Ferroelectric Layer Research Articles

Влияние магнитострикционной фазы на мультикалорический эффект в магнитострикционно-сегнетоэлектрическом композите

The ferroelectric volume fraction dependence of the multicaloric effect in a bilayer of barium titanate and cobalt ferrite was studied using the Landau-Ginzburg thermodynamic approach. It is shown that the electrocaloric effect can be controlled using the volume fraction of the ferroelectric. Magnetic field application is an effective tool to enhance the multicaloric effect in a magnetostrictive-ferroelectric structure. The choice of an optimal magnetic component which has a magnetocaloric effect in the appropriate temperature range and enables generating the necessary mechanical stress in ferroelectric layer, will increase the multicaloric effect in a bilayer.

Электропроводность и интерфейсные явления в тонкопленочных гетероструктурах на основе ниобата лития и танталата лития

In this work, the electrophysical properties of metal-ferroelectric-semiconductor structures – Cu/LiNbO₃/Si and Ag/LiTaO₃/Si – with a ferroelectric layer thickness of 200 nm have been studied. The ferroelectric layers were deposited by RF‐magnetron sputtering. A topography study of thin film surface revealed a grain structure. The electrical conductivity mechanisms in Cu/LiNbO₃/Si and Ag/LiTaO₃/Si were considered. In a dependence of bias voltage value, there are a space charge-limited current, hopping conduction, and Schottky emission in Cu/LiNbO₃/Si structures. For Ag/LiTaO₃/Si structures, the space charge-limited current and hopping conduction were observed. An asymmetry of the current-voltage characteristics may indicate the presence of a potential barrier at the interface. For the studied structures, the value of the potential barrier was determined.

Effect of Floating Gate Insertion on the Analog States of Ferroelectric Field-Effect Transistors

In this work, we propose a structural approach to mitigate device-to-device variation and performance degradation of ferroelectric (FE) field-effect transistors (FeFETs) due to the inhomogeneity of FE and dielectric (DE) phases of the FE layer. We found that by inserting a floating gate below the FE layer, the polarization effect of FE grains is equalized, thus suppressing the formation of an undesired current percolation path through the channel of the FeFET. This also results in a wider memory window and improved device variation, which ultimately improves the accuracy of in-memory computing. We believe that the proposed approach could be an important strategy enabling reliable and unified operation of FeFETs with the scaling of device.

Implementation of high-performance and high-yield nanoscale hafnium zirconium oxide based ferroelectric tunnel junction devices on 300 mm wafer platform

In this work, hafnium zirconium oxide (HZO)-based 100 × 100 nm2 ferroelectric tunnel junction (FTJ) devices were implemented on a 300 mm wafer platform, using a baseline 65 nm CMOS process technology. FTJs consisting of TiN/HZO/TiN were integrated in between metal 1 (M1) and via 1 (V1) layers. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy analysis confirmed the targeted thickness and composition of the FTJ film stack, while grazing incidence, in-plane x-ray diffraction analysis demonstrated the presence of orthorhombic phase Pca21 responsible for ferroelectric polarization observed in HZO films. Current measurement, as a function of voltage for both up- and down-polarization states, yielded a tunneling electroresistance (TER) ratio of 2.28. The device TER ratio and endurance behavior were further optimized by insertion of thin Al2O3 tunnel barrier layer between the bottom electrode (TiN) and ferroelectric switching layer (HZO) by tuning the band offset between HZO and TiN, facilitating on-state tunneling conduction and creating an additional barrier layer in off-state current conduction path. Investigation of current transport mechanism showed that the current in these FTJ devices is dominated by direct tunneling at low electric field (E < 0.4 MV/cm) and by Fowler–Nordheim (F–N) tunneling at high electric field (E > 0.4 MV/cm). The modified FTJ device stack (TiN/Al2O3/HZO/TiN) demonstrated an enhanced TER ratio of ∼5 (2.2× improvement) and endurance up to 106 switching cycles. Write voltage and pulse width dependent trade-off characteristics between TER ratio and maximum endurance cycles (Nc) were established that enabled optimal balance of FTJ switching metrics. The FTJ memory cells also showed multi-level-cell characteristics, i.e., 2 bits/cell storage capability. Based on full 300 mm wafer statistics, a switching yield of >80% was achieved for fabricated FTJ devices demonstrating robustness of fabrication and programming approach used for FTJ performance optimization. The realization of CMOS-compatible nanoscale FTJ devices on 300 mm wafer platform demonstrates the promising potential of high-volume large-scale industrial implementation of FTJ devices for various nonvolatile memory applications.

