Coercive Voltage Research Articles

In-Plane Ferroelectric Domain Wall Memory with Embedded Electrodes on LiNbO3 Thin Films.

With the formation of mesa-like cells at their surfaces, LiNbO3 thin films are useful for integrating high-density domain wall memory. However, the material is too hard and inert to etch the cells with inclined side edges that help to diminish polarization retention. Moreover, etching could damage the ferroelectricity of the film. To overcome these drawbacks in forming memory cells directly, we developed a technique to deposit two gapped electrodes in the film surface, without needing to etch the film. While applying an in-plane write voltage above a coercive voltage, the domain within the gap is reversibly switched along with the creation/erasure of conducting domain walls against the peripheral unswitched domain. This technique enables "on"/"off" current read of the written information. Unfortunately, the switched domain within the gap generally has poor retention and a weak wall current arises from the presence of a strong depolarization field. To overcome this problem, we fabricated a type of embedded electrode that diffuses thickness-wise into the LiNbO3 thin film to form a parallel-plate-like structure to screen the depolarization field. The switched domains now had good retention and carry large wall currents. Alternatively, without the embedded electrodes, the switched domains within the cells can be stabilized with increasing gap distance above a critical length of 320 nm. The two methods foreshadow the possibility in the future to fabricate damage-free LiNbO3 memory cells without etching.

Assembly of Close-Packed Ferroelectric Polymer Nanowires via Interface-Epitaxy with ReS2.

The flexible, transparent, and low-weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization-enabled device functionalities, large-scale fabrication of polymer thin films with well-controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir-Blodgett, spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) thin films composed of close-packed crystalline nanowires via interface-epitaxy with 1T'-ReS2 . Upon controlled thermal treatment, uniform P(VDF-TrFE) films restructure into about 10 and 35 nm-wide (010)-oriented nanowires that are crystallographically aligned with the underlying ReS2 , as revealed by high-resolution transmission electron microscopy. Piezoresponse force microscopy studies confirm the out-of-plane polar axis of the nanowire films and reveal coercive voltages as low as 0.1 V. Reversing the polarization can induce a conductance switching ratio of >108 in bilayer ReS2 , over six orders of magnitude higher than that achieved by an untreated polymer gate. This study points to a cost-effective route to large-scale processing of high-performance ferroelectric polymer thin films for flexible energy-efficient nanoelectronics.

Voltage controlled awakening of memristor-like dynamic current–voltage loops of ferroelectric triglycine sulfate

For the study of polarization reversal features in the structure of "mixed ionic-electronic conductor - on - ferroelectric" type, the hydrogen bonded molecular ferroelectric crystal triglycine sulphate (TGS) is used. The dynamic currant-voltage (I-V-) loops of thin TGS plates of polar (010) cut with dissimilar electrodes on the opposite surfaces (vacuum deposited Ag and Ag-paste diluted with a water-ethanol mixture) are investigated. After electroforming in the low-voltage vicinity of coercive voltage, during further cycling, a reversible transformation of I-V-loop shape occurs from typical of non-linear dielectrics to typical of memristive systems and than to typical of ferroelectrics (and vice versa) with a sequential increase (decrease) of drawing voltage amplitude. Such "awakening" and "falling asleep" of I-V-loops of the memristor type is apparently associated with reversible electromigration and accumulation of charged species (e.g., protons), in the hydrated surface layer of TGS as a component of electrochemical capacitor-on-ferroelectric structure. The observed transformation can be explained by considering the coupled ion transfer and polarization reversal taking into account the swinging of the boundary of the built-in chemical inhomogeneity under conditions of linear and/or nonlinear dynamics of ion drift and ionic concentration polarization caused by charged species transfer under the applied voltage.

