to prepare Sr:HfO2 ferroelectric thin films via chemical solution deposition with inorganic salt precursors.

Ferroelectric and magnetic properties in 85 nm-thick Bi6Fe2Ti3O18 thin films by a modified sol-gel processing

Bi6Fe2Ti3O18 thin film on Pt/Ti/SiO2/Si substrate with a thickness of 85 nm is successfully prepared by a monoethanolamine-modified sol-gel processing. The prepared thin film is polycrystalline and shows dense morphology. The room temperature remnant polarization is higher than 10 μC cm−2 at 5 V driving voltage both from polarization-voltage hysteresis loop and positive-up negative-down measurements. The polarization after 107 switching cycles and a retention time of 104 s shows no obvious degradation, suggesting good fatigue-resistant characteristics. Weak ferromagnetism is observed at 5 K with a saturated magnetic moment of 11.7 emu/cm3. The results will provide an effective route to optimize Bi6Fe2Ti3O18 thin films with thickness below 100 nm by low-voltage driving.

Ferroelectricity and ferroelectric resistive switching in sputtered Hf0.5Zr0.5O2 thin films

Ferroelectric properties and ferroelectric resistive switching (FE-RS) of sputtered Hf0.5Zr0.5O2 (HZO) thin films were investigated. The HZO films with the orthorhombic phase were obtained without capping or post-deposition annealing. Ferroelectricity was demonstrated by polarization-voltage (P-V) hysteresis loops measured in a positive-up negative-down manner and piezoresponse force microscopy. However, defects such as oxygen vacancies caused the films to become leaky. The observed ferroelectricity and semiconducting characteristics led to the FE-RS effect. The FE-RS effect may be explained by a polarization modulated trap-assisted tunneling model. Our study not only provides a facile route to develop ferroelectric HfO2-based thin films but also explores their potential applications in FE-RS memories.

Nanosecond Characterization of Regional Domain Imprint from Fast Domain Switching Currents in Pb(Zr,Ti)O3 Thin Films**Supported by the National Key Basic Research Program of China under Grant No 2014CB921004, the National Natural Science Foundation of China under Grant Nos 61225020 and 61176121, and the Program for Professor of Special Appointment (Eastern Scholar) in Shanghai.

The traditional imprint characterization of ferroelectric thin films estimates imprint time dependence of the mean coercive voltage of all domains from a polarization-voltage hysteresis loop, which shows a semilogarithmic time dependence above an initial imprint time of τ0 > 1 μs at room temperature. Below τ0, the imprint effect is believed to be weak. In consideration of region-by-region domain reversal under a rising pulsed voltage with ordered coercive voltages increasing from zero up to the maximum applied voltage during capacitor charging time, we can estimate the imprinted coercive voltage of each domain from domain switching current transient separately with imprint time as short as 20 ns. In disagreement with the previous observations, all imprinted coercive voltages for the domains in Pt/Pb(Zr0.4Ti0.6)O3/Pt thin-film capacitors show step-like increases at two characteristic times of 300 ns and 0.27s. The imprint effect is surprisingly strong enough even at shortened time down to 20 ns without any evidence of weakening.