Ferroelectrics-Integrated Two-Dimensional Devices toward Next-Generation Electronics

Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

Independent Electrical Control of Spin and Valley Degrees in 2D Breathing Kagome Ta3I8 with Intrinsic Triferroicity

Independent electrical control of spin and valley degrees of freedom (DOFs) in 2D materials is difficult due to the coupling of spin and valley DOFs. Here we propose that spin-filter transport and valley polarization can be independently manipulated by an electric field in 2D breathing kagome Ta3I8 due to the possession of both triferroic (ferromagnetism, ferroelectric, and ferrovalley) and bipolar magnetic semiconducting characteristics. The spin-filter transport can be realized by applying a bias voltage without altering the semiconducting characteristic. The flip of valley polarization is fulfilled by switching the ferroelectric polarization with a gate voltage. Our results demonstrate the potential to control different DOFs independently by adjusting the direction of the electric field.

Out-of-Plane Ferroelectricity in Two-Dimensional 1T‴-MoS2 Above Room Temperature

Two-dimensional (2D) molybdenum disulfide (MoS2), one of the most extensively studied van der Waals (vdW) materials, is a significant candidate for electronic materials in the post-Moore era. MoS2 exhibits various phases, among which the 1T‴ phase possesses noncentrosymmetry. 1T‴-MoS2 was theoretically predicted to be ferroelectric a decade ago, but this has not been experimentally confirmed until now. Here, we have prepared high-purity 2D 1T‴-MoS2 crystals and experimentally confirmed the room-temperature out-of-plane ferroelectricity. The noncentrosymmetric crystal structure in 2D 1T‴-MoS2 was convinced by atomically resolved transmission electron microscopic imaging and second harmonic generation (SHG) measurements. Further, the ferroelectric polarization states in 2D 1T‴-MoS2 can be switched using piezoresponse force microscopy (PFM) and electrical gating in field-effect transistors (FETs). The ferroelectric-to-paraelectric transition temperature is measured to be about 350 K. Theoretical calculations have revealed that the ferroelectricity of 2D 1T‴-MoS2 originates from the intralayer charge transfer of S atoms within the layer. The discovery of intrinsic ferroelectricity in the 1T‴ phase of MoS2 further enriches the properties of this important vdW material, providing more possibilities for its application in the field of next-generation electronic devices.

The Investigation of Neuromimetic Dynamics in Ferroelectrics via In Situ TEM

The core task of neuromorphic devices is to effectively simulate the behavior of neurons and synapses. Based on the functionality of ferroelectric domains with the advantages of low power consumption and high-speed response, great progress has been made in realizing neuromimetic behaviors such as ferroelectric synaptic devices. However, the correlation between the ferroelectric domain dynamics and neuromimetic behavior remains unclear. Here, we reveal the correlation between domain/domain wall dynamics and neuromimetic behaviors from a microscopic perspective in real-time by using high temporal and spatial resolution in situ transmission electron microscopy. Furthermore, we propose utilizing ferroelectric microstructures for the simultaneous simulation of neuronal and synaptic plasticity, which is expected to improve the integration and performance of ferroelectric neuromorphic devices. We believe that this work to study neuromimetic behavior from the perspective of domain dynamics is instructive for the development of ferroelectric neuromorphic devices.

Emerging 2D Ferroelectric Devices for In-Sensor and In-Memory Computing

The quantity of sensor nodes within current computing systems is rapidly increasing in tandem with the sensing data. The presence of a bottleneck in data transmission between the sensors, computing, and memory units obstructs the system's efficiency and speed. To minimize the latency of data transmission between units, novel in-memory and in-sensor computing architectures are proposed as alternatives to the conventional von Neumann architecture, aiming for data-intensive sensing and computing applications. The integration of 2D materials and 2D ferroelectric materials has been expected to build these novel sensing and computing architectures due to the dangling-bond-free surface, ultra-fast polarization flipping, and ultra-low power consumption of the 2D ferroelectrics. Here, the recent progress of 2D ferroelectric devices for in-sensing and in-memory neuromorphic computing is reviewed. Experimental and theoretical progresses on 2D ferroelectric devices, including passive ferroelectrics-integrated 2D devices and active ferroelectrics-integrated 2D devices, are reviewed followed by the integration of perception, memory, and computing application. Notably, 2D ferroelectric devices have been used to simulate synaptic weights, neuronal model functions, and neural networks for image processing. As an emerging device configuration, 2D ferroelectric devices have the potential to expand into the sensor-memory and computing integration application field, leading to new possibilities for modern electronics.

