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Elemental tellurium electrical switch, relying on a transient crystal–liquid–crystal phase transition, has recently been proposed as a promising selector candidate for the next‐generation 3D high‐density memory, bridging performance gap in today's computer. Further miniaturization of the switch cell to increase memory density strongly depends on the scalability of the tellurium film, which, however, has not been experimentally studied. Herein, the tellurium films are prepared with the thickness downscaled from 400 to 2 nm and a significant increase is found in the bandgap from 0.29 to 0.91 eV, as predicted by ab initio molecular dynamics. Interestingly, the as‐deposited tellurium films with a thickness above 3 nm are in the crystalline trigonal phase, whereas 2 nm thick film suddenly becomes amorphous, observed by both Raman and transmission electron microscopies. In this finding, since the leakage current of the elemental tellurium switch is determined by both the Schottky barrier between tellurium/electrode interface and the bandgap of the tellurium film, a reduction in leakage current is predicted with further miniaturization. [ABSTRACT FROM AUTHOR]

Chalcogenide selector devices are key components of the high-density cross-point memory/memristor arrays. However, their threshold switching mechanism remains elusive. Apart from the commonly cited purely electronic models, the filamentary models have recently been supported by experiment. In the context of filamentary switching, we study the native filament:dielectric interfaces in typical selector materials, namely, GeSe and SiTe. Local crystallization induced filaments and electrochemically induced filaments are considered. The filament:dielectric contacts are found to have low n-type and p-type Schottky barriers for GeSe and SiTe, respectively. The filamentary mechanism and purely electronic mechanism can be synergetic: thermal excitation of carriers from the filament to the host dielectric can induce electronic and structural changes of the host dielectric, which may in turn facilitate the growth of filament. This work promotes the understanding of filamentary-type switching in chalcogenide selector devices. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) materials-based tunneling field-effect transistors (TFETs) suffer from low driving currents. In contrast to the prevailing wisdom that defects are detrimental, we proposed to harness the ubiquitous defects in MoS2 to overcome the problem of the low on-state current in TFET. The existence of certain molybdenum-related vacancies and sulfur vacancy in appropriate positions confers the higher driving currents without compromising the low-power benefits. Such performance enhancements are related to the defect-assisted resonant Zener tunneling mechanism introduced by the mid-gap states of the vacancy defects. These unveiled hidden defect benefits could provide new opportunities for boosting the performance of 2D TFETs. The defect-assisted resonant Zener tunneling mechanism in TFET introduced by the mid-gap states of the vacancies in MoS2 is beneficial for enhancing the on-state current. [ABSTRACT FROM AUTHOR]

Neuronal oscillation is critically involved in almost every cognitive task, including information coding, memory formation, and perception. This phenomenon is rooted in the spiking activity of neuron that is mediated by two principal ion‐channel mechanisms, i.e., fast Na+ mediated depolarization and delayed K+ mediated repolarization. In neuromorphic circuits, however, subcellular level of biocomplexity has routinely been traded off for smaller hardware overhead. Even so, the simplest artificial oscillatory neuron by far is still made of no less than two devices, a volatile memristor and a parallelly (serially) connected capacitor (resistor) that typically occupies relatively large circuit area. Herein, a simplest imaginable implementation is reported of the neuronal oscillation function in a single threshold switching device based on tellurium (Te). Thanks to the unique electrochemical‐thermal properties combined in Te, this device can exhibit self‐sustained oscillatory behavior when stimulated by a steady DC voltage as a result of the repetitive electrochemical growth and the ensuing Joule heat‐induced thermal melting of the Te filament, which phenomenologically capture the Na+ and K+ ion‐channel functions, respectively. This work broadens the neuromorphic applications of Te resistive switching devices from synaptic array devices to artificial neurons. [ABSTRACT FROM AUTHOR]