Polarization switching in Bi0.8La0.2FeO3 films with ferroelectric/semiconductor heterojunctions

Bi0.8La0.2FeO3 (BLFO) films are deposited directly on low-resistance Si (100) substrates by the pulsed laser deposition method. Compared with the typical hysteresis loops of ferroelectric films, those of the BLFO/Si samples, with almost horizontal upper and lower ends, severely shrink in the center part, and the coercive voltage is extremely small. A systematic analysis of the dielectric and ferroelectric characteristics reveals that a p–n junction forms between BLFO and Si, with oxygen vacancies accumulating near the interface. The polarization switching is accompanied by the change of the interface barrier and the migration of charged defects. This work provides a reference for the direct growth of ferroelectric films on silicon substrates and is of great significance for the development of the silicon-based electronic devices.

Design space exploration of hysteretic negative capacitance ferroelectric FETs based on static solutions of Landau–Khalatnikov model for nonvolatile memory applications

Systematic design space exploration of negative capacitance ferroelectric field-effect transistors (FeFETs) for nonvolatile memory operations was performed, combining load line analyses and circuit simulations. Unlike those FeFETs aiming at a steep subthreshold slope, the key design target here is to achieve bi-stable current versus voltage FET characteristics with appropriate hysteresis width (i.e. memory window). Remanent polarization, coercive voltage, and interfacial layer thickness were selected as design parameters. The results show that, if a ferroelectric gate dielectric film obeying ideal single-domain Landau–Khalatnikov model dynamics with reduced remanent polarization is available, ultralow voltage nonvolatile memories operating with sub-one volt voltage swing would become possible. An interesting feature of the negative capacitance FeFETs is that, unlike conventional multiple domain FeFETs, the memory window can be adjusted to a much smaller value than twice the coercive voltage. The lowered remanent polarization is required to suppress the depolarization field to an acceptable level for reliability. It is proposed that considering the abrupt polarization switching, three-transistor and two-transistor memory cells would be suitable for working and code storage memories, respectively.

Enhanced ferroelectric properties of P(VDF-TrFE) thin film on single-layer graphene simply adjusted by crystallization condition

Ferroelectric polymer/graphene hybrid systems have received considerable attention due to their promising utilization in flexible electronics. The crystallization of ferroelectric polymers such as poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) can obviously affect the electronic properties of the hybrid systems. Here, the effects of thermal treatment temperature and crystallization temperature on the structure, morphology, and piezoelectric/ferroelectric properties of P(VDF-TrFE) on graphene substrate are reported. Moreover, the dependence of coercive voltage and piezoelectric coefficient (d33) on thermal treatment temperature or crystallization temperature is explored. It is identified that coercive voltage is only relying on the crystallization temperature other than thermal treatment temperature. Once crystallization temperature changes, both coercive voltage and d33 alter significantly and an optimum crystallization temperature exists at which the sample shows the best properties. The relationship between structure and piezoelectric/ferroelectric properties is built. These findings provide useful guidance for fabricating piezoelectric/ferroelectric P(VDF-TrFE)/graphene devices with high performance.

Uniform Crystal Formation and Electrical Variability Reduction in Hafnium-Oxide-Based Ferroelectric Memory by Thermal Engineering

In this paper, we achieved excellent variation control, endurance enhancement, and leakage reduction in zirconium (Zr)-doped hafnium oxide (Hf1–xZrxO2) based ferroelectric films by the germination of large ferroelectric grains through extending the duration of rapid thermal annealing without increasing the temperature beyond 700 °C. The pivotal point of this work is to reduce electrical variations in ferroelectric capacitors, which we attribute to the random variation in ferroelectric and dielectric phases. The motivation for this research originally stemmed from Johnson–Mehl–Avrami–Kolmogorov’s theory of nucleation, which predicts that a sufficient time is required for forming any particular phase during phase transformation. Instead of increasing the temperature beyond 700 °C, extending the duration of rapid thermal annealing allows uniform crystal to form, which agrees with the theory. A 10 nm thick Hf1–xZrxO2 film (x = 0.2) has been fabricated. The duration and temperature of rapid thermal annealing (RTA) under a nitrogen (N2) atmosphere at 1 atmospheric pressure (atm) is varied to observe its effect on crystal formation and the electrical properties of the devices. It was observed that very low temperature and short (30 s) duration annealing at 1 atm pressure cannot infuse ferroelectricity in Hf1–xZrxO2. Structural analysis clearly showed the formation of large and uniform ferroelectric domains with negligible impact on surface roughness upon extending the annealing duration up to 180 s. The device-to-device variation in terms of standard deviation of coercive voltage and peak capacitance are reduced from 400 to 17 mV and from 20 to 4 fF/cm2, respectively, by increasing the RTA duration at 700 °C. The devices have also displayed endurance above 1 million cycles and leakage current density below 10 pA/μm2. The simple physics-based process discussed here reduces the thermal budget required for Hf1–xZrxO2, mitigates the randomness in the domain distribution, and infuses deterministic switching. This improvement paves the way for implementing Hf1–xZrxO2-based, deeply scaled ferroelectric devices for memory and steep slope device applications.