Persistent triboelectrification-induced electroluminescence for self-powered all-optical wireless user identification and multi-mode anti-counterfeiting.

Persistent triboelectrification-induced electroluminescence (TIEL) is highly desirable to break the constraints in the transient-emitting behavior of existing TIEL technologies as it addresses the hindrance caused by incomplete information in optical communication. In this work, a novel self-powered persistent TIEL material (SP-PTM) has been created for the first time, by incorporating the long-afterglow phosphors SrAl2O4:Eu2+, Dy3+ (SAOED) in the material design. It was found that the blue-green transient TIEL derived from ZnS:Cu, Al serves as a reliable excitation source to trigger the persistent photoluminescence (PL) of SAOED. Notably, the aligned dipole moment formed along the vertical direction in the bottom ferroelectric ceramics layer acts as an "optical antenna" to promote variation in the electric field of the upper luminescent layer. Accordingly, the SP-PTM exhibits intense and persistent TIEL for about 10 s in the absence of a continuous power supply. Due to such unique TIEL afterglow behavior, the SP-PTM is applicable in many fields, such as user identification and multi-mode anti-counterfeiting. The SP-PTM proposed in this work not only represents a breakthrough in TIEL materials due to its recording capability and versatile responsivity but also contributes a new strategy to the development of high-performance mechanical-light energy-conversion systems, which may inspire various functional applications.

Manipulating two-dimensional magnetic states via electric field and pressure.

Topological spin configurations have been an intriguing topic due to the exotic transport properties and promising applications in spintronic devices. The discovery of two-dimensional (2D) magnetic materials such as CrI3 provides new platforms for manipulating magnetic structures. Here, by first-principles calculations and Monte Carlo methods, we investigated the exchange interaction and magnetic states of 2D van der Waals ferromagnetic/ferroelectric heterostructure CrI3/In2Se3. By switching the polarization in the ferroelectric In2Se3 layer under an electric field and changing the interlayer distance between CrI3 and In2Se3 under pressure, four spin configurations, ferromagnetic states, topological domain wall skyrmions, topological bimerons, and stripe domains can be realized. These striking tunable magnetic states can be understood from the Dzyaloshinskii-Moriya interaction and single-ion anisotropy parameters being modified by switching the polarization and changing the interlayer distance. Our results of controllable topological/non-topological spin states broaden the spin phenomena and potential of spintronic applications in van der Waals heterostructures.

The effect of switchable electronic polarization states on the electronic properties of two-dimensional multiferroic TMBr2/Ga2SSe2 (TM = V-Ni) heterostructures.

Multiferroic van der Waals (vdW) heterostructures (HSs) prepared by combining different ferroic materials offer an exciting platform for next-generation nanoelectronic devices. In this work, we investigate the magnetoelectric coupling properties of multiferroic vdW HSs consisting of a magnetic TMBr2 (TM = V-Ni) monolayer and a ferroelectric Ga2SSe2 monolayer using first-principles theory calculations. It is found that the magnetic orderings in the magnetic TMBr2 layers are robust and the band alignment of these TMBr2/Ga2SSe2 HSs can be altered by reversing the polarization direction of the ferroelectric layer. Among them, VBr2/Ga2SSe2 and FeBr2/Ga2SSe2 HSs can be switched from a type-I to a type-II semiconductor, which allows the generation of spin-polarized and unpolarized photocurrent. Besides, CrBr2/Ga2SSe2, CoBr2/Ga2SSe2 and NiBr2/Ga2SSe2 exhibit a type-II band alignment in reverse ferroelectric polarization states. Moreover, the magnetic configuration and band alignment of these TMBr2/Ga2SSe2 HSs can be further modulated by applying an external strain. Our findings suggest the potential of TMBr2/Ga2SSe2 HSs in 2D multiferroic and spintronic applications.