The complementary resistive switching (CRS) memristor has originally been proposed for use as the storage element or artificial synapse in large-scale crossbar array with the capability of solving the sneak path problem, but its usage has mainly been hampered by the inherent destructiveness of the read operation (switching ‘1’ state to ‘ON’ or ‘0’ state). Taking a different perspective on this ‘undesired’ property, we here report on the inherent behavioral similarity between the CRS memristor and a leaky integrate-and-fire (LIF) neuron which is another basic neural computing element, in addition to synapse. In particular, the mechanism behind the undesired read destructiveness for storage element and artificial synapse can be exploited to naturally realize the LIF and the ensuing spontaneous repolarization processes, followed by a refractory period. By means of this biological similarity, we demonstrate a Pt/Ta2O5âˆ' x /TaO y /Ta CRS memristor that can exhibit these neuronal behaviors and perform various fundamental neuronal operations, including additive/subtractive operations and coincidence detection. These results suggest that the CRS neuron, with its bio-interpretability, is a useful addition to the family of memristive neurons. [ABSTRACT FROM AUTHOR]

What precludes lowering the supply voltage and overall power consumption of the transistor is the infamous Boltzmann tyranny, a fundamental thermionic limit preventing the subthreshold slope (SS) of the transistor from being lower than 60 mV dec−1 at room temperature (RT). Internal voltage amplification through the use of an additional threshold switch (TS) serially connected to the transistor channel has been shown to be highly effective to break the Boltzmann barrier. This approach, however, is typically heterogeneous by employing TS materials that are foreign to the transistor. Recently, semiconducting tellurium (Te) has gained considerable research interest for its use as an emerging low-dimensional p-type channel material with high hole mobility, air-stability, and the ability to be processed at near ambient temperature. In this work, we unveil a hidden advantage of Te for a steep-slope transistor in which Te serves as the channel material and the TS material at the same time. In a homogeneous and compact device architecture, RT-SS lower than 30 mV dec−1 can be achieved. This is enabled by a combination of unique electrical–electrochemical–thermal properties in Te. This work augments the functionality of Te and provides a low-power transistor solution. [ABSTRACT FROM AUTHOR]

The success of lithium–sulfur batteries will reduce the expected Co, Ni resource challenges from the wide adoption of lithium‐ion batteries. Unfortunately, the shuttle effect of soluble polysulfides brings many problems. Anchoring or blocking polysulfides on the cathode side using functional separators is the dominant strategy for addressing this. However, the blocked polysulfides gradually aggregate on the separator to form the so‐called "dead sulfur" and withdraw from cycling. Herein, a multipurpose separator is proposed that enables catalytic activation of the blocked polysulfides to prevent the formation of "dead sulfur", and contribute to capacity. The multifunctionality is supported by montmorillonite (MMT) that provides sufficient channels for lithium‐ion transport, and selenium‐doped sulfurized‐polyacrylonitrile (Se0.06SPAN) that catalyzes conversion of "dead sulfur" and simultaneously contributes capacity. The theoretical calculations reveal Se0.06SPAN/MMT has a low migration barrier for Li+ and a low decomposition barrier for Li2S, facilitating the conversion and minimizing "dead sulfur". Consequently, the Li–S battery with the Se0.06SPAN/MMT@PP (polypropylene) separator shows a low fading rate of 0.034% during 1000 cycles and achieves a super‐high areal capacity (33.07 mAh cm–2) under high sulfur loading (26.75 mg cm–2) and lean electrolyte conditions (4.5 µL mg–1). Moreover, the multipurpose separator has encouraging performance in stability, flexibility, and sustainability. [ABSTRACT FROM AUTHOR]

Chalcogenide superlattice (CSL) is one of the emerging material technologies for ultralow-power phase change memories. However, the resistance switching mechanism of the CSL-based device is still hotly debated. Early electrical measurements and recent materials characterizations have suggested that the Kooi-phase CSL is very likely to be the as-fabricated low-resistance state. Due to the difficulty in in situ characterization at atomic resolution, the structure of the electrically switched CSL in its high-resistance state is still unknown and mainly investigated by theoretical modelings. So far, there has been no simple model that can unify experimental results obtained from device-level electrical measurements and atomic-level materials characterizations. In this work, we carry out atomistic transport modelings of the CSL-based device and propose a simple mechanism accounting for its high resistance. The modeled high-resistance state is based on the interfacial SbTe bilayer flipped CSL that has previously been mistaken for the low-resistance state. This work advances the understanding of CSL for emerging memory applications. [ABSTRACT FROM AUTHOR]