Photo-Assisted Ferroelectric Domain Control for α-In2Se3 Artificial Synapses Inspired by Spontaneous Internal Electric Fields

α-In2Se3 semiconductor crystals realize artificial synapses by tuning in-plane and out-of-plane ferroelectricity with diverse avenues of electrical and optical pulses. While the electrically induced ferroelectricity of α-In2Se3 shows synaptic memory operation, the optically assisted synaptic plasticity in α-In2Se3 has also been preferred for polarization flipping enhancement. Here, the synaptic memory behavior of α-In2Se3 is demonstrated by applying electrical gate voltages under white light. As a result, the induced internal electric field is identified at a polarization flipped conductance channel in α-In2Se3/hexagonal boron nitride (hBN) heterostructure ferroelectric field effect transistors (FeFETs) under white light and discuss the contribution of this built-in electric field on synapse characterization. The biased dipoles in α-In2Se3 toward potentiation polarization direction by an enhanced internal built-in electric field under illumination of white light lead to improvement of linearity for long-term depression curves with proper electric spikes. Consequently, upon applying appropriate electric spikes to α-In2Se3/hBN FeFETs with illuminating white light, the recognition accuracy values significantly through the artificial learning simulation is elevated for discriminating hand-written digit number images.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection

A two-dimensional (2D) ferroelectric semiconductor, which is coupled with photosensitivity and room-temperature ferroelectricity, provides the possibility of coordinated conductance modulation by both electric field and light illumination and is promising for triggering the revolution of optoelectronics for monolithic multifunctional integration. Here, we report that semiconducting Sn2P2S6 crystals can be achieved in a 2D morphology using a chemical vapor transport approach with the assistant of space confinement and experimentally demonstrate the robust ferroelectricity in atomic-thin Sn2P2S6 nanosheet at room temperature. The intercorrelated programming of ferroelectric order along out-of-plane (OOP) and in-plane (IP) directions enables a tunable bulk photovoltaic (BPV) effect through multidirectional electrical control. By combining the capability of anisotropic in-plane optical absorption, a highly integrated Sn2P2S6 optoelectronic device vertically sandwiched with graphene electrodes yields the polarization-dependent open-circuit photovoltage with a dichroic ratio of 2.0 under 405 nm light illumination. The reintroduction of ferroelectric Sn2P2S6 to the 2D asymmetric semiconductor family provides possibilities to hardware implement of the self-powered polarization-sensitive photodetection and spotlights the promising applications for next-generation photovoltaic devices.

Ferroelectric 2D SnS2 Analog Synaptic FET

In this study, the development and characterization of 2D ferroelectric field-effect transistor (2D FeFET) devices are presented, utilizing nanoscale ferroelectric HfZrO2 (HZO) and 2D semiconductors. The fabricated device demonstrated multi-level data storage capabilities. It successfully emulated essential biological characteristics, including excitatory/inhibitory postsynaptic currents (EPSC/IPSC), Pair-Pulse Facilitation (PPF), and Spike-Timing Dependent Plasticity (STDP). Extensive endurance tests ensured robust stability (107 switching cycles, 105 s (extrapolated to 10 years)), excellent linearity, and high Gmax/Gmin ratio (>105), all of which are essential for realizing multi-level data states (>7-bit operation). Beyond mimicking synaptic functionalities, the device achieved a pattern recognition accuracy of ≈94% on the Modified National Institute of Standards and Technology (MNIST) handwritten dataset when incorporated into a neural network, demonstrating its potential as an effective component in neuromorphic systems. The successful implementation of the 2D FeFET device paves the way for the development of high-efficiency, ultralow-power neuromorphic hardware which is in sub-femtojoule (48 aJ/spike) and fast response (1 µs), which is 104 folds faster than human synapse (≈10 ms). The results of the research underline the potential of nanoscale ferroelectric and 2D materials in building the next generation of artificial intelligence technologies.