Nonvolatile electrical control of valley splitting by ferroelectric polarization switching in a two-dimensional AgBiP2S6/CrBr3 multiferroic heterostructure.

The generation and controllability of valley splitting are the major challenge in effectively utilizing valley degrees of freedom in valleytronics. Using first-principles calculations, we propose a novel multiferroic system, a AgBiP2S6/CrBr3 van der Waals heterostructure, with ferromagnetism, ferroelectricity and ferrovalley behaviors. The ferroelectric monolayer AgBiP2S6 originally has two degenerate valleys with a large spin splitting (∼423.1 meV) at the conduction band minimum of K/K' points, due to inversion symmetry breaking combined with strong spin orbit coupling. Magnetic proximity coupling with the ferromagnetic layer CrBr3 breaks the time-reversal symmetry, damaging the degeneracy of K/K' valleys and causing valley splitting (∼30.5 meV). The transition energy barrier between two ferroelectric states with opposite polarization direction of the heterostructure is sufficient to prevent the spontaneous transition at room temperature, and the large intermediate barrier suggests that the ferroelectric state should be observed experimentally under ambient conditions. Nonvolatile electrical control of the valley degrees of freedom is achieved by switching the polarization direction of the ferroelectric layer in the heterostructure. The modulation of valley splitting can also be achieved by applying an external electric field and biaxial strain, as well as changing the magnetization direction. The research of nonvolatile electrical control of valley splitting in the two-dimensional AgBiP2S6/CrBr3 multiferroic heterostructure is crucial for designing all-in-one valleytronic devices, and has important theoretical significance and practical value.

Simulation Analysis of Noise Components in NCTFET with Ferroelectric Layer in Gate Stack

Tunnel FETs (TFETs) are a strong contender for replacing conventional MOSFETs in lower power applications due to their low off-state current and sub-threshold swing being much lower than 60 mV/decade. The major trade-off is the shortcomings of TFET, which are ambipolar behaviour, low on-state current and large miller capacitance. The various novel structures of TFETs have been proposed by researchers. This work presents one such structure of heterojunction negative capacitance TFET with raised source and gate material overlapping. With the intervention of the ferroelectric layer in the gate stack of TFET, the overall performance of the device has been improved. Further AC DC analysis and electrical noise analysis are carried out for the proposed device to see the impact of improvisations being done in the structure on the input–output characteristics of the device. Synopsys TCAD tool is used to carry out the simulations at low and high frequencies. As per the results obtained, the raised source device using Ge as source material exhibits a high ION of 4.295 × 10−5 A/μm, low IOFF of 6.01 × 10−15 A/μm and sub-threshold slope of 53.75 mV/decade. The proposed device structure is having least ambipolarity and can be used in ultra-low power circuits.

Wavelength dependence of polarization-resolved second harmonic generation from ferroelectric SnS few layers

Two-dimensional group-IV monochalcogenides such as GeS, GeSe, SnS, and SnSe were theoretically predicted as multiferroic materials with two or more ferroic properties. However, most of their bulk crystals are stacked layer by layer with an antiferroelectric manner, which lose the macroscopic in-plane ferroelectricity. In this work, we studied SnS in which the layers are stacked in a ferroelectric manner both experimentally and theoretically. We utilized polarization-resolved second harmonic generation (SHG) microscopy to investigate numerous flakes of ferroelectric SnS few layers on mica substrates. We found the SHG polar patterns dramatically varied in the range of 800 nm and 1000 nm due to the frequency-dependent SHG susceptibilities. First-principles calculations have been performed to study the frequency-dependent and layer-dependent SHG susceptibilities in the ferroelectric SnS with AA and AC stacking orders. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results. Since polarization-resolved SHG is a noncontact and nondestructive technique to determine the crystal orientation, understanding of its properties is important, especially for monitoring the transition of different ferroic phases.

