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Ovonic threshold switching (OTS) selectors play a critical role in suppressing the sneak-path current of three-dimensional crossbar integration circuits. Compared to conventional nonmetal-telluride OTS selectors, selectors based on AlxTey glass are found to have both satisfactory on-state current and selectivity. However, it is unclear why the Al-rich AlxTey glass-based OTS selectors have robust insulation properties for reducing the off-state current. This work reveals the structure–property correlations of amorphous AlxTey at the atomic scale by first-principles calculations. It is found that the stoichiometric Al2Te3 glass tends to have a clean bandgap owing to the covalent and dative bonds formed by non-equivalent sp3-hybridized Al orbitals and the lone-pair electrons of Te. Unexpectedly, for Al-rich AlxTey glass (Al2.21Te2.79), the Al–Al bonds formed by redundant Al-atoms have an integrated crystal orbital bond index (ICOBI) of 0.8–0.9, which is much larger than that of Al–Al bonds in pure metals (0.227), indicating they are covalent. It is the covalent Al–Al bonds that ensure the robust insulation characteristics of Al-rich AlxTey glass, while the Te–Te interaction in the Al-poor AlxTey glass (Al1.79Te3.21) produces midgap states, thereby reducing the insulativity. The presented atomic and electronic pictures here will provide useful theoretical insights for designing OTS selectors with improved performances. [ABSTRACT FROM AUTHOR]

Disorder‐induced electronic localization is responsible for the OFF state of the Ovonic threshold switching (OTS) device, which is an indispensable component in the present 3D‐crossbar‐architecture phase‐change memory circuit. However, the atomic mechanism of the OTS device, especially the role of thermal effect, remains a long‐term open question. Recent researches suggest that the working current of the OTS ON‐state is often large enough to melt the adjacent phase‐change memory material in the OTS+PCM devices. Thus, Joule heating‐induced atomic/electronic structure transition in OTS materials must be seriously considered. Taking the typical OTS material GeSe as an example, first‐principles calculations reveal an unexpected electronic delocalization induced by the enhanced structural disorder upon solid‐liquid transition. Meanwhile, as the temperature rises, the band gap decreases or even closes, leading to an increase in carrier concentration. Therefore, the melting filament with high conductivity could be responsible for the holding‐ON state of OTS materials. The results in this study may provide possible explanations for some puzzles of OTS devices, such as the reasons for holding‐ON state and limited endurance. [ABSTRACT FROM AUTHOR]

The non-volatile resistive switching process of a MoS2 based atomristor with a vertical structure is investigated by first-principles calculations. It is found that the monolayer MoS2 with a S vacancy defect ( V S ) could maintain an insulation characteristic and a high resistance state (HRS) is remained. As an electrode metal atom is adsorbed on the MoS2 monolayer, the semi-conductive filament is formed with the assistance of V S . Under this condition, the atomristor presents a low resistance state (LRS). The ON state current of this semi-filament is increased close to two orders of magnitude larger than that without the filament. The energy barrier for an Au-atom to penetrate the monolayer MoS2 via V S is as high as 6.991 eV. When it comes to a double S vacancy ( V S 2 ), the energy barrier is still amounted to 3.554 eV, which manifests the bridge-like full conductive filament cannot form in monolayer MoS2 based atomristor. The investigation here promotes the atomic level understanding of the resistive switching properties about the monolayer MoS2 based memristor. The physics behind should also work in atomristors based on other monolayer transition-metal dichalcogenides, like WSe2 and MoTe2. The investigation will be a reference for atomristor-device design or optimization. [ABSTRACT FROM AUTHOR]