Negative capacitance in epitaxial ferroelectric capacitors evidenced by dynamic dielectric characterization

A simple dynamic method was developed to evaluate the components in the equivalent circuit of a ferroelectric capacitor. The method is based on the application of short trapezoidal voltage pulses of variable amplitude and the analysis of the resulting current by considering the ferroelectric capacitor as a parallel Rp-Cp equivalent circuit. The values of Rp and Cp components are obtained in relation to different stages of polarization switching as the amplitude of the applied pulses increases. The most important result obtained by applying the present method is the evidence of an abrupt decrease of the Rp value (about 2 orders of magnitude) at the coercive voltage, while the equivalent Cp does not present a dramatic variation during polarization switching. This is interpreted in the context of negative capacitance obtained for ferroelectrics when polarization passes through zero value. The results obtained by using the proposed dynamic characterization method that will be referred to as “dynamic dielectric characterization”, are in good agreement with those obtained by classic capacitance-voltage measurements. A method to stabilize the negative capacitance for longer periods of time is also presented by adding an external resistance in series with the ferroelectric capacitor.

Depolarization field tuning of nanoscale ferroelectric domains in (001)PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 epitaxial heterostructures

The effect of tuning the depolarization field in (001)-oriented ultrathin epitaxial PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 ferroelectric heterostructures is investigated. The thickness of the dielectric spacer (SrTiO3) is maintained constant at 2 unit cells. The ferroelectric layer thickness in the heterostructure (each PbZr0.4Ti0.6O3) layer varied from 8 to 15 nm is exploited as the parameter to tune the depolarization field. Piezoresponse force microscopy reveals a domain evolution from continuous labyrinthine domains to individual nanoscale bubble domains under the influence of an increasing depolarization field. A statistical analysis of the domain features (i.e., domain wall length and domain fraction) reveals that this change in domain morphology also affects the wall roughness and its associated disorder. The local coercive voltage obtained using switching spectroscopy piezoresponse force microscopy finds the 15 nm film to have the lowest coercive voltage. This is attributed to both a strain-induced increase in tetragonality and the depolarization field-induced changes in the domain morphology.

Domain modulation in LiNbO3 films using litho piezoresponse force microscopy

Domain engineering plays a pivotal role in the development of ferroelectric non-volatile memory devices. In this work, we mainly focus on the domain kinetic in ion-sliced single crystal LiNbO3 thin films under tip-induced electric fields using piezoresponse force microscope (PFM). Polarization reversal takes place when the electric fields are above threshold value (coercive voltage V c) of films. Besides, the dependence of domain dynamic on pulse duration and amplitude were investigated in detail, and specific local domain reversal (5 μm) was completed by the optimized poling condition. All the results reveal that tip-induced polarization reversal could be an effective way to domain engineering, which gives much more promising prospects regarding to the high density non-volatile ferroelectric memory devices.

