Tunable non-volatile memories based on 2D InSe/h-BN/GaSe heterostructures towards potential multifunctionality

Floating-gate memories based on two-dimensional van der Waal (2D vdW) heterostructures play an important role in the development of next-generation information technology. The diversity of 2D vdW materials and their heterostructures provides flexibility in the design of novel storage architectures. However, 2D InSe/h-BN/GaSe heterostructures are rarely reported in the field of tunable non-volatile memories, probably due to the quality limitation of materials and complex interfaces from stackings. Herein, a floating-gate 2D InSe/h-BN/GaSe memory with high performance and atmosphere stability is demonstrated. It exhibits both a large ON/OFF current ratio of ∼105 and a good extinction ratio of ∼103, with an estimated maximum storage capacity of 5.1 × 1012 cm-2. Moreover, the storage performance can be regulated by optimizing the thickness of the insulating h-BN layer. Different device configurations have been explored to validate the working mechanism. Furthermore, a simulation of biological synaptic behavior is achieved on the same prototype device. The enhanced non-volatile characteristics enable the exploration of the integrated 2D memory and potential multifunctionality.

Van der Waals Ferroelectric Semiconductor Field Effect Transistor for In-Memory Computing

In-memory computing is a highly efficient approach for breaking the bottleneck of von Neumann architectures, i.e., reducing redundant latency and energy consumption during the data transfer between the physically separated memory and processing units. Herein we have designed a in-memory computing device, a van der Waals ferroelectric semiconductor (InSe) based metal-oxide-ferroelectric semiconductor field-effect transistor (MOfeS-FET). This MOfeS-FET integrates memory and logic functions in the same material, in which the out-of-plane (OOP) ferroelectric polarization in InSe is used for data storage and the semiconducting property is used for the logic computation. The MOfeS-FET shows a long retention time with high on/off ratios (>10⁶), high program/erase (P/E) ratios (10³), and stable cyclic endurance. Moreover, inverter, programmable NAND, and NOR Boolean logic operations with nonvolatile storage of the results have all been demonstrated using our approach. These findings highlight the potential of van der Waals ferroelectric semiconductor-based MOfeS-FETs in the in-memory computing and their potential of achieving size scaling beyond Moore's law.

Revealing the origin of PL evolution of InSe flake induced by laser irradiation

Two-dimensional InSe has been considered as a promising candidate for novel optoelectronic devices owing to large electron mobility and a near-infrared optical band gap. However, its widespread applications suffer from environmental instability. A lot of theoretical studies on the degradation mechanism of InSe have been reported whereas the experimental proofs are few. Meanwhile, the role of the extrinsic environment is still obscure during the degradation. As a common technique of studying the degradation mechanism of 2D materials, laser irradiation exhibits many unique advantages, such as being fast, convenient, and offering in situ compatibility. Here, we have developed a laser-treated method, which involves performing repeated measurements at the same point while monitoring the evolution of the resulting PL, to systematically study the photo-induced degradation process of InSe. Interestingly, we observe different evolution behavior of PL intensity under weak irradiation and strong irradiation. Our experimental results indicate the vacancy passivation and degrading effect simultaneously occurring in InSe under a weak laser irradiation, resulting in the PL increasing first and then decreasing during the measurement. Meanwhile we also notice that the passivation has a stronger effect on the PL than the degrading effect of weak oxidation. In contrast, under a strong laser irradiation, the InSe suffers serious destruction caused by excess heating and intense oxidation. This leads to a direct decrease of PL and corresponding oxidative products. Our work provides a reliable experimental supplement to the photo oxidation study of InSe and opens up a new avenue to regulate the PL of InSe.

Two-Dimensional CIPS-InSe van der Waal Heterostructure Ferroelectric Field Effect Transistor for Nonvolatile Memory Applications

Channel current conduction modulation with the spontaneous polarization of ferroelectric films in ferroelectric field-effect transistors (FeFETs) has been widely investigated. Low interface quality and thermodynamic instability owing to the presence of dangling bonds in the conventional ferroelectrics have limited the memory retention and endurance of FeFETs. This, in turn, prevents their commercialization. However, the atomically thin nature of 2D ferroelectric, semiconducting, and insulating films facilitate the achievement of trap-free interfaces as van der Waal heterostructures (vdWHs) to develop FeFETs with long data retention and endurance characteristics. Here, we demonstrate a 2D vdWH FeFET fabricated with ferroelectric CuInP₂S₆ (CIPS), hexagonal boron nitride (h-BN) as the dielectric, and InSe as the ferroelectric semiconductor channel. The device shows an excellent performance as nonvolatile memory (NVM) with its large memory window (4.6 V at a voltage sweep of 5 V), high drain current on/off ratio (>10⁴), high endurance, and long data retention (>10⁴ s). These results demonstrate the considerable potential of vdWHs for the development of FeFETs for logic and NVM applications.