Emerging van der Waals Dielectrics of Inorganic Molecular Crystals for 2D Electronics

In the landscape of continuous downscaling metal-oxide-semiconductor field-effect transistors, two-dimensional (2D) semiconductors with atomic thinness emerge as promising channel materials for ultimate scaled devices. However, integrating compatible dielectrics with 2D semiconductors, particularly in a scalable way, remains a critical challenge that hinders the development of 2D devices. Recently, 2D inorganic molecular crystals (IMCs), which are free of dangling bonds and possess excellent dielectric properties and simplicity for scalable fabrication, have emerged as alternatives for gate dielectric integration in 2D devices. In this Perspective, we start with the introduction of structure and synthesis methods of IMCs and then discuss the explorations of using IMCs as the dielectrics, as well as some remaining relevant issues to be unraveled. Moreover, we look at the future opportunities of IMC dielectrics in 2D devices both for practical applications and fundamental research.

Nanotube ferroelectric tunnel junctions with an ultrahigh tunneling electroresistance ratio

Low-dimensional ferroelectric tunnel junctions are appealing for the realization of nanoscale nonvolatile memory devices due to their inherent advantages of device miniaturization. Those based on current mechanisms have limitations, including low tunneling electroresistance (TER) effects and complex heterostructures. Here, we introduce an entirely new TER mechanism to construct a nanotube ferroelectric tunnel junction with ferroelectric nanotubes as the tunneling region. When rolling a ferroelectric monolayer into a nanotube, due to the coexistence of its intrinsic ferroelectric polarization with the flexoelectric polarization induced by bending, a metal-insulator transition occurs depending on the radiative polarization states. For the pristine monolayer, its out-of-plane polarization is tunable by an in-plane electric field, and the conducting states of the ferroelectric nanotube can thus be tuned between metallic and insulating states via axial electric means. Using α-In2Se3 as an example, our first-principles density functional theory calculations and nonequilibrium Green's function formalism confirm the feasibility of the TER mechanism and indicate an ultrahigh TER ratio that exceeds 9.9 × 1010% of the proposed nanotube ferroelectric tunnel junctions. Our findings provide a promising approach based on simple homogeneous structures for high density ferroelectric microelectric devices with excellent ON/OFF performance.

Chemical Vapor Deposition Synthesis of Intrinsic High-Temperature Ferroelectric 2D CuCrSe2

Ultrathin 2D ferroelectrics with high Curie temperature are critical for multifunctional ferroelectric devices. However, the ferroelectric spontaneous polarization is consistently broken by the strong thermal fluctuations at high temperature, resulting in the rare discovery of high-temperature ferroelectricity in 2D materials. Here, a chemical vapor deposition method is reported to synthesize 2D CuCrSe2 nanosheets. The crystal structure is confirmed by scanning transmission electron microscopy characterization. The measured ferroelectric phase transition temperature of ultrathin CuCrSe2 is about ≈800 K. Significantly, the switchable ferroelectric polarization is observed in ≈5.2 nm nanosheet. Moreover, the in-plane and out-of-plane ferroelectric response are modulated by different maximum bias voltage. This work provides a new insight into the construction of 2D ferroelectrics with high Curie temperature.

