Ferroelectric Diodes Research Articles

Artificial photoelectric synaptic devices with ferroelectric diode effect for high-performance neuromorphic computing

With the rapid advancement of artificial intelligence and machine learning, the demand for neuromorphic computing systems has intensified. High-performance artificial synaptic devices are crucial to achieving this objective. Ferroelectric diode artificial synaptic devices were prepared using aluminum-doped BaTiO3 (BTAO) thin films by a low-cost sol-gel method. These devices mimic the basic properties of biological synapses, such as long-term plasticity (LTP) and short-term plasticity (STP), under electrical stimulation. Additionally, the devices exhibit an excellent UV light response, enabling the transition from STP to LTP by adjusting the light pulse intensity, width, and number of pulses. Aluminum doping significantly enhances the ferroelectricity of BaTiO3 films, increasing the Pr from 2.31 µC/cm² to 9.08 µC/cm². By modulating the Schottky barrier through polarization, the BTAO films exhibit a switchable diode effect, which facilitates fine-tuning of the synaptic connection strength while maintaining synaptic stability. Recognition accuracies of 97.32% for the MNIST dataset and 87.48% for the Fashion-MNIST dataset were achieved by convolutional neural network simulations. These results suggest new possibilities for brain-like processing with ferroelectric diodes.

Multistate Ferroelectric Diodes with High Electroresistance Based on van der Waals Heterostructures.

Some van der Waals (vdW) materials exhibit ferroelectricity, making them promising for novel nonvolatile memories (NVMs) such as ferroelectric diodes (FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been integrated with graphene for memory devices. Here we demonstrate FeDs with self-rectifying, hysteretic current-voltage characteristics based on vertical heterostructures of 10 nm thick CIPS and graphene. By using vdW indium-cobalt top electrodes and graphene bottom electrodes, we achieve a high electroresistance (on- and off-state resistance ratios) of ∼106, an on-state rectification ratio of 2500 for read/write voltages of 2 V/0.5 V, and a maximum output current density of 100 A/cm2. These metrics compare favorably with state-of-the-art FeDs. Piezoresponse force microscopy measurements show that stabilization of intermediate net polarization states in CIPS leads to stable multibit data retention at room temperature. The combination of two-terminal design, multibit memory, and low-power operation in CIPS-based FeDs is potentially interesting for compute-in-memory and neuromorphic computing applications.

Two-Dimensional Ferroelectric Materials: Synthesis, Characterization and Applications

In recent years, the continuous advancements in microelectronics have driven the evolution of electronic devices towards miniaturization and integration. However, at the nanoscale level, surface and size effects become significant, imposing constraints on the use of conventional bulk ferroelectric materials in contemporary industry. As a result, in the field of materials research, two-dimensional (2D) ferroelectric materials with stable spontaneous polarization and minimal size effects have gained significant attention. These novel 2D ferroelectric materials have great potential for future nano-level ferroelectric applications, enabling high levels of device integration. To begin with, this paper divides 2D ferroelectric materials into two categories: intrinsic ferroelectrics and sliding ferroelectrics. It also discusses the features and current state of research on each of these categories. Next, typical 2D ferroelectric material preparation and characterization techniques are outlined. Additionally, 2D ferroelectric memory devices like ferroelectric diodes (FD), ferroelectric field effect transistors (FeFET), ferroelectric semiconductor field-effect transistors (FeSFET), and ferroelectric tunnel junctions (FTJ) are introduced. Finally, this review also presents expectations and potential challenges in the domain of 2D ferroelectric materials.

Multistate, Ultrathin, Back-End-of-Line-Compatible AlScN Ferroelectric Diodes.