Atomic-scale understanding of HfO2 ferroelectricity is important to help address many challenges in developing reliable and high-performance ferroelectric HfO2 (fe-HfO2) based devices. Though investigated from different angles, a factor that is real device-relevant and clearly deserves more attention has largely been overlooked by previous research, namely, the fe-HfO2/dielectric interface. Here, we investigate the electronic structures of several typical interfaces formed between ultrathin fe-HfO2 and oxide dielectrics in the sub-3-nm region. We find that interface formation introduces strong depolarizing fields in fe-HfO2, which is detrimental for ferroelectric polarization but can be a merit if tamed for tunneling devices, as recently demonstrated. Asymmetric oxygen distribution-induced polarity, intertwined with ferroelectric polarization or not, is also investigated as a relevant interfacial effect in the real device. Though considered detrimental from certain aspects, such as inducing build-in field (independent of ferroelectric polarization) and exacerbating depolarization (intertwined with ferroelectric polarization), it can be partly balanced out by other effects, such as annealing (extrinsic) and polarity-induced defect formation (intrinsic). This work provides insights into ferroelectric-HfO2/dielectric interfaces and some useful implications for the development of devices. [ABSTRACT FROM AUTHOR]

A two‐terminal selector is an essential element for high‐density 3D stackable memory arrays. Suppressing sneak current, the device also needs to provide extremely high current density to melt‐quench the storage layer in phase‐change memory (PCM). Recently, an ovonic threshold switching (OTS) selector based on amorphous GeS demonstrated a large current density, which seems to be promising for 3D stackable PCM application. Herein, the scalability of GeS OTS selectors is investigated as the device scales down from 200 to 60 nm. Interestingly, the ON/OFF current, threshold voltage, and switching speed hardly change as the device scales down, whereas the ON current density exponentially grows. A large current density, ≈35.4 MA cm−2, is achieved in 60 nm‐sized devices. The high current density endows the GeS selector with great potential for use in high‐density 3D stackable memory applications. [ABSTRACT FROM AUTHOR]

The popularization of silicon transistor technology has arisen because of the high‐quality native oxide passivation of the silicon surface. The high‐k oxide solution to the continuous scaling of the devices is another technology milestone. In the new era of More Moore and More than Moore, 2D semiconductor alternatives as the channel materials are of considerable interest. Major 2D candidates, however, have not demonstrated thermally induced semiconductor/native oxide interfaces, analogous to Si:SiO2, for workable devices. Recently, a prototype 2D transistor based on a native oxide high‐k stack was showcased. The transistor is based on Bi2O2Se, an emerging layered 2D semiconductor, with its native high‐k oxide, Bi2O5Se, grown by thermal oxidation. This holds great promise for the development of 2D electronics. Here, comprehensive first‐principles investigations are carried out on the fundamental electrical properties of the native oxygen and selenium vacancies in Bi2O5Se, and two other less explored oxidized forms of Bi2O2Se, i.e., Bi2O9Se3 and Bi2O10Se3, which are found to have comparable bandgap values to that of Bi2O5Se. The corresponding vacancy defect landscapes are provided as an important guideline for the developmental applications of the Bi2O2Se:Bi2OxSey stack, as a 2D analogue of Si:SiO2. [ABSTRACT FROM AUTHOR]

"The interface is the device," a famous phrase coined by Nobel laureate Herbert Kroemer, has highlighted the importance of materials interfaces for the functionalities and performance of electronic devices. Semiconductor:dielectric interfaces play central roles in transistors: one of the main reasons of the success of silicon technology and the bottlenecks of many proposed alternatives, among which are the 2D transistors. Major 2D semiconductors, such as MoS2, WSe2, HfSe2, and ZrSe2, have not yet given rise to workable devices based on native high‐K stacks by thermal oxidation which is highly technologically desired. Recently, this feasibility has been demonstrated in Bi2O2Se, an emerging layered 2D material, with its native high‐K oxide, Bi2O5Se. Understanding the atomic‐scale properties of the Bi2O2Se:Bi2O5Se interface is crucial for the developmental applications of this emerging 2D analogue of Si:SiO2. Herein, the atomic structures and band alignment properties of various Bi2O2Se:Bi2O5Se interfaces with different configurations of the interfacial anion layers are comprehensively studied. The correlation between band offset and interfacial atom intermixing is observed and rationalized. The interfacial anion vacancies are also investigated and it is found that oxygen vacancies are detrimental. [ABSTRACT FROM AUTHOR]