Strain tuning of negative capacitance in epitaxial PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub> thin films

In order to continue Moore’s Law, the reducing of power consumption is concerned by many researchers, and the discovery of ferronegative negative capacitance effect (NCE) provides a solution. Strain engineering has been widely studied as an effective means to regulate the physical properties of ferroelectric thin films. But the relevant mechanism of strain to ferroelectric negative capacitance regulation is not clear. Recently, the experimental results have shown that it is possible to stabilize the transient NCE in resistance-ferroelectric networks. In this work, we use the Landau-Khalatnikov theory to study the microscopic domain evolution and the influence of strain and temperature on NCE in a ferroelectric film. It is shown that compressive strain enhances NCE while NCE becomes weaker under a tensile strain. However, a larger compressive strain will give rise to a higher coercive voltage that hinders the NCE from forming. In addition, under a certain strain, the NCE becomes stronger at lower temperature. This work provides the theoretical basis for designing the negative capacitance devices and scaling towards nanoscale dimensions in future.

The microstructure and ferroelectric properties of PbZr0.52Ti0.48O3 films on mica substrates

PbZr0.52Ti0.48O3 (PZT) tetragonal ferroelectric films have been fabricated on flexible mica and rigid silicon substrates respectively with a bottom electrode of LaNiO3 (LNO) films via a low-cost sol-gel process. The microstructure and ferroelectric properties of both films were systematically investigated. The quantitative analysis results of XRD and TEM indicate that the PZT film on Si substrate suffer from residual in-plane tensile stress, while a virtually strain-free film can be obtained on mica substrate. The PZT film on mica substrate has a lower local coercive voltage studied by a piezoelectric response force microscope. It is the weak van der Waals interaction between the film and the two-dimensional mica substrate that reduce the substrate clamping effect and contribute to a more free response to the electric field for domain walls. A large measured residual polarization of ~52 μC/cm2 was obtained for the film on mica substrate, and the P−E loops can remain stable after bending cycles of 1000 times. The flexible PZT thin film is expected to have a strong potential application in the next generation of wearable devices.

A computational search for wurtzite-structured ferroelectrics with low coercive voltages

Ferroelectricity has recently been observed in wurtzite-structured Sc-doped AlN thin films, five years after our initial prediction of ferroelectricity in wurtzite compounds based on first-principles calculations. The thin films exhibited a much higher coercive voltage (3 MV/cm) than that of conventional perovskite-structured ferroelectric material PbTiO3, however, making it difficult to switch the films’ polarity and limiting their practical application. To identify tetrahedral ferroelectric materials with low coercive voltages, we have carried out a wider exploration of candidate binary compounds, from halides to chalcogenides to pnictogenides, using first-principles methods. The overall trend is for polarization switching barriers to decrease with decreasing anion-to-cation radius ratio, with the lowest barriers found in monovalent compounds such as the copper and silver halides; e.g., CuCl is calculated to have a switching barrier of 0.17 eV/f.u. and that of AgI is 0.22 eV/f.u., values similar in magnitude to that of PbTiO3 (0.20 eV/f.u.). Applying an epitaxial tensile strain to the basal plane is also effective for lowering the potential barrier further, with barriers in both AgI and CuCl decreasing to 0.04 eV/f.u. when a 5% in-plane expansion is applied. The results suggest that tetrahedral ferroelectrics with moderate coercive voltages (below 100 kV/cm) should be achievable.

Simulations of single event effects in 6T2C-based ferroelectric non-volatile static random access memory

The single event effects (SEEs) on ferroelectric Hf0.5Zr0.5O2 capacitor-based non-volatile static random access memory (nvSRAM) were investigated by simulation. A nvSRAM cell integrated with two ferroelectric Hf0.5Zr0.5O2 capacitors is proposed in this study. A macro-model of the ferroelectric Hf0.5Zr0.5O2 capacitor, extracted from the real fabricated devices, is utilized for simulation analysis. Fundamental store and recall operations of the proposed nvSRAM design have been demonstrated. An independent double exponential current source was utilized and injected into specific circuit nodes to simulate the heavy ion induced single event transient current. The simulation results show that the transient pulse current is possible to upset the logic state of the memory cell from 1 to 0, but whether it can recover in a short time period after the upset errors depends on the exact value of linear energy transfer for the injected particles. In addition, increasing the remnant polarization (P r) and decreasing the coercive voltage (V c) and film thickness of ferroelectric capacitors can mitigate the influence of SEEs, which provides guidance for process hardening techniques aiming at space applications.