Ferroelectric memory is a promising candidate for next‐generation nonvolatile memory owing to its outstanding performance such as low power consumption, fast speed, and high endurance. However, the ferroelectricity of conventional ferroelectric materials will be eliminated by the depolarization field when the size drops to the nanometer scale. As a result, the miniaturization of ferroelectric devices was hindered, which makes ferroelectric memory unable to keep up with the development of integrated‐circuit (IC) miniaturization. Recently, a two‐dimensional (2D) In2Se3 was reported to maintain stable ferroelectricity at the ultrathin scale, which is expected to break through the bottleneck of miniaturization. Soon, devices based on 2D In2Se3, including the ferroelectric field‐effect transistor, ferroelectric channel transistor, synaptic ferroelectric semiconductor junction, and ferroelectric memristor were demonstrated. However, a comprehensive understanding of the structures and the ferroelectric‐switching mechanism of 2D In2Se3 is still lacking. Here, the atomic structures of different phases, the dynamic mechanism of ferroelectric switching, and the performance/functions of the latest devices of 2D In2Se3 are reviewed. Furthermore, the correlations among the structures, the properties, and the device performance are analyzed. Finally, several crucial problems or challenges and possible research directions are put forward. We hope that this review paper can provide timely knowledge and help for the research community to develop 2D In2Se3 based ferroelectric memory and computing technology for practical industrial applications. [ABSTRACT FROM AUTHOR]

First-principles calculations reveal a Mexican-hat potential energy surface (PES) for two-dimensional (2D) In2Se3. This unique PES leads to a pseudo-centrosymmetric paraelectric β phase that resolves the current controversy between theory and experiment. We further show that while the α-to-β (ferroelectric-to-paraelectric) phase transition is fast and coherent, assisted by an in-plane shear phonon mode, a random distribution of the atoms in the trough of the PES acts as an entropy barrier against the reverse β-to-α transition. This will be the origin of the speed limitation of current In2Se3 ferroelectric devices. However, if one orders the β phase (due to the formation of in-plane ferroelectric domains), the reverse transition can take place within tens of picoseconds in the presence of a perpendicular electric field. Finally, the Mexican-hat PES is a general feature for the entire family of 2D III2-VI3 materials. Our finding offers a critical physical picture in controlling the ultrafast reversible phase transition in 2D In2Se3 and other III2-VI3 materials, which will benefit their practical industrial development for advanced ferroelectric devices. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high‐density data integration. In this work, by first‐principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one‐step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh‐density data storage. [ABSTRACT FROM AUTHOR]

Reversible phase transitions between crystalline and amorphous phases of phase‐change memory (PCM) materials are the physical basis of PCM. Applications of PCM in multilevel memory, in‐memory computing, and neuromorphic computing require a precise control of material's structure/property to achieve signals beyond binary (0/1), where recrystallization is the key process because it is smooth compared with violent amorphization. By molecular dynamic simulations and electronic structure analyses based on density functional theory, nucleation dynamics during recrystallization are investigated from the point of view of atomic motions and property evolutions. Atomic motions are examined individually rather than statistically. According to the results, an atomic picture of the growth of the nucleus is described in detail, where the competition between the potential seeds of nucleus plays a critical role. The electron polarization during recrystallization is mapped to the linearly aligned atom chains. Further analyses of the Born effective charge confirm the delocalization of the electrons in the atom chains. Also, a large p‐orbital‐axial Born effective charge is observed in long‐range atom chains, which suggests a kind of resonance enhancement of electronic delocalization. These dynamics of nucleation in recrystallization provide references for the fine control of PCM performances. [ABSTRACT FROM AUTHOR]

Defects play a central role in controlling the electronic properties of two‐dimensional (2D) materials and realizing the industrialization of 2D electronics. However, the evaluation of charged defects in 2D materials within first‐principles calculation is very challenging and has triggered a recent development of the WLZ (Wang, Li, Zhang) extrapolation method. This method lays the foundation of the theoretical evaluation of energies of charged defects in 2D materials within the first‐principles framework. Herein, the vital role of defects for advancing 2D electronics is discussed, followed by an introduction of the fundamentals of the WLZ extrapolation method. The ionization energies (IEs) obtained by this method for defects in various 2D semiconductors are then reviewed and summarized. Finally, the unique defect physics in 2D dimensions including the dielectric environment effects, defect ionization process, and carrier transport mechanism captured with the WLZ extrapolation method are presented. As an efficient and reasonable evaluation of charged defects in 2D materials for nanoelectronics and other emerging applications, this work can be of benefit to the community. [ABSTRACT FROM AUTHOR]