PdSe2 is a unique layered two-dimensional (2D) material with pentagonal structural motif and anisotropic properties. In addition, its strong interlayer interaction leads to new 2D form of the exfoliated monolayer, that is, Pd2Se3. Despite the increasing interest in these emerging 2D materials, the landscape of the native point defects, as a fundamental materials property, has not been revealed. In this work, we systematically investigate different types of defects in mono- and bi-layer PdSe2 and monolayer Pd2Se3. In contrast to the common expectation, Se vacancy is not the readily formed defect. Instead, Se-excess defects, such as SePd antisite and Se interstitial, are more likely to form over a majority of the allowed range of the atomic chemical potentials. Se-deficiency defect, Pd interstitial, is able to form under the Se-poor condition in bilayer PdSe2. The defect-mediated interlayer fusion model in the formation of monolayer Pd2Se3 from bilayer PdSe2 is reformulated. These dominant defects are found to stay in the neutral charge state, partly explaining the ambipolar behavior of the PdSe2 transistors. Finally, the stacked and lateral contacts between these few-layer semiconductors and the native Pd17Se15 metal are also studied. All these interfaces show p-type contact properties. This work reveals the important materials properties of few-layer PdSe2 and Pd2Se3 for the better development of new functional devices. [ABSTRACT FROM AUTHOR]

Metal halide based perovskite materials have attracted tremendous interest in optoelectronics. Although conventional lead (Pb) based organic/inorganic halide perovskites have been extensively studied and have realized highly efficient devices like solar cells and light emitting diodes, they are plagued with the toxicity of the Pb element, restraining their applications in environmentally and biologically friendly scenarios. Substitutes for Pb ions have been suggested, yet most of the Pb-free perovskites suffer from low stability and inferior efficiencies compared to their Pb based counterparts. In this work, we explore the synthesis of cesium titanium halide perovskites (Cs2TiX6, X = Cl, Br). At room-temperature, aqueous solution-based processes are developed to grow thermally stable, high quality Cs2Ti(BrxCl1−x)6 (0 < x < 1) crystals and films in large volume. Their structural, thermal and optical properties are systematically studied in experiments and compared with first principles calculations. The produced materials demonstrate a tunable, quasi-direct band gap from ∼1.7 eV to ∼2.5 eV and photoluminescence peaked from ∼535 nm to ∼670 nm. Furthermore, the materials are stable in an ambient environment up to ∼500 °C. These Pb-free perovskite materials will create opportunities for novel optical and electronic devices used in eco-friendly and bio-friendly environments. [ABSTRACT FROM AUTHOR]

Understanding the nature of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials is crucial to guide the development of multi-level memories, selectors, and neuromorphic devices. Currently, however, the knowledge of the mid-gap defects is fragmented and the known structural and electrical characteristics of the defects cannot be unified in a single atomic model. In this work, a Ge–Ge chain defect model is built by artificial bond rearrangement in an amorphous GeTe network generated by atomic distortion, revealing a possible valence alternation mechanism in its formation. Atoms in the Ge–Ge chain are found to have a crystalline-like environment. The chain is found to have a negative-U property and to introduce mid-gap states. This model unifies the main known properties of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials. [ABSTRACT FROM AUTHOR]

Ovonic threshold switches are a favored choice for chalcogenide-based amorphous (a-) GeSex selector devices used in cross-point arrays of nonvolatile memories. Previous models of their nonlinear high-field conduction proposed a largely electronic-only switching mechanism, within a fixed density of electronic states. Here, we use a density functional molecular-dynamics supercell calculation to show that the high-current excited state configuration of a-GeSex has structural changes such as additional Ge-Ge bonds and overcoordinated Ge sites, giving lower effective mass, more delocalized conduction states, and a lower ON resistance. [ABSTRACT FROM AUTHOR]

There are two general approaches to developing artificial general intelligence (AGI)1: computer-science-oriented and neuroscience-oriented. Because of the fundamental differences in their formulations and coding schemes, these two approaches rely on distinct and incompatible platforms2–8, retarding the development of AGI. A general platform that could support the prevailing computer-science-based artificial neural networks as well as neuroscience-inspired models and algorithms is highly desirable. Here we present the Tianjic chip, which integrates the two approaches to provide a hybrid, synergistic platform. The Tianjic chip adopts a many-core architecture, reconfigurable building blocks and a streamlined dataflow with hybrid coding schemes, and can not only accommodate computer-science-based machine-learning algorithms, but also easily implement brain-inspired circuits and several coding schemes. Using just one chip, we demonstrate the simultaneous processing of versatile algorithms and models in an unmanned bicycle system, realizing real-time object detection, tracking, voice control, obstacle avoidance and balance control. Our study is expected to stimulate AGI development by paving the way to more generalized hardware platforms. The 'Tianjic' hybrid electronic chip combines neuroscience-oriented and computer-science-oriented approaches to artificial general intelligence, demonstrated by controlling an unmanned bicycle. [ABSTRACT FROM AUTHOR]

Memristor crossbar arrays have great potential in brain inspired computing and the next generation high-density memories. The sneak current issue, however, seriously degrades their performance with increasing array size. Selectors with volatile threshold switching (TS) behavior have become important components of the arrays to suppress this issue. Ag/Ta2O5/Pt diffusive devices have promising TS characteristics as selectors, including high ON/OFF ratio and low OFF current. However, their unidirectional TS excludes their application in arrays consisting of bipolar memristors. Bipolar memristors require voltage biases of different polarities to enable the device programming, thus selectors with bidirectional TS are essential for them. In this study, we realize reproducible bidirectional TS behavior on Ag/Ta2O5/Pt diffusive devices. The ON/OFF ratio and the OFF current of the device are ∼107 A and ∼ 10−12 A, respectively. The switching voltage is ∼ 0.35 V and the hold voltage is ∼ 0.125 V. The TS behavior can be also optimized by choosing suitable compliance current during a voltage sweep. Simulations of nanoparticles diffusion are also carried out to study the mechanism of this bidirectional TS process. The simulations show that this behavior can be attributed to the outer diffusion of Ag nanoparticles from an Ag electrode and their accumulation near the Pt electrode under the voltage sweep, which can serve as an additional counter active electrode. This work illustrates that Ag/Ta2O5/Pt diffusive devices are promising candidates for selector applications in bipolar memristor crossbar arrays. [ABSTRACT FROM AUTHOR]

Abstract: The important synaptic functionalities, such as long‐term memory, short‐term memory, and experience‐dependent learning, are extensively emulated by the emerging memristive devices, which provide a main step toward brain inspired computing. The majority of the electronic synapses based on memristors usually focus on the emulation of the physiological behaviors at the cellular level. However, the research on the subcellular synaptic dynamic properties of electronic synapses is rarely reported. In this article, Ag/Ta2O5/Pt‐based memristors are utilized to emulate both the long‐term memory and short‐term facilitation of biological synapse. Under pulse‐train measurements, the device exhibits short‐term facilitation depending on varying the electrical pulse intensity, duration, and frequency. Based on the experimental results, one‐to‐one correspondence is established between the filament dynamics in our device and the subcellular synaptic dynamics during short‐term facilitation. By fitting the experimental results into the biological model, the dynamic displacements of the rear and head ends of the Ag filament are drawn analogy to the neurotransmitter and Ca2+ dynamics. The dynamic process is explained by simulations of the nanoparticle dynamics in which the Ag filament is simplified to a 1D “stick.” This research enables the emulation of synaptic function with improved biofidelity and broad applications to neuromorphic computing. [ABSTRACT FROM AUTHOR]

Elemental tellurium, a prototypical one‐dimensional van der Waals material, has recently been found to crystallize quickly from the liquid on a nanosecond timescale, yet the inherent mechanism is not clear. Here, by combining in situ high‐energy synchrotron radiation X‐ray diffraction with ab initio molecular‐dynamics simulation, it is found that trigonal crystalline Te completely melts into the liquid phase at 450 °C, and recrystallizes into the trigonal phase for temperatures lower than 380 °C without the formation of any other phase. This directly confirms the recent proposal of a crystal‐liquid‐crystal phase transition in this material underlying the observed electrical‐switching process. Atomic‐scale, melt‐quench computer simulations show that liquid Te is capable of crystallizing within a time of 25 ps in the vicinity of templating crystallization interfaces. This ultrafast crystallization ability of Te can be understood as being due to delayed Peierls distortions during a quench and therefore a high atomic mobility over a wide range of temperature. This finding opens the way to develop a crystal‐liquid‐crystal, phase‐transition‐based selector switch with an ultrafast switching speed. [ABSTRACT FROM AUTHOR]