Reliability aspects of ferroelectric TiN/Hf0.5Zr0.5O2/Ge capacitors grown by plasma assisted atomic oxygen deposition

The reliability of Hf0.5Zr0.5O2 (HZO) metal–ferroelectric–semiconductor capacitors grown by plasma-assisted atomic oxygen deposition on Ge substrates is investigated with an emphasis on the influence of crystallization annealing. The capacitors show very weak wake-up and imprint effects, allowing reliable operation in excess of 10 years, which is attributed partly to the clean, oxide-free Ge/HZO bottom interface. The weak temperature dependence and the observed asymmetries between polarization up and down states and between positive and negative coercive voltage shifts lead to the conclusion that imprint is controlled by carrier injection at the top electrode interface. The latter mechanism is associated with trapping at interfacial oxygen-vacancy defects. On the other hand, using ultrafast (millisecond) flash annealing improves the leakage current by at least an order of magnitude and the endurance by a factor of 3 compared to conventional rapid thermal annealing, which makes them suitable for low power nonvolatile memory applications where (ultra)thin HZO is an essential requirement.

Tunable Microwave Filters Using HfO2-Based Ferroelectrics.

In this paper, we present microwave filters that are based on 6-nm-thick ferroelectric thin films of hafnium oxide doped with zirconium (HfZrO), which are tunable continuously in targeted bands of interest within the frequency range 0.1–16 GHz, when the applied direct current (DC) voltage is swept between 0 V and 4 V. Here, we exploit the orthorhombic polar phase in HfO2 through a careful doping using zirconium in an Atomic Layer Deposition (ALD) process, in order to guarantee phase stabilization at room temperature. Polarization versus voltage characterization has been carried out, showing a remanent polarization (Pr) of ~0.8 μC/cm2 and the coercive voltage at ~2.6 V. The average roughness has been found to be 0.2 nm for HfZrO films with a thickness of 6 nm. The uniform topography, without holes, and the low surface roughness demonstrate that the composition and the structure of the film are relatively constant in volume. Three filter configurations (low-pass, high-pass, and band-pass) have been designed, modelled, fabricated, and fully characterized in microwaves, showing a frequency shift of the minimum of the reflection coefficient between 90 MHz and 4.4 GHz, with a minimum insertion loss of approximately 6.9 dB in high-pass configuration.

Investigation of Imprint in FE-HfO₂ and Its Recovery

Ferroelectric (FE)-HfO2-based FETs (FEFETs) are one of the most promising candidates for emerging memories. However, the FE material suffers from a unique reliability phenomenon known as imprint: the coercive voltage shifts during data retention, which has been regarded as a major issue for memory operation, while the mechanism causing it is still under research. In this article, imprint and its recovery in FE-HfO2 are investigated in detail by comprehensive electrical measurements to reveal its underlying mechanism including the cause of asymmetric coercive voltage shifts. The recovery measurements clarify that domain switching is indispensable for the recovery from imprint. The subloop imprint effect shows that imprint and its recovery must be independent for each domain. In addition, switching time measurements and corresponding fitting results with the nucleation-limited-switching (NLS) model strongly indicate that imprint is caused by domain-seeds-pinning. Based on these results, we conclude that charge trapping and detrapping affecting activation barriers for domain switching, accompanied by domain switching, is responsible for imprint and its recovery.

