The Integration of Two-Dimensional Materials and Ferroelectrics for Device Applications

In recent years, there has been growing interest in functional devices based on two-dimensional (2D) materials, which possess exotic physical properties. With an ultrathin thickness, the optoelectrical and electrical properties of 2D materials can be effectively tuned by an external field, which has stimulated considerable scientific activities. Ferroelectric fields with a nonvolatile and electrically switchable feature have exhibited enormous potential in controlling the electronic and optoelectronic properties of 2D materials, leading to an extremely fertile area of research. Here, we review the 2D materials and relevant devices integrated with ferroelectricity. This review starts to introduce the background about the concerned themes, namely 2D materials and ferroelectrics, and then presents the fundamental mechanisms, tuning strategies, as well as recent progress of the ferroelectric effect on the optical and electrical properties of 2D materials. Subsequently, the latest developments of 2D material-based electronic and optoelectronic devices integrated with ferroelectricity are summarized. Finally, the future outlook and challenges of this exciting field are suggested.

Nonvolatile Memory Organic Light-Emitting Transistors

In the field of active-matrix organic light emitting display (AMOLED), large-size and ultra-high-definition AMOLED applications have escalated the demand for the integration density of driver chips. However, as Moore's Law approaches the limit, the traditional technology of improving integration density that relies on scaling down device dimension is facing a huge challenge. Thus, developing a multifunctional and highly integrated device is a promising route for improving the integration density of pixel circuits. Here, a novel nonvolatile memory ferroelectric organic light-emitting transistor (Fe-OLET) device which integrates the switching capability, light-emitting capability and nonvolatile memory function into a single device is reported. The nonvolatile memory function of Fe-OLET is achieved through the remnant polarization property of ferroelectric polymer, enabling the device to maintain light emission at zero gate bias. The reliable nonvolatile memory operations are also demonstrated. The proof-of-concept device optimized through interfacial modification approach exhibits 20 times improved field-effect mobility and five times increased luminance. The integration of nonvolatile memory, switching and light-emitting capabilities within Fe-OLET provides a promising internal-storage-driving paradigm, thus creating a new pathway for deploying storage capacitor-free circuitry to improve the pixel aperture ratio and the integration density of circuits toward the on-chip advanced display applications.

Inorganic Perovskite Quantum Dot-Mediated Photonic Multimodal Synapse

Artificial synapse is the basic unit of a neuromorphic computing system. However, there is a need to explore suitable synaptic devices for the emulation of synaptic dynamics. This study demonstrates a photonic multimodal synaptic device by implementing a perovskite quantum dot charge-trapping layer in the organic poly(3-hexylthiophene-2,5-diyl) (P3HT) channel transistor. The proposed device presents favorable band alignment that facilitates spatial separation of photogenerated charge carriers. The band alignment serves as the basis of optically induced charge trapping, which enables nonvolatile memory characteristics in the device. Furthermore, high photoresponse and excellent synaptic characteristics, such as short-term plasticity, long-term plasticity, excitatory postsynaptic current, and paired-pulse facilitation, are obtained through gate voltage regulation. Photosynaptic characteristics obtained from the device showed a multiwavelength response and a large dynamic range (∼10³) that is suitable for realizing a highly accurate artificial neural network. Moreover, the device showed nearly linear synaptic weight update characteristics with incremental depression electric gate pulse. The simulation based on the experimental data showed excellent pattern recognition accuracy (∼85%) after 120 epochs. The results of this study demonstrate the feasibility of the device as an optical synapse in the next-generation neuromorphic system.

Recent Advances in Ferroelectric-Enhanced Low-Dimensional Optoelectronic Devices

Ferroelectric (FE) materials, including BiFeO₃ , P(VDF-TrFE), and CuInP₂ S₆ , are a type of dielectric material with a unique, spontaneous electric polarization that can be reversed by applying an external electric field. The combination of FE and low-dimensional materials produces synergies, sparking significant research interest in solar cells, photodetectors (PDs), nonvolatile memory, and so on. The fundamental aspects of FE materials, including the origin of FE polarization, extrinsic FE materials, and FE polarization quantification are first discussed. Next, the state-of-the-art of FE-based optoelectronic devices is focused. How FE materials affect the energy band of channel materials and how device structures influence PD performance are also summarized. Finally, the future directions of this rapidly growing field are discussed.

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

Ferroelectric materials play an important role in a wide spectrum of semiconductor technologies and device applications. Two-dimensional (2D) van der Waals (vdW) ferroelectrics with surface-insensitive ferroelectricity that is significantly different from their traditional bulk counterparts have further inspired intensive interest. Integration of ferroelectrics into 2D-layered-material-based devices is expected to offer intriguing working principles and add desired functionalities for next-generation electronics. Herein, fundamental properties of ferroelectric materials that are compatible with 2D devices are introduced, followed by a critical review of recent advances on the integration of ferroelectrics into 2D devices. Representative device architectures and corresponding working mechanisms are discussed, such as ferroelectrics/2D semiconductor heterostructures, 2D ferroelectric tunnel junctions, and 2D ferroelectric diodes. By leveraging the favorable properties of ferroelectrics, a variety of functional 2D devices including ferroelectric-gated negative capacitance field-effect transistors, programmable devices, nonvolatile memories, and neuromorphic devices are highlighted, where the application of 2D vdW ferroelectrics is particularly emphasized. This review provides a comprehensive understanding of ferroelectrics-integrated 2D devices and discusses the challenges of applying them into commercial electronic circuits.

High-Speed Switching and Giant Electroresistance in an Epitaxial Hf0.5Zr0.5O2-Based Ferroelectric Tunnel Junction Memristor

HfO₂-based ferroelectric materials are good candidates for constructing next-generation nonvolatile memories and high-performance electronic synapses and have attracted extensive attention from both academia and industry. Here, a Hf_0.5Zr_0.5O₂-based ferroelectric tunnel junction (FTJ) memristor is successfully fabricated by epitaxially growing a Hf_0.5Zr_0.5O₂ film on a 0.7 wt % Nb-doped SrTiO₃ (001) substrate with a buffer layer of La_2/3Sr_1/3MnO₃ (∼1 u.c.). The FTJ shows a high switching speed of 20 ns, a giant electroresistance ratio of ∼834, and multiple states (eight states or three bits) with good retention >10⁴ s. As a solid synaptic device, tunable synapse functions have also been obtained, including long-term potentiation, long-term depression, and spike-timing-dependent plasticity. These results highlight the promising applications of Hf_0.5Zr_0.5O₂-based FTJ in ultrafast-speed and high-density nonvolatile memories and artificial synapses.

Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications

Nanostructured In₂Se₃ compounds have been widely used in electronics, optoelectronics, and thermoelectrics. Recently, the revelation of ferroelectricity in low-dimensional (low-D) In₂Se₃ has caused a new upsurge of scientific interest in nanostructured In₂Se₃ and advanced functional devices. The ferroelectric, thermoelectric, and optoelectronic properties of In₂Se₃ are highly correlated with the crystal structure. In this review, we summarize the crystal structures and electronic band structures of the widely interested members of the In₂Se₃ compound family. Recent achievements in the preparation of low-D In₂Se₃ with controlled phases are discussed in detail. General principles for obtaining pure-phased In₂Se₃ nanostructures are described. The excellent ferroelectric, optoelectronic, and thermoelectric properties having been demonstrated using nanostructured and heterostructured In₂Se₃ with different phases are also summarized. Progress and challenges on the applications of In₂Se₃ nanostructures in nonvolatile memories, photodetectors, gas sensors, strain sensors, and photovoltaics are discussed in detail. In the last part of this review, perspectives on the challenges and opportunities in the preparation and applications of In₂Se₃ materials are presented.

Emerging Opportunities for 2D Semiconductor/Ferroelectric Transistor-Structure Devices

Semiconductor technology, which is rapidly evolving, is poised to enter a new era for which revolutionary innovations are needed to address fundamental limitations on material and working principle level. 2D semiconductors inherently holding novel properties at the atomic limit show great promise to tackle challenges imposed by traditional bulk semiconductor materials. Synergistic combination of 2D semiconductors with functional ferroelectrics further offers new working principles, and is expected to deliver massively enhanced device performance for existing complementary metal-oxide-semiconductor (CMOS) technologies and add unprecedented applications for next-generation electronics. Herein, recent demonstrations of novel device concepts based on 2D semiconductor/ferroelectric heterostructures are critically reviewed covering their working mechanisms, device construction, applications, and challenges. In particular, emerging opportunities of CMOS-process-compatible 2D semiconductor/ferroelectric transistor structure devices for the development of a rich variety of applications are discussed, including beyond-Boltzmann transistors, nonvolatile memories, neuromorphic devices, and reconfigurable nanodevices such as p-n homojunctions and self-powered photodetectors. It is concluded that 2D semiconductor/ferroelectric heterostructures, as an emergent heterogeneous platform, could drive many more exciting innovations for modern electronics, beyond the capability of ubiquitous silicon systems.

The Role of Ferroelectric Polarization in Resistive Memory Properties of Metal/Insulator/Semiconductor Tunnel Junctions: A Comparative Study

Recently, tunnel junction devices adopting semiconducting Nb:SrTiO₃ electrodes have attracted considerable attention for their potential applications in resistive data storage and neuromorphic computing. In this work, we report on a comparative study of Pt/insulator/Nb:SrTiO₃ tunnel junctions between ferroelectric BaTiO₃ and nonferroelectric SrTiO₃ and LaAlO₃ barriers to reveal the role of polarization in resistance switching properties. Although hysteretic behaviors appear in current-voltage measurements of all devices regardless of the barrier character, significantly improved current ratios by more than three orders of magnitude are observed in the Pt/BaTiO₃/Nb:SrTiO₃ tunnel junctions due to the dominance of polarization in modulation of junction barrier profiles between the low and high resistance states. The switchable polarization also gives rise to enhanced resistance retention since the electron diffusion that smears the barrier contrast of the bistable resistance states is suppressed by the polar BaTiO₃/Nb:SrTiO₃ interface associated with the ferroelectric bound charges. These polarization-induced effects are absent in the nonferroelectric Pt/SrTiO₃/Nb:SrTiO₃ and Pt/LaAlO₃/Nb:SrTiO₃ devices in which serious resistance state decay, described by Fick's second law, is observed since there are no switchable interface charges on SrTiO₃/Nb:SrTiO₃ and LaAlO₃/Nb:SrTiO₃ to block the electron diffusion. In addition, the Pt/BaTiO₃/Nb:SrTiO₃ device also exhibits an excellent switching endurance up to ∼4.0 × 10⁶ bipolar cycles. These enhancements indicate the importance of ferroelectric polarization for achieving high-performance resistance switching and suggest that metal/ferroelectric/Nb:SrTiO₃ tunnel junctions are promising candidates for nonvolatile memory applications.

Ultralow Voltage Manipulation of Ferromagnetism

Spintronic elements based on spin transfer torque have emerged with potential for on-chip memory, but they suffer from large energy dissipation due to the large current densities required. In contrast, an electric-field-driven magneto-electric storage element can operate with capacitive displacement charge and potentially reach 1-10 µJ cm^-2 switching operation. Here, magneto-electric switching of a magnetoresistive element is shown, operating at or below 200 mV, with a pathway to get down to 100 mV. A combination of phase detuning is utilized via isovalent La substitution and thickness scaling in multiferroic BiFeO₃ to scale the switching energy density to ≈10 µJ cm^-2 . This work provides a template to achieve attojoule-class nonvolatile memories.

Organic Transistor Nonvolatile Memory with Three-Level Information Storage and Optical Detection Functions

By the current processing technology, it is a challenge to obtain ultrahigh-density information storage in the conventional binary floating-gate-based organic field-effect transistor (FG-OFET) nonvolatile memories (NVMs). To develop a multilevel memory in one cell is a feasible solution. In this work, we demonstrate FG-OFET NVMs with an integrated polymer floating-gate/tunneling (I-FG/T) layer consisting of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and polystyrene. The photoelectric effect of organic/polymer semiconductors is used to improve the controllability of the polarity and the number of the charges stored in the floating-gate. The FG-OFET NVMs integrate light sensitivity and nonvolatile information storage functions. By selecting suitable optical and electrical programming/erasing conditions, three-level information storage states, corresponding to electron storage, approximate neutrality, and hole storage in the floating-gate, are achieved and freely switched to each other. The memory mechanism and the dependence of the memory performances on the F8BT contents in I-FG/T layers are investigated. As a result, good memory performances, with mobility larger than 1.0 cm² V^-1 s^-1, reliable three-level switching endurance over 100 cycles, and stable three-level retention capability over 20 000 s, are achieved in our memory. Furthermore, an imaging system with a nonvolatile information storage function is demonstrated in a 16 × 5 array of FG-OFET NVMs.

Nonvolatile Memories Based on Graphene and Related 2D Materials

The pervasiveness of information technologies is generating an impressive amount of data, which need to be accessed very quickly. Nonvolatile memories (NVMs) are making inroads into high-capacity storage to replace hard disk drives, fuelling the expansion of the global storage memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory cell layers, innovative device concepts, and novel materials are being investigated. In this context, emerging 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous, offer a host of physical and chemical properties, which could both improve existing memory technologies and enable the next generation of low-cost, flexible, and wearable storage devices. Herein, an overview of graphene and related 2D materials (GRMs) in different types of NVM cells is provided, including resistive random-access, flash, magnetic and phase-change memories. The physical and chemical mechanisms underlying the switching of GRM-based memory devices studied in the last decade are discussed. Although at this stage most of the proof-of-concept devices investigated do not compete with state-of-the-art devices, a number of promising technological advancements have emerged. Here, the most relevant material properties and device structures are analyzed, emphasizing opportunities and challenges toward the realization of practical NVM devices.

Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li_x CoO₂ layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.

Single Atomic Layer Ferroelectric on Silicon

A single atomic layer of ZrO₂ exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance-voltage measurements of a ZrO₂ gate stack demonstrate that a reversible polarization of the ZrO₂ interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states. This monolayer ferroelectric represents a new class of materials for achieving devices that transcend conventional complementary metal oxide semiconductor (CMOS) technology. Significantly, a single atomic layer ferroelectric allows for more aggressively scaled devices than bulk ferroelectrics, which currently need to be thicker than 5-10 nm to exhibit significant hysteretic behavior (Park, et al. Adv. Mater. 2015, 27, 1811).

High-Performance Single-Active-Layer Memristor Based on an Ultrananocrystalline Oxygen-Deficient TiOx Film

The theoretical and practical realization of memristive devices has been hailed as the next step for nonvolatile memories, low-power remote sensing, and adaptive intelligent prototypes for neuromorphic and biological systems. However, the active materials of currently available memristors need to undergo an often destructive high-bias electroforming process in order to activate resistive switching. This limits their device performance in switching speed, endurance/retention, and power consumption upon high-density integration, due to excessive Joule heating. By employing a nanocrystalline oxygen-deficient TiO_x switching matrix to localize the electric field at discrete locations, it is possible to resolve the Joule heating problem by reducing the need for electroforming at high bias. With a Pt/TiO_x/Pt stacking architecture, our device follows an electric field driven, vacancy-modulated interface-type switching that is sensitive to the junction size. By scaling down the junction size, the SET voltage and output current can be reduced, and a SET voltage as low as +0.59 V can be obtained for a 5 × 5 μm² junction size. Along with its potentially fast switching (over 10⁵ cycles with a 100 μs voltage pulse) and high retention (over 10⁵ s) performance, memristors based on these disordered oxygen-deficient TiO_x films promise viable building blocks for next-generation nonvolatile memories and other logic circuit systems.

Flexible Nonvolatile Transistor Memory with Solution-Processed Transition Metal Dichalcogenides

Nonvolatile field-effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating-gate organic-FET memory with an all-in-one floating-gate/tunneling layer of the solution-processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS₂ ) is efficiently liquid-exfoliated by amine-terminated polystyrene with a controlled amount of MoS₂ nanosheets in an all-in-one floating-gate/tunneling layer, allowing for systematic investigation of concentration-dependent charge-trapping and detrapping properties of MoS₂ nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 10⁴ , a program/erase endurance cycle over 400 times, and data retention longer than 7 × 10³ s. All-in-one floating-gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically-flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer-bending events over 500 times at the bending radius of 4.0 mm.

A CMOS-compatible poly-Si nanowire device with hybrid sensor/memory characteristics for System-on-Chip applications

This paper reports a versatile nano-sensor technology using “top-down” poly-silicon nanowire field-effect transistors (FETs) in the conventional Complementary Metal-Oxide Semiconductor (CMOS)-compatible semiconductor process. The nanowire manufacturing technique reduced nanowire width scaling to 50 nm without use of extra lithography equipment, and exhibited superior device uniformity. These n type polysilicon nanowire FETs have positive pH sensitivity (100 mV/pH) and sensitive deoxyribonucleic acid (DNA) detection ability (100 pM) at normal system operation voltages. Specially designed oxide-nitride-oxide buried oxide nanowire realizes an electrically V_th-adjustable sensor to compensate device variation. These nanowire FETs also enable non-volatile memory application for a large and steady V_th adjustment window (>2 V Programming/Erasing window). The CMOS-compatible manufacturing technique of polysilicon nanowire FETs offers a possible solution for commercial System-on-Chip biosensor application, which enables portable physiology monitoring and in situ recording.