Ferroelectric-Optoelectronic Hybrid System for Photodetection

Photodetectors (PDs), as functional devices based on photon-to-electron conversion, are an indispensable component for the next-generation Internet of Things system. The research of advanced and efficient PDs that meet the diverse demands is becoming a major task. Ferroelectric materials can develop a unique spontaneous polarization due to the symmetry-breaking of the unit cell, which is switchable under an external electric field. Ferroelectric polarization field has the intrinsic characteristics of non-volatilization and rewritability. Introducing ferroelectrics to effectively manipulate the band bending and carrier transport can be non-destructive and controllable in the ferroelectric-optoelectronic hybrid systems. Hence, ferroelectric integration offers a promising strategy for high-performance photoelectric detection. This paper reviews the fundamentals of optoelectronic and ferroelectric materials, and their interactions in hybrid photodetection systems. The first section introduces the characteristics and applications of typical optoelectronic and ferroelectric materials. Then, the interplay mechanisms, modulation effects, and typical device structures of ferroelectric-optoelectronic hybrid systems are discussed. Finally, in summary and perspective section, the progress of ferroelectrics integrated PDs is summed up and the challenges of ferroelectrics in the field of optoelectronics are considered.

Integrated In-Memory Sensor and Computing of Artificial Vision Based on Full-vdW Optoelectronic Ferroelectric Field-Effect Transistor

The development and application of artificial intelligence have led to the exploitation of low-power and compact intelligent information-processing systems integrated with sensing, memory, and neuromorphic computing functions. The 2D van der Waals (vdW) materials with abundant reservoirs for arbitrary stacking based on functions and enabling continued device downscaling offer an attractive alternative for continuously promoting artificial intelligence. In this study, full 2D SnS2 /h-BN/CuInP2 S6 (CIPS)-based ferroelectric field-effect transistors (Fe-FETs) and utilized light-induced ferroelectric polarization reversal to achieve excellent memory properties and multi-functional sensing-memory-computing vision simulations are designed. The device exhibits a high on/off current ratio of over 105 , long retention time (>104  s), stable cyclic endurance (>350 cycles), and 128 multilevel current states (7-bit). In addition, fundamental synaptic plasticity characteristics are emulated including paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP), long-term potentiation, and long-term depression. A ferroelectric optoelectronic reservoir computing system for the Modified National Institute of Standards and Technology (MNIST) handwritten digital recognition achieved a high accuracy of 93.62%. Furthermore, retina-like light adaptation and Pavlovian conditioning are successfully mimicked. These results provide a strategy for developing a multilevel memory and novel neuromorphic vision systems with integrated sensing-memory-processing.

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

High-performance van der Waals antiferroelectric CuCrP2S6-based memristors

Layered thio- and seleno-phosphate ferroelectrics, such as CuInP2S6, are promising building blocks for next-generation nonvolatile memory devices. However, because of the low Curie point, the CuInP2S6-based memory devices suffer from poor thermal stability (<42 °C). Here, exploiting the electric field-driven phase transition in the rarely studied antiferroelectric CuCrP2S6 crystals, we develop a nonvolatile memristor showing a sizable resistive-switching ratio of ~ 1000, high switching endurance up to 20,000 cycles, low cycle-to-cycle variation, and robust thermal stability up to 120 °C. The resistive switching is attributed to the ferroelectric polarization-modulated thermal emission accompanied by the Fowler-Nordheim tunneling across the interfaces. First-principles calculations reveal that the good device performances are associated with the exceptionally strong ferroelectric polarization in CuCrP2S6 crystal. Furthermore, the typical biological synaptic learning rules, such as long-term potentiation/depression and spike amplitude/spike time-dependent plasticity, are also demonstrated. The results highlight the great application potential of van der Waals antiferroelectrics in high-performance synaptic devices for neuromorphic computing.

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

Emerging reconfigurable devices are fast gaining popularity in the search for next-generation computing hardware, while ferroelectric engineering of the doping state in semiconductor materials has the potential to offer alternatives to traditional von-Neumann architecture. In this work, we combine these concepts and demonstrate the suitability of reconfigurable ferroelectric field-effect transistors (Re-FeFETs) for designing nonvolatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe2) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP2S6) layer. Controlling the state encoded in the program gate enables switching between p, n, and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 106 and hysteresis windows of up to 10 V width. The homojunction can change from Ohmic-like to diode behavior with a large rectification ratio of 104. When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy-harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with a long retention time exceeding 104 s. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key nonvolatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates high compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit promising potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.