Electrically driven transient and permanent phase transformations in highly strained epitaxial BiFeO3 thin films

Electric-field-driven phase transformation phenomena in multiferroic BiFeO3 are directly linked to the functionalities of electronic devices based on multiferroic materials. Understanding how the transformation evolves at the nanoscale under the influence of an electric field will provide fascinating insights into key parameters that utilize the transformation features. Here, we report both the electric-field-driven transient and permanent phase transformations in highly strained BiFeO3 thin films and their transformation dynamics at the nanoscale. We found that two distinct transient and permanent phase transformations were triggered below and above a coercive voltage of the polymorphic phase, indicating that ferroelectric polarization switching could promote permanent phase transformations. We also found that the transient transformations evolve via complex phase boundary motions between the coexisting phases, whereas permanent transformations occurred via nucleation of the other phases.

Multidomain Negative Capacitance Effect in P(VDF-TrFE) Ferroelectric Capacitor and Passive Voltage Amplification

This article demonstrates low supply voltage ( ${V}_{{\text {DD}}}{)}$ and high-frequency polarization switching in an organic ferroelectric capacitor (OFEC). The Landau–Khalatnikov (LK) equation is used to model the organic ferroelectric (OFE) in the Sentaurus technology computer-aided design (TCAD) environment considering multidomain (MD) dynamics and interaction factor ( ${K}_{{\text {OFE}}}{)}$ , resulting in a strong resemblance with the reported experimental results. The operation of OFEC at gigahertz frequency is examined with the experimentally validated MD-LK model. We study the impact of multiple domains in the OFEC on the transient response considering three separate sizes of the top metal plate (TMP). Our findings show that OFEC with a larger TMP area charges slowly than the other OFEC with smaller and proposed area TMP because of the enormous amount of domains in the larger area TMP. Various effects of ${V}_{{\text {DD}}}$ , external resistor ( ${R}{)}$ , and LK static parameters on negative capacitance signature (NCS) are investigated. We observe the NCS effect at 1 V because the coercive voltage of the OFEC is around 0.56 V, which is 73.33% lower than the conventional ferroelectric (FE) lead zirconate titanate (PZT), resulting in a minimum energy-delay product (EDP) of $2.7\times 10^{-{26}}$ Js. We observe that as the value of ${K}_{{\text {OFE}}}$ increases, the nature of domain wall switches from hard to soft, that is, coercive voltage reduces, resulting in a smaller hysteresis loop. Finally, a passive voltage amplifier (PVA) is implemented which shows voltage amplification of 1.45 at 500 MHz with low viscosity ( $\rho _{{\text {OFE}}} = {4.5}\,\,\Omega $ cm), which is still a challenge for PZT-based NC field-effect transistor (NCFET) reported in the literature.

Polarization Switching and Negative Capacitance in Epitaxial PbZr0.2Ti0.8O3 Thin Films

The negative-capacitance effect in devices based on combined ferroelectric-dielectric gate oxides is thought to be a potential solution to break free from the so-called Boltzmann tyranny. To lower the power consumption in field-effect transistors, the subthreshold swing factor $S$ should be reduced below the thermodynamic limit of $60\phantom{\rule{0.2em}{0ex}}\mathrm{mV}$ per decade. Yet, despite numerous studies dedicated to this effect in the past decade, its origin in ferroelectric capacitors or ferroelectric-based superlattices remains unclear, being considered either a transitory product of polarization switching or an intrinsic phenomenon related to the presence of ferroelectric polarization. In this study it is shown, starting from simple electrostatic considerations, that negative capacitance is present during polarization switching and is accompanied by a significant increase of the current flowing through the ferroelectric capacitor. Coupled with piezo-force microscopy results, it is shown that the polarization orientation suddenly changes at the coercive voltage, accompanied by a complete reconfiguration of the potential barriers at the Schottky-like contacts present at the electrode-ferroelectric interfaces. A method to estimate the polarization-switching time, as the time associated with the presence of the negative-capacitance effect, is proposed. Values in the range from 100 to 1000 ns are obtained for epitaxial $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ films. These findings suggest that negative capacitance may be an intrinsic effect in ferroelectrics but that it is a transitory effect, present only when ferroelectric polarization passes through zero (switching).