Maximum Power of Thin-Film Capacitive Electrostatic Micromotors Based on Nanogaps

This paper is devoted to the determination of the value of the maximum specific power of capacitive electrostatic micromotors based on nanometer working gaps. The motors under consideration were developed earlier. They have the following structure: a metal—thin crystalline ferroelectric layer with a high dielectric constant—a nanometer working gap—a movable electrode. The mechanism limiting the magnitude of the maximum field in the gap have also been determined in previous works. The mechanism is the stripping of atoms from the surface of the movable electrode under the action of electrostatic forces. It was shown that the maximum energy density in the working gap can be as high as 1.6 × 109 J/m3. In the presented paper, a maximum frequency of electromechanical energy conversion as high as 10 MHz is estimated for these motors, with a maximum specific power of 5 × 108 W/kg. The application of the proposed motors for micromachines is discussed.

Boosting the Sensitivity of WSe2 Phototransistor via Janus Interfaces with 2D Perovskite and Ferroelectric Layers

Hybrid systems have recently attracted increasing attention, which combine the special attributes of each constitute and create interesting functionalities through multiple heterointerface interactions. Here, we design a two-dimensional (2D) hybrid phototransistor utilizing Janus-interface engineering, in which the WSe2 channel combines light-sensitive perovskite and spontaneously polarized ferroelectrics, achieving collective ultrasensitive detection performance. The top perovskite (BA2(MA)3Pb4I13) layer can absorb the light efficiently and provide generous photoexcited holes to WSe2. WSe2 exhibit p-type semiconducting states of different degrees due to the selective light-operated doping effect, which also enables the ultrahigh photocurrent of the device. The bottom ferroelectric (Hf0.5Zr0.5O2) layer dramatically decreases the dark current, which should be attributed to the ferroelectric polarization assisted charge trapping effect and improved gate control. As a whole, our phototransistors show excellent photoelectric performances across the ultraviolet to near-infrared range (360-1050 nm), including an ultrahigh ON/OFF current ratio > 109 and low noise-equivalent power of 1.3 fW/Hz1/2, all of which are highly competitive in 2D semiconductor-based optoelectronic devices. In particular, the devices show excellent weak light detection ability, where the distinguishable photoswitching signal is obtained even under a record-low light intensity down to 1.6 nW/cm2, while showing a high responsivity of 2.3 × 105 A/W and a specific detectivity of 4.1 × 1014 Jones. Our work demonstrates that Janus-interface design makes the upper and lower interfaces complement each other for the joint advancement into high-performance optoelectronic applications, providing a picture to realize the integrated engineering on carrier dynamics by light irradiation, electric field, interfacial trapping, and band alignment.

In-situ organic-inorganic ferroelectric layer growth for efficient perovskite solar cells with high photovoltage

Halide perovskite solar cells (PSCs) have shown outstanding performance, which can be further improved through enhancing the built-in electric field (Vbi) and reducing non-radiative recombination pathways. Here we in-situ grow an organic-inorganic ferroelectric layer on the perovskite film, which can be easily and quickly processed even at room-temperature within one minute. The in-situ process endows intimate growth of ferroelectric layer on the original perovskite grains, which can reduce the formation of grain boundaries and thus minimize the recombination of electron and hole at grain boundaries. The Vbi of the PSCs is enhanced from 1.02 V to 1.28 V due to the widened space charge depletion region after the formation of ferroelectric layer. Finally, an output photovoltage up to 1198 mV is achieved with a champion efficiency of 22.45% and a low voltage deficit of 352 mV. The strategy offers a new platform of growing intimate ferroelectric layers on perovskite photoactive films for integration between ferroelectrics and PSCs.