Negative Capacitance Field Effect Transistors based on Van der Waals 2D Materials

Steep subthreshold swing (SS) is a decisive index for low energy consumption devices. However, the SS of conventional field effect transistors (FETs) has suffered from Boltzmann Tyranny, which limits the scaling of SS to sub-60 mV dec-1 at room temperature. Ferroelectric gate stack with negative capacitance (NC) is proved to reduce the SS effectively by the amplification of the gate voltage. With the application of 2D ferroelectric materials, the NC FETs can be further improved in performance and downscaled to a smaller dimension as well. This review introduces some related concepts for in-depth understanding of NC FETs, including the NC, internal gate voltage, SS, negative drain-induced barrier lowering, negative differential resistance, single-domain state, and multi-domain state. Meanwhile, this work summarizes the recent advances of the 2D NC FETs. Moreover, the electrical characteristics of some high-performance NC FETs are expressed as well. The factors which affect the performance of the 2D NC FETs are also presented in this paper. Finally, this work gives a brief summary and outlook for the 2D NC FETs.

p-Type Two-Dimensional Semiconductors: From Materials Preparation to Electronic Applications

Two-dimensional (2D) materials are regarded as promising candidates in many applications, including electronics and optoelectronics, because of their superior properties, including atomic-level thickness, tunable bandgaps, large specific surface area, and high carrier mobility. In order to bring 2D materials from the laboratory to industrialized applications, materials preparation is the first prerequisite. Compared to the n-type analogs, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in practical applications such as complementary logic circuits. So far, many efforts have been made in the preparation of p-type 2D semiconductors. In this review, we overview recent progresses achieved in the preparation of p-type 2D semiconductors and highlight some promising methods to realize their controllable preparation by following both the top-down and bottom-up strategies. Then, we summarize some significant application of p-type 2D semiconductors in electronic and optoelectronic devices and their superiorities. In end, we conclude the challenges existed in this field and propose the potential opportunities in aspects from the discovery of novel p-type 2D semiconductors, their controlled mass preparation, compatible engineering with silicon production line, high-κ dielectric materials, to integration and applications of p-type 2D semiconductors and their heterostructures in electronic and optoelectronic devices. Overall, we believe that this review will guide the design of preparation systems to fulfill the controllable growth of p-type 2D semiconductors with high quality and thus lay the foundations for their potential application in electronics and optoelectronics.

Voltage-Mode Ferroelectric Synapse for Neuromorphic Computing

Ferroelectric materials with a modulable polarization extent hold promise for exploring voltage-driven neuromorphic hardware, in which direct current flow can be minimized. Utilizing a single active layer of an insulating ferroelectric polymer, we developed a voltage-mode ferroelectric synapse that can continuously and reversibly update its states. The device states are straightforwardly manifested in the form of variable output voltage, enabling large-scale direct cascading of multiple ferroelectric synapses to build a deep physical neural network. Such a neural network based on potential superposition rather than current flow is analogous to the biological counterpart driven by action potentials in the brain. A high accuracy of over 97% for the simulation of handwritten digit recognition is achieved using the voltage-mode neural network. The controlled ferroelectric polarization, revealed by piezoresponse force microscopy, turns out to be responsible for the synaptic weight updates in the ferroelectric synapses. The present work demonstrates an alternative strategy for the design and construction of emerging artificial neural networks.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Two-Dimensional Layered Materials Meet Perovskite Oxides: A Combination for High-Performance Electronic Devices

As the Si-based transistors scale down to atomic dimensions, the basic principle of current electronics, which heavily relies on the tunable charge degree of freedom, faces increasing challenges to meet the future requirements of speed, switching energy, heat dissipation, and packing density as well as functionalities. Heterogeneous integration, where dissimilar layers of materials and functionalities are unrestrictedly stacked at an atomic scale, is appealing for next-generation electronics, such as multifunctional, neuromorphic, spintronic, and ultralow-power devices, because it unlocks technologically useful interfaces of distinct functionalities. Recently, the combination of functional perovskite oxides and two-dimensional layered materials (2DLMs) led to unexpected functionalities and enhanced device performance. In this paper, we review the recent progress of the heterogeneous integration of perovskite oxides and 2DLMs from the perspectives of fabrication and interfacial properties, electronic applications, and challenges as well as outlooks. In particular, we focus on three types of attractive applications, namely field-effect transistors, memory, and neuromorphic electronics. The van der Waals integration approach is extendible to other oxides and 2DLMs, leading to almost unlimited combinations of oxides and 2DLMs and contributing to future high-performance electronic and spintronic devices.

Photoferroelectric All-van-der-Waals Heterostructure for Multimode Neuromorphic Ferroelectric Transistors

Interface-driven effects in ferroelectric van der Waals (vdW) heterostructures provide fresh opportunities in the search for alternative device architectures toward overcoming the von Neumann bottleneck. However, their implementation is still in its infancy, mostly by electrical control. It is of utmost interest to develop strategies for additional optical and multistate control in the quest for novel neuromorphic architectures. Here, we demonstrate the electrical and optical control of the ferroelectric polarization states of ferroelectric field effect transistors (FeFET). The FeFETs, fully made of ReS₂/hBN/CuInP₂S₆ vdW materials, achieve an on/off ratio exceeding 10⁷, a hysteresis memory window up to 7 V wide, and multiple remanent states with a lifetime exceeding 10³ s. Moreover, the ferroelectric polarization of the CuInP₂S₆ (CIPS) layer can be controlled by photoexciting the vdW heterostructure. We perform wavelength-dependent studies, which allow for identifying two mechanisms at play in the optical control of the polarization: band-to-band photocarrier generation into the 2D semiconductor ReS₂ and photovoltaic voltage into the 2D ferroelectric CIPS. Finally, heterosynaptic plasticity is demonstrated by operating our FeFET in three different synaptic modes: electrically stimulated, optically stimulated, and optically assisted synapse. Key synaptic functionalities are emulated including electrical long-term plasticity, optoelectrical plasticity, optical potentiation, and spike rate-dependent plasticity. The simulated artificial neural networks demonstrate an excellent accuracy level of 91% close to ideal-model synapses. These results provide a fresh background for future research on photoferroelectric vdW systems and put ferroelectric vdW heterostructures on the roadmap for the next neuromorphic computing architectures.

Impact of annealing temperature on the remanent polarization and tunneling electro-resistance of ferroelectric Al-doped HfOx tunnel junction memory

The ferroelectric characteristics of a metal-ferroelectric-metal (MFM) ferroelectric tunneling junction (FTJ) capacitor device are investigated herein. The device consists of an aluminum-doped hafnium oxide (HAO) insulator sandwiched between tungsten (W) and titanium nitride (TiN) metal electrodes. Rapid thermal annealing (RTA) is performed for 20 s under a nitrogen atmosphere at temperatures of 750 °C, 800 °C, and 850 °C to find that ferroelectricity with a large remanent polarization (P_r) of 41.28 μC cm^-2 can be obtained at the optimum annealing temperature of 800 °C. The presence of ferroelectricity is confirmed by polarization-switching positive-up-negative-down (PUND) measurements and by the hysteric polarization-voltage (P-V) loop. All devices exhibit excellent reliability, with an endurance of up to ∼10⁶ cycles and long retention characteristics. In addition, the interfacial paraelectric capacitance (C_i) values of the three HAO FTJs are investigated via pulse-switching measurements. The results indicate that the HAO film annealed at 800 °C for 20 s exhibits an excellent tunneling electro-resistance (TER) ratio of 186% and this is attributed to the extra paraelectric layer formed between the ferroelectric layer and the bottom electrode. The detailed findings of this study are expected to assist in the development of hafnium oxide-based ferroelectric non-volatile memory applications.