Jiuxiang Dai

The two-dimensional/three-dimensional (2D/3D) van der Waals heterostructures (vdWH) combine the advantages of the 2D layer materials (2DLMs) and traditional 3D semiconductor materials simultaneously, which provide a new platform for developing high-performance optoelectronic devices. Herein, we present a comprehensive review on the structure categories, working mechanisms, construction methods and recent progress of 2D/3D vdWH based photodetectors. At the end of this review, we highlight the current challenges and prospects of the heterointegration of 2DLMs with traditional 3D semiconductors toward photodetection applications.

Abstract

In the last decade, two-dimensional layered materials (2DLMs) have been drawing extensive attentions due to their unique properties, such as absence of surface dangling bonds, thickness-dependent bandgap, high absorption coefficient, large specific surface area, and so on. But the high-quality growth and transfer of wafer-scale 2DLMs films is still a great challenge for the commercialization of pure 2DLMs-based photodetectors. Conversely, the material growth and device fabrication technologies of three-dimensional (3D) semiconductors photodetectors tend to be gradually matured. However, the further improvement of the photodetection performance is limited by the difficult heterogeneous integration or the inferior crystal quality via heteroepitaxy. Fortunately, 2D/3D van der Waals heterostructures (vdWH) combine the advantages of the two types of materials simultaneously, which may provide a new platform for developing high-performance optoelectronic devices. Here, we first discuss the unique advantages of 2D/3D vdWH for the future development of photodetection field and simply introduce the structure categories, working mechanisms, and the typical fabrication methods of 2D/3D vdWH photodetector. Then, we outline the recent progress on 2D/3D vdWH-based photodetection devices integrating 2DLMs with the traditional 3D semiconductor materials, including Si, Ge, GaAs, AlGaN, SiC, and so on. Finally, we highlight the current challenges and prospects of heterointegrating 2DLMs with traditional 3D semiconductors toward photodetection applications.

A deterministic method is presented to enhance MoTe₂ device performance by inducing tensile strain through substrate engineering and encapsulation. This structure significantly improves hole and electron mobilities in MoTe₂ field-effect transistors, reaching up to 130 and 160 cm² V s, respectively. Remarkably, the electron mobility increases by 6 times compared to the unstrained devices. The technique holds potential for 2D-electronics.

Abstract

Molybdenum ditelluride (MoTe₂) exhibits immense potential in post-silicon electronics due to its bandgap comparable to silicon. Unlike other 2D materials, MoTe₂ allows easy phase modulation and efficient carrier type control in electrical transport. However, its unstable nature and low-carrier mobility limit practical implementation in devices. Here, a deterministic method is proposed to improve the performance of MoTe₂ devices by inducing local tensile strain through substrate engineering and encapsulation processes. The approach involves creating hole arrays in the substrate and using atomic layer deposition grown Al₂O₃ as an additional back-gate dielectric layer on SiO₂. The MoTe₂ channel is passivated with a thick layer of Al₂O₃ post-fabrication. This structure significantly improves hole and electron mobilities in MoTe₂ field-effect transistors (FETs), approaching theoretical limits. Hole mobility up to 130 cm⁻² V⁻¹ s⁻¹ and electron mobility up to 160 cm⁻² V⁻¹ s⁻¹ are achieved. Introducing local tensile strain through the hole array enhances electron mobility by up to 6 times compared to the unstrained devices. Remarkably, the devices exhibit metal–insulator transition in MoTe₂ FETs, with a well-defined critical point. This study presents a novel technique to enhance carrier mobility in MoTe₂ FETs, offering promising prospects for improving 2D material performance in electronic applications.

By combining thin Au film with monolayer graphene, thin hybrid drain electrodes are realized with low sheet resistance, leading to the construction of ultra-short vertical field effect transistor with pitch size down to 20 nm and on-state current of 730 A cm⁻².

Abstract

Vertical field effect transistors (VFETs) have attracted considerable interest for developing ultra-scaled devices. In particular, individual VFET can be stacked on top of another and does not consume additional chip footprint beyond what is needed for a single device at the bottom, representing another dimension for high-density transistors. However, high-density VFETs with small pitch size are difficult to fabricate and is largely limited by the trade-offs between drain thickness and its conductivity. Here, a simple approach is reported to scale the drain to sub-10 nm. By combining 7 nm thick Au with monolayer graphene, the hybrid drain demonstrates metallic behavior with low sheet resistance of ≈100 Ω sq⁻¹. By van der Waals laminating the hybrid drain on top of 3 nm thick channel and scaling gate stack, the total VFET pitch size down to 20 nm and demonstrates a higher on-state current of 730 A cm⁻². Furthermore, three individual VFETs together are vertically stacked within a vertical distance of 59 nm, representing the record low pitch size for vertical transistors. The method pushes the scaling limit and pitch size limit of VFET, opening up a new pathway for high-density vertical transistors and integrated circuits.

Light: Science & Applications, Published online: 09 August 2023; doi:10.1038/s41377-023-01244-w

Tunable optical topological transitions of plasmon polaritons in WTe₂ van der Waals films

By boosting the emission strength by a factor of 100, a new hybrid photonic‒plasmonic architecture is successfully demonstrated to realize low-threshold room-temperature UV nanolasers that can be monolithically integrated into silicon. The synergistic engineering of the metal‒semiconductor heterojunction, the modal reflectivity, and the intermodal coupling engender the realization of electrically driven nanolasers.

Abstract

The metal‒semiconductor heterojunction is imperative for the realization of electrically driven nanolasers for chip-level platforms. Progress in developing such nanolasers has hitherto rarely been realized, however, because of their complexity in heterojunction fabrication and the need to use noble metals that are incompatible with microelectronic manufacturing. Most plasmonic nanolasers lase either above a high threshold (10¹‒10³ MW cm⁻²) or at a cryogenic temperature, and lasing is possible only after they are removed from the substrate to avoid the large ohmic loss and the low modal reflectivity, making monolithic fabrication impossible. Here, for the first time, record-low-threshold, room-temperature ultraviolet (UV) lasing of plasmon-coupled core‒shell nanowires that are directly grown on silicon is demonstrated. The naturally formed core‒shell metal‒semiconductor heterostructure of the nanowires leads to a 100-fold improvement in growth density over previous results. This unprecedentedly high nanowire density creates intense plasmonic resonance, which is outcoupled to the resonant Fabry‒Pérot microcavity. By boosting the emission strength by a factor of 100, the hybrid photonic‒plasmonic system successfully facilitates a record-low laser threshold of 12 kW cm⁻² with a spontaneous emission coupling factor as high as ≈0.32 in the 340‒360 nm range. Such architecture is simple and cost-competitive for future UV sources in silicon integration.

We report the magnetic properties of epitaxial bismuth-substituted yttrium iron garnet (BiYIG) films. Growth-induced anisotropy is found to be the major contributor to perpendicular magnetic anisotropy (PMA) in films. Field cycling reveals triangular domains in films with higher PMA. Low damping, tunable anisotropy, and high magnetooptical contrast make BiYIG advantageous for a range of spintronic applications.

Abstract

Iron garnets that combine robust perpendicular magnetic anisotropy (PMA) with low Gilbert damping are desirable for studies of magnetization dynamics as well as spintronic device development. This paper reports the magnetic properties of low-damping bismuth-substituted iron garnet thin films (Bi_0.8Y_2.2Fe₅O₁₂) grown on a series of single-crystal gallium garnet substrates. The anisotropy is dominated by magnetoelastic and growth-induced contributions. Both stripe and triangular domains form during field cycling of PMA films, with triangular domains evident in films with higher PMA. Ferromagnetic resonance measurements show damping as low as 1.3 × 10⁻⁴ with linewidths of 2.7 to 5.0 mT. The lower bound for the spin-mixing conductance of BiYIG/Pt bilayers is similar to that of other iron garnet/Pt bilayers.

Nature Communications, Published online: 10 August 2023; doi:10.1038/s41467-023-40602-5

Mid-infrared light emitting diodes (LEDs) based on black phosphorus (BP) have shown promising performance, but they are usually limited by the environmental instability of the material. Here, the authors extrapolate a room-temperature operational lifetime of BP LEDs up to ~ 15,000 h via Al2O3 passivation and nitrogen seal packaging.

Abstract

The bulk, pristine sp² carbons, such as graphite, carbon nanotubes, and graphene, are usually assumed to be typical diamagnetic materials. However, over the past two decades, there have been many reports about the ferromagnetism in these sp² carbon materials, which have attracted intense interest for basic research and potential applications. In this review, we focus on the evidence and developments of the emergent ferromagnetism in sp² carbon revealed by nine kinds of experimental methods: magnetic force microscopy (MFM), magnetization measurements with physical property measurement system (PPMS), X-ray magnetic circular dichroism (XMCD), scanning tunneling microscopy (STM), miniaturized magnetic particle inspection (MPI), anomalous Hall effect (AHE), mechanical deflection of carbon nanotube cantilevers, magnetoresistance, and spin-related devices (spin field effect transistor and spin memory). The advantages, conclusions, challenges, and future of these methods are discussed. The ferromagnetism in sp² carbon will open a door to explore exotic physical phenomena and lay the basis for the development of integrated circuit of spintronics, which is fundamentally different from charge-based conventional electronics.

In-plane (IP) ferroelectric memory effect of a 100 nm channel-length 2D ferroelectric semiconductor α-In₂Se₃ stamped onto nanogap electrodes on Si/SiO₂ is demonstrated. The non-volatile memory characteristics with IP polarization including the on/off ratio reaching 10³, stable retention lasting 17 h, and endurance over 1200 cycles suggest a wide range of memory applications utilizing the lateral bottom contact structure.

Abstract

Owing to the emerging trend of non-volatile memory and data-centric computing, the demand for more functional materials and efficient device architecture at the nanoscale is becoming stringent. To date, 2D ferroelectrics are cultivated as channel materials in field-effect transistors for their retentive and switchable dipoles and flexibility to be compacted into diverse structures and integration for intensive production. This study demonstrates the in-plane (IP) ferroelectric memory effect of a 100 nm channel-length 2D ferroelectric semiconductor α-In₂Se₃ stamped onto nanogap electrodes on Si/SiO₂ under a lateral electric field. As α-In₂Se₃ forms the bottom contact of the nanogap electrodes, a large memory window of 13 V at drain voltage between ±6.5 V and the on/off ratio reaching 10³ can be explained by controlled IP polarization. Furthermore, the memory effect is modulated by the bottom gate voltage of the Si substrate due to the intercorrelation between IP and out-of-plane (OOP) polarization. The non-volatile memory characteristics including stable retention lasting 17 h, and endurance over 1200 cycles suggest a wide range of memory applications utilizing the lateral bottom contact structure.

Abstract

During the development of ultrathin two-dimensional (2D) materials, the appearance of ripples has been widely observed. However, the formation mechanisms and their influences are still rarely investigated, especially their contributions to the electronic structures and optical properties. To compensate for the knowledge gap, we have carried out comprehensive theoretical studies on the monolayer WSe₂ with a series of ripple structures from 0 to 12 Å in different lattice sizes. The sensitivity of the formation energy, band structures, electronic structures, and optical properties to the ripple structures have been performed systematically for the first time. The formation of ripples in Armchair and Zigzag simultaneously are more energetically favorable, leading to more flexible optimizations of the optoelectronic properties. The improved charge-locking effect and extension of absorption ranges indicate the significant role of ripple structures. The spontaneous formation of ripples is associated with orbital rearrangements and structural distortions. This leads to the unique charge carrier correlate inversion between W-5d and Se-4p orbitals, resulting in the pinning of the Fermi level. This work has supplied significant references to understand ultrathin 2D structures and benefit their future developments and applications in high-performance optoelectronic devices.

Efforts toward atom-by-atom fabrication have gathered momentum. Recent results with electron-beam manipulation illustrate a significant advance toward atomic-scale patterning. In this work, the case is made that future efforts in this direction will benefit from reconceptualizing the scanning transmission electron microscope as a combined synthesis and manipulation platform, and simple strategies to incorporate synthesis processes with existing hardware are provided.

Abstract

The scanning transmission electron microscope, a workhorse instrument in materials characterization, is being transformed into an atomic-scale material-manipulation platform. With an eye on the trajectory of recent developments and the obstacles toward progress in this field, a vision for a path toward an expanded set of capabilities and applications is provided. The microscope is reconceptualized as an instrument for fabrication and synthesis with the capability to image and characterize atomic-scale structural formation as it occurs. Further development and refinement of this approach may have substantial impact on research in microelectronics, quantum information science, and catalysis, where precise control over atomic-scale structure and chemistry of a few “active sites” can have a dramatic impact on larger-scale functionality and where developing a better understanding of atomic-scale processes can help point the way to larger-scale synthesis approaches.

Nature Electronics, Published online: 14 August 2023; doi:10.1038/s41928-023-00991-3

Individual graphene nanoribbons synthesized by an on-surface approach can be contacted with carbon nanotubes—with diameters as small as 1 nm—and used to make multigate devices that exhibit quantum transport effects such as Coulomb blockade and single-electron tunnelling.

Nature Materials, Published online: 14 August 2023; doi:10.1038/s41563-023-01618-w

An alloy engineering approach is developed to reliably grow atomically thin bilayers with predictable and tunable moiré patterns.

Nature Materials, Published online: 14 August 2023; doi:10.1038/s41563-023-01619-9

Ferroelectricity