Design optimization of a silicon-germanium heterojunction negative capacitance gate-all-around tunneling field effect transistor based on a simulation study

The steep sub-threshold swing of a tunneling field-effect transistor (TFET) makes it one of the best candidates for low-power nanometer devices. However, the low driving capability of TFETs prevents their application in integrated circuits. In this study, an innovative gate-all-around (GAA) TFET, which represents a negative capacitance GAA gate-to-source overlap TFET (NCGAA-SOL-TFET), is proposed to increase the driving current. The proposed NCGAA-SOL-TFET is developed based on technology computer-aided design (TCAD) simulations. The proposed structure can solve the problem of the insufficient driving capability of conventional TFETs and is suitable for sub-3-nm nodes. In addition, due to the negative capacitance effect, the surface potential of the channel can be amplified, thus enhancing the driving current. The gate-to-source overlap (SOL) technique is used for the first time in an NCGAA-TFET to increase the band-to-band tunneling rate and tunneling area at the silicon–germanium heterojunction. By optimizing the design of the proposed structure via adjusting the SOL length and the ferroelectric layer thickness, a sufficiently large on-state current of 17.20 μA can be achieved and the threshold voltage can be reduced to 0.31 V with a sub-threshold swing of 44.98 mV/decade. Finally, the proposed NCGAA-SOL-TFET can overcome the Boltzmann limit-related problem, achieving a driving current that is comparable to that of the traditional complementary metal–oxide semiconductor devices.

Untwining polar contributions from light-polarization dependent photovoltaic response of LuMnO3-based ferroelectric capacitors

Narrow bandgap ferroelectrics are receiving a renewed interest for photovoltaic applications aiming at exploiting new functionalities arising from bulk photovoltaic effect (BPE). We report on the photovoltaic response of vertical capacitors of ferroelectric hexagonal LuMnO3 films sandwiched between semitransparent top electrodes (Pt, Co, Ti) and a common bottom (Pt) bottom electrode. Our results show that the presence of electrodes, other than their optical transmittance, crucially determines the imprint in the ferroelectric layer and ultimately the sensitivity of short circuit current density (Jsc) to the ferroelectric polarization direction. The use of ultrathin (7 nm) Pt top electrodes allowed to obtain a large Jsc (up to 100 mA/cm2) and an open circuit voltage of Voc ≈ 0.52 V, with a responsivity of 2 × 10−3 A/W. Yet, polarization back-switching due to imprint largely washes out the dependence of Jsc on the direction of the polarization and thus, at first sight, Jsc seemingly appears to be ruled by conventional photovoltaic response. However, a pioneering analysis of the light-polarization dependent photosensitivity, allowed to disentangle for the first time, a genuine contribution of BPE from a ubiquitous Fresnel-like contribution arising from interfaced optical media.

The Opportunity of Negative Capacitance Behavior in Flash Memory for High‐Density and Energy‐Efficient In‐Memory Computing Applications

AbstractFlash memory is a promising candidate for use in in‐memory computing (IMC) owing to its multistate operations, high on/off ratio, non‐volatility, and the maturity of device technologies. However, its high operation voltage, slow operation speed, and string array structure severely degrade the energy efficiency of IMC. To address these challenges, a novel negative capacitance‐flash (NC‐flash) memory‐based IMC architecture is proposed. To stabilize and utilize the negative capacitance (NC) effect, a HfO2‐based reversible single‐domain ferroelectric (RSFE) layer is developed by coupling the flexoelectric and surface effects, which generates a large internal field and surface polarization pinning. Furthermore, NC‐flash memory is demonstrated for the first time by introducing a RSFE and dielectric heterostructure layer in which the NC effect is stabilized as a blocking layer. Consequently, an energy‐efficient and high‐throughput IMC is successfully demonstrated using an AND flash‐like cell arrangement and source‐follower/charge‐sharing vector‐matrix multiplication operation on a high‐performance NC‐flash memory.

Improved detectivity and response speed of MoS2 phototransistors based on the negative-capacitance effect and defect engineering.