The growth in data generation necessitates efficient data processing technologies to address the von Neumann bottleneck in conventional computer architecture. Memory-driven computing, which integrates nonvolatile memory (NVM) devices in a 3D stack, is gaining attention, with CMOS back-end-of-line (BEOL)-compatible ferroelectric (FE) diodes being ideal due to their two-terminal design and inherently selector-free nature, facilitating high-density crossbar arrays. Here, we demonstrate BEOL-compatible, high-performance FE diodes scaled to 5, 10, and 20 nm FE Al0.72Sc0.28N/Al0.64Sc0.36N films. Through interlayer (IL) engineering, we show substantial improvements in the on/off ratios (>166 times) and rectification ratios (>176 times) in these scaled devices. These characteristics also enable 5-bit multistate operation with a stable retention. We also experimentally and theoretically demonstrate the counterintuitive result that the inclusion of an IL can lead to a decrease in the ferroelectric switching voltage of the device. An in-depth analysis into the device transport mechanisms is performed, and our compact model aligns seamlessly with the experimental results. Our results suggest the possibility of using scaled AlxSc1-xN FE diodes for high-performance, low-power, embedded NVM.

Resistance Switching and Carrier Transport Mechanisms of HfO2-Based Ferroelectric Diode

Resistance Switching and Carrier Transport Mechanisms of HfO₂-Based Ferroelectric Diode

A pure pyrochlore phase ferroelectric thin film diode for optoelectric artificial synapse

In this work, Au/Bi2Ti2O7/ITO ferroelectric diodes based on Schottky barrier were fabricated on ITO substrates by the sol-gel method. The device has a simple structure and good diode performance. Under UV light irradiation at 23.5 mW/cm2, the device exhibits an open-circuit voltage of 0.31 V and a short-circuit current of −0.0425 μA. It shows that the device has good optoelectronic properties. Au/Bi2Ti2O7/ITO ferroelectric diodes also exhibited behaviors similar to biological synapses, including long-term plasticity, short-term plasticity, paired-pulse facilitation, and spike-timing-dependent plasticity when stimulated by pulses. We also build convolutional neural networks and use the MNIST dataset for handwritten digit recognition.

Domain-modified engineering for low-power resistive switching in ferroelectric diodes

Neuromorphic devices based on ferroelectric resistive switching (RS) effects are promising to simulate the information recognition and memory of the human brain. However, the high power of RS elements in crossbar arrays is still an issue, limiting the neuromorphic applications. Here, we propose a domain-modified engineering for low-power RS in ferroelectric diodes by locally introducing relaxor ferroelectric units to lower domain switching barriers. A low-power RS of ∼ 70 μW, with large OFF/ON resistance ratio and high endurance, is achieved in Au/0.8BaTiO3-0.1Ba0.7Sr0.3TiO3-0.1BaTi0.7Zr0.3O3/Pt diodes, which is about 48.5% lower than that in Au/BaTiO3/Pt diodes. The interaction between macrodomains is depressed by domain modification engineering, lowering domain switching barriers, thereby operating voltage and power are significantly modulated. Meanwhile, good nonvolatility is obtained since the remanent polarization is partially maintained by the initial macrodomains and its decrease is slowed down by the relaxor units. This work provides a strategy to lower RS power by domain modification engineering for developing memristors and neuromorphic computing devices.

A ferroelectric fin diode for robust non-volatile memory

Among today’s nonvolatile memories, ferroelectric-based capacitors, tunnel junctions and field-effect transistors (FET) are already industrially integrated and/or intensively investigated to improve their performances. Concurrently, because of the tremendous development of artificial intelligence and big-data issues, there is an urgent need to realize high-density crossbar arrays, a prerequisite for the future of memories and emerging computing algorithms. Here, a two-terminal ferroelectric fin diode (FFD) in which a ferroelectric capacitor and a fin-like semiconductor channel are combined to share both top and bottom electrodes is designed. Such a device not only shows both digital and analog memory functionalities but is also robust and universal as it works using two very different ferroelectric materials. When compared to all current nonvolatile memories, it cumulatively demonstrates an endurance up to 1010 cycles, an ON/OFF ratio of ~102, a feature size of 30 nm, an operating energy of ~20 fJ and an operation speed of 100 ns. Beyond these superior performances, the simple two-terminal structure and their self-rectifying ratio of ~ 104 permit to consider them as new electronic building blocks for designing passive crossbar arrays which are crucial for the future in-memory computing.