Atomic insights into the influence of Bi doping on the optical properties of two-dimensional van der Waals layered InSe

As a narrow-gap semiconductor, III-VI two-dimensional (2D) van der Waals layered indium selenide (InSe) has attracted a lot of attention due to excellent physical properties. For potential optoelectronic applications, the tunability of the optical property is challenging, e.g., the modulation of optical bandgap commonly by element doping. However, the deep understanding of the influence of element doping on the microstructure and the optical properties lacks of systematic investigation. In this work, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we investigate the influence of Bi doping on controlling of the microstructure and optical properties of InSe single crystal in detail. The results show that Bi doping can introduce additional stacking faults in InSe single crystal, and more importantly, the atomic spacing and lattice constant of Bi-doped InSe are changed a lot as compared to that of the undoped one. Further optical characterizations including photoluminescence and transmission spectra reveal that Bi-doping can broaden the transmission wavelength range of InSe and make its optical bandgap blue-shift, which can also be physically interpreted from the doping-induced structure change. Our work expands new ideas for the optical property modulation of 2D thin-layer materials and brings new possibilities for the development of thin-layer InSe optical devices.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Photoluminescence enhancement at a high generation rate induced by exciton localization

The photoluminescence (PL) efficiency of two-dimensional (2D) transition metal dichalcogenides (TMDs) is extremely low under high power excitation, limiting its potential in display and light-emission application. This arises from the much shorter lifetime of non-radiative recombination than radiative recombination, wherein photo-carriers tend to decay through non-radiative processes. Herein, a "molecular state" near the valence band is successfully introduced into the ${{\rm MoS}_2}$ monolayer to increase the density of radiative states and speed up the exciton relaxation. This reduces the recombination lifetime of excitons by two orders of magnitude and forms vigorous competition with non-radiative decays. As a consequence, dozens of times enhancement of PL in ${{\rm MoS}_2}$ monolayers under high excitation power (${\rm G}\sim{{10}^{19}}\;{{\rm cm}^{- 2}}\cdot{{\rm S}^{- 1}}$) is realized. These results provide an effective method to improve PL efficiency under high injection levels for applications of 2D materials in light-emission industry.

Photo-excited carrier relaxation dynamics in two-dimensional InSe flakes

Carrier relaxation dynamics of InSe flakes is investigated by using time-resolved pump-probe reflectivity measurement. The photocarriers associated with the P _xy orbital band-edge transition at 2.40 eV, which is coupled to the in-plane polarized light, is observed to possess a lifetime of ∼19 ps at room temperature and ∼99 ps at 10 K. The temperature and power dependent carrier lifetime suggests that Shockley-Read-Hall process is the dominant nonradiative recombination mechanism responsible for the carrier relaxation. In addition, the electron scattering with a 14.5 meV optical phonon plays an active role in the carrier relaxation with increasing temperatures. A broad absorption around 1.65-1.90 eV is observed. The photocarriers associated with this broad transition show a long lifetime of ∼200 ps that is nearly independent of temperature and photon energy. This is indicative of bound carriers by defects. Our experimental results provide essential information for the characteristics of carrier dynamics and defects in InSe flakes. The experimental findings are fundamentally important for further development of microelectronics and optoelectronics based on InSe.

Broadband Nonlinear Optical Response of InSe Nanosheets for the Pulse Generation From 1 to 2 μm

Few-layered InSe nanosheets were fabricated by the simple liquid-phase exfoliation method. The morphology and crystal structure features of InSe nanosheet sample were characterized comprehensively. The photoluminescence (PL) spectrum indicated that the liquid-phase exfoliated InSe nanosheets contained variously layered nanoflakes, where eight layers nanosheets dominate. In addition, the first-principle simulation was carried out to describe the electron density of states (DOS) and the electronic band structures. Moreover, the few-layered InSe nanosheets performed excellent nonlinear absorption properties in a broad spectral band. As an application, the stable passively Q-switched (PQS) lasers with few-layered InSe nanosheets saturable absorbers (SAs) were realized with the operating wavelengths at 1.06, 1.34, and 1.91 μm. The shortest pulse durations were 599, 520, and 210 ns, respectively. Our results confirmed that the few-layered InSe nanosheets could be an excellent candidate for pulsed lasers in wide spectral bands.