Due to the unique crystal structure, outstanding optoelectronic properties and a tunable band gap from 1.2-1.8 eV, two-dimensional molybdenum disulfide (MoS2) has attracted extensive attention as a promising candidate for future photodetectors. In this work, a negative-capacitance (NC) MoS2 phototransistor is fabricated by using H f 0.5 Z r 0.5 O 2 (HZO) as ferroelectric layer and Al2O3 as matching layer, and a low subthreshold swing (SS) of 39 mV/dec and an ultrahigh detectivity of 3.736×1014 cmHz1/2W-1 are achieved at room temperature due to the NC effect of the ferroelectric HZO. Moreover, after sulfur (S) treatment on MoS2, the transistor obtained a lower SS of 33 mV/dec, a detectivity of 1.329×1014 cmHz1/2W-1 and specially a faster response time of 3-4 ms at room temperature, attributed to the modulation of photogating effect induced by S-vacancy passivation in MoS2 by the S treatment. Therefore, the combination of the defect engineering on MoS2 and the NC effect from ferroelectric thin film could provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.

Write Disturb-Free Ferroelectric FETs With Non-Accumulative Switching Dynamics

HfO2 based ferroelectric field-effect transistors (FeFETs) are promising for low-power and high-speed non-volatile memories. However, FeFETs can be susceptible to write disturbs due to their accumulative switching behavior, which limits their application in large scale memory arrays. Here we show that FeFETs with a thin HfO2 based ferroelectric and SiNx interfacial layer are immune to pulsed write disturbs under both <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\textit {dd}}$ </tex-math></inline-formula> /2 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\textit {dd}}$ </tex-math></inline-formula> /3 inhibit schemes, but not to continuous write disturb. This contrasts with previous reports which found no difference between pulsed and continuous write disturbs. Our findings suggest that accumulative polarization switching is not an intrinsic property of HfO2 based FeFETs, but might be related to charge trapping dynamics instead.

Trap Generation in Whole Gate Stacks of FeFET With TiN/Hf0.5Zr0.5O2/SiOx/Si (MFIS) Gate Structure During Endurance Fatigue

This work investigates trap generation in gate stacks of ferroelectric field-effect transistor (FeFET) with TiN/Hf0.5Zr0.5O2/SiOx/Si gate structure during endurance fatigue by using the low-frequency noise method. We find that the traps are generated not only at Hf0.5Zr0.5O2/SiOx interface but also in both the Hf0.5Zr0.5O2 and SiOx. Our work provides evidence of defect generation inside the ferroelectric layer of FeFET during cycling. Furthermore, the traps in the SiOx are more detrimental to endurance fatigue. And the trap generation in the SiOx is more important than its initial trap density for endurance fatigue. Our work is helpful in deeply understanding the endurance of FeFET.

Modulating multiple leakage current mechanisms in the [LaNiO3/Ba0.67Sr0.33TiO3]3 multilayer heterostructure thin films via dielectrics/electrode interface modifications

Ferroelectric (FE) multilayer heterostructure films have attracted significant attention due to their superior dielectric performance, which shows increasing opportunities in the application of energy storage capacitors or high-capacitance density systems. However, the leakage current density in FE multilayer heterostructure thin films is closely linked to the thickness of each single FE layer and the number of hetero-structure interfaces. In Pt/[LaNiO3/Ba0.67Sr0.33TiO3]3 (Pt/[LNO/BST]3) multilayer thin films, the dominant leakage current mechanism is Poole–Frenkel (PF) emission at room temperature. The space charge limited current (SCLC) and the co-dominated leakage current mechanism from the PF and Schottky (SC) emissions were observed under higher operating temperatures. After inserting an ultrathin SrTiO3 (STO) layer between the Pt/[LNO/BST]3 multilayer thin films and the top Au electrode, the SCLC region was replaced by an Ohmic region, and the SC emission was not detected in the temperature range of 288–368 K. Moreover, the calculated zero-field energy barriers ϕPF of PF emission are higher than the prepared multilayer thin films without the STO layer. Consequently, the leakage current density of Pt/[LNO/BST]3 thin films showed a significant decrease.