An adjustable multistage resistance switching behavior of a photoelectric artificial synaptic device with a ferroelectric diode effect for neuromorphic computing.

Neuromorphic computing, which mimics biological neural networks, is widely regarded as the optimal solution for addressing the limitations of traditional von Neumann computing architecture. In this work, an adjustable multistage resistance switching ferroelectric Bi2FeCrO6 diode artificial synaptic device was fabricated using a sol-gel method with a simple process. The device exhibits nonlinearity in its electrical characteristics, demonstrating tunable multistage resistance switching behavior and a strong ferroelectric diode effect through the manipulation of ferroelectric polarization. One of its salient advantages resides in its capacity to dynamically regulate its polarization state in response to an external electric field, thereby facilitating the fine-tuning of synaptic connection strength while maintaining synaptic stability. The device is capable of accurately simulating the fundamental properties of biological synapses, including long/short-term plasticity, paired-pulse facilitation, and spike-timing-dependent plasticity. Additionally, the device exhibits a distinctive photoelectric response and is capable of inducing synaptic plasticity by light signal activation. The utilization of a femtosecond laser for the scrutiny of carrier transport mechanisms imparts profound insights into the intricate dynamics governing the optical memory effect. Furthermore, utilizing a convolutional neural network (CNN) architecture, the recognition accuracy of the MNIST and fashion MNIST datasets was improved to 95.6% and 78%, respectively, through the implementation of improved random adaptive algorithms. These findings present a new opportunity for utilizing Bi2FeCrO6 materials in the development of artificial synapses for neuromorphic computation.

Electronic Synapses Enabled by an Epitaxial SrTiO3‐δ / Hf0.5Zr0.5O2 Ferroelectric Field‐Effect Memristor Integrated on Silicon

AbstractSynapses play a vital role in information processing, learning, and memory formation in the brain. By emulating the behavior of biological synapses, electronic synaptic devices hold the promise of enabling high‐performance, energy‐efficient, and scalable neuromorphic computing. Ferroelectric memristive devices integrate the characteristics of both ferroelectric and memristive materials and present a far‐reaching potential as artificial synapses. Here, it is reported on a new ferroelectric device on silicon, a field‐effect memristor, consisting of an epitaxial ultrathin ferroelectric Hf0.5Zr0.5O2 film sandwiched between an epitaxial highly doped oxide semiconductor SrTiO3‐δ and a top metal. Upon a low voltage of less than 2 V, the field‐effect modulation in the semiconductor enables to access multiple states. The device works in a large time domain ranging from milliseconds down to tens of nanoseconds. By gradually switching the polarization by identical pulses, the ferroelectric diode devices can dynamically adjust the synaptic strength to mimic short‐ and long‐term memory plasticity. Ionic contributions due to redox processes in the oxide semiconductor beneficially influence the device operation and retention.

0D van der Waals interfacial ferroelectricity

The dimensional limit of ferroelectricity has been long explored. The critical contravention is that the downscaling of ferroelectricity leads to a loss of polarization. This work demonstrates a zero-dimensional ferroelectricity by the atomic sliding at the restrained van der Waals interface of crossed tungsten disufilde nanotubes. The developed zero-dimensional ferroelectric diode in this work presents not only non-volatile resistive memory, but also the programmable photovoltaic effect at the visible band. Benefiting from the intrinsic dimensional limitation, the zero-dimensional ferroelectric diode allows electrical operation at an ultra-low current. By breaking through the critical size of depolarization, this work demonstrates the ultimately downscaled interfacial ferroelectricity of zero-dimensional, and contributes to a branch of devices that integrates zero-dimensional ferroelectric memory, nano electro-mechanical system, and programmable photovoltaics in one.

Reversible charge injection-controlled resistance switching in BiFeO3 ferrodiodes

The ferroelectric diode effect is a promising candidate for resistive memory applications, but the precise role of defects in the current switching mechanism remains unclear. Here, we investigated ferroelectric SrRuO3/BiFeO3/SrRuO3 capacitors and observed strong diode current. The capacitors exhibited preferred polarization orientation toward the bottom electrode in the presence of an imprint field, as evidenced by poor polarization retention of upward polarizations at a bias voltage of 1 V. Interfacial defect-mediated charge injection and trapping enabled by negative voltage poling reduced the built-in field and improved the retention property at the expense of reduced diode current. This phenomenon can be reversed by long-time positive voltage poling, allowing the deeply trapped charges to be expelled out of the trap for the rejuvenation of the diode current. Our study provides experimental evidence that interfacial defects modify the diode current in a manner opposite to that of the switched polarization.

Transport mechanism in Hf0.5Zr0.5O2-based ferroelectric diodes

Transport mechanism in Hf0.5Zr0.5O2-based ferroelectric diodes

Ferroelectric Schottky diodes of CuInP2S6 nanosheet

Ferroelectricity in van der Waals (vdW) layered material has attracted a great deal of interest recently. CuInP2S6 (CIPS), the only vdW layered material whose ferroelectricity in the bulk was demonstrated by direct polarization measurements, was shown to remain ferroelectric down to a thickness of a few nanometers. However, its ferroelectric properties have just started to be explored in the context of potential device applications. We report here the preparation and measurements of metal-ferroelectric semiconductor-metal heterostructures using nanosheets of CIPS obtained by mechanical exfoliation. Four bias voltage and polarization dependent resistive states were observed in the current–voltage characteristics, which we attribute to the formation of ferroelectric Schottky diode, along with switching behavior.

Schottky Barrier Control of Self-Polarization for a Colossal Ferroelectric Resistive Switching.

Controlling the domain evolution is critical both for optimizing ferroelectric properties and for designing functional electronic devices. Here we report an approach of using the Schottky barrier formed at the metal/ferroelectric interface to tailor the self-polarization states of a model ferroelectric thin film heterostructure system SrRuO3/(Bi,Sm)FeO3. Upon complementary investigations of the piezoresponse force microscopy, electric transport measurements, X-ray photoelectron/absorption spectra, and theoretical studies, we demonstrate that Sm doping changes the concentration and spatial distribution of oxygen vacancies with the tunable host Fermi level which modulates the SrRuO3/(Bi,Sm)FeO3 Schottky barrier and the depolarization field, leading to the evolution of the system from a single domain of downward polarization to polydomain states. Accompanied by such modulation on self-polarization, we further tailor the symmetry of the resistive switching behaviors and achieve a colossal on/off ratio of ∼1.1 × 106 in the corresponding SrRuO3/BiFeO3/Pt ferroelectric diodes (FDs). In addition, the present FD also exhibits a fast operation speed of ∼30 ns with a potential for sub-nanosecond and an ultralow writing current density of ∼132 A/cm2. Our studies provide a way for engineering self-polarization and reveal its strong link to the device performance, facilitating FDs as a competitive memristor candidate used for neuromorphic computing.

All-ferroelectric implementation of reservoir computing

Reservoir computing (RC) offers efficient temporal information processing with low training cost. All-ferroelectric implementation of RC is appealing because it can fully exploit the merits of ferroelectric memristors (e.g., good controllability); however, this has been undemonstrated due to the challenge of developing ferroelectric memristors with distinctly different switching characteristics specific to the reservoir and readout network. Here, we experimentally demonstrate an all-ferroelectric RC system whose reservoir and readout network are implemented with volatile and nonvolatile ferroelectric diodes (FDs), respectively. The volatile and nonvolatile FDs are derived from the same Pt/BiFeO3/SrRuO3 structure via the manipulation of an imprint field (Eimp). It is shown that the volatile FD with Eimp exhibits short-term memory and nonlinearity while the nonvolatile FD with negligible Eimp displays long-term potentiation/depression, fulfilling the functional requirements of the reservoir and readout network, respectively. Hence, the all-ferroelectric RC system is competent for handling various temporal tasks. In particular, it achieves an ultralow normalized root mean square error of 0.017 in the Hénon map time-series prediction. Besides, both the volatile and nonvolatile FDs demonstrate long-term stability in ambient air, high endurance, and low power consumption, promising the all-ferroelectric RC system as a reliable and low-power neuromorphic hardware for temporal information processing.

Tuning Local Conductance to Enable Demonstrator Ferroelectric Domain Wall Diodes and Logic Gates

AbstractFundamentally, lithium niobate is a good electrical insulator. However, this can change dramatically when 180° domain walls are present, as they are often found to be strongly conducting. Conductivities depend on the inclination angles of walls with respect to the polarization axis and so, if these angles can be altered, then electrical conduction can be tuned, or toggled on and off. In ≈500 nm thick z‐cut ion‐sliced thin films, localized wall angle variations can be controlled by both the sense and magnitude of applied electrical bias. It is shown that this results in diode‐like behaviour, allowing half‐wave rectification at modest frequencies. Importantly, it is experimentally demonstrated that these domain wall diodes can be used to construct “AND” and inclusive “OR” logic gates, where “0” and “1” output states are clearly distinguishable. Extrapolation to more complex arrangements shows that output states can still be distinguished in two‐level cascade logic. Insights show that simple logic circuits can be realized by localized manipulation of domain wall conductivity. Our research complements that by Jie Sun et al. (Adv. Funct. Mater. 2207418 (2022)), where NOT, NOR, and NAND gates are realized by moving conducting domain walls to make or break electrical contacts.

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.

Diode-Like Behavior Based on Conductive Domain Wall in LiNbO Ferroelectric Single-Crystal Thin Film

The ferroelectric diode is a promising candidate for memory, rectifier system, and logic devices. Conductive domain wall (CDW) is essential to ferroelectric diodes. A permanent CDW-based unipolar two-terminal ferroelectric diode-like cell with enhanced conductivity is demonstrated here. Through poling engineering on LiNbO3 ferroelectric single-crystal thin film, diode-like cells with much smaller size than conventional diode is created in significantly efficient way. Throughout 5880 h, the CDW has maintained consistent operation around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{A}$ </tex-math></inline-formula> rather than pA. The ferroelectric diode-like cell is fatigue-resistant for 500 cycles and retains its conductivity for 2400 h. In addition, the conductivity of a diode-like cell has increased from 0.2 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16 ~\mu \text{A}$ </tex-math></inline-formula> via charge injection and parallel connection of multiple cells. Finally, the “OR” logic gate is assembled using the cells. These ferroelectric cells can be used as a diode-like component in large-scale integrated circuits and have potential applications in nanoelectronic logic devices.

Ultradense One-Memristor Ternary-Content-Addressable Memory Based on Ferroelectric Diodes

In this Letter, for the first time, one-memristor (1M)-based ternary-content-addressable memory (TCAM) with an ultradense 4F2 cell area is proposed on a single reconfigurable TiN/hafnium zirconium oxide (HZO)/indium gallium zinc oxide (IGZO)/TiN ferroelectric (FE) diode. By modulating the FE-polarization-controlled Schottky junction that exists at the interface, reconfigured P–N, N–P, and ohmic like junctions of the FE diodes were designed for ‘0’, ‘1’, and ‘X’ storage states in the TCAM cell, respectively. In addition to non-volatile FE polarization for data storage, ambipolar-like behavior induced by the symmetrical junction characteristics was obtained on the diodes for searching queries. Using both non-volatile and ambipolar-like characteristics, typical TCAM functions with a maximum driving on/off ratio of ~500 were confirmed experimentally on a single FE diode, indicating the great potential of this memory device in area-efficient artificial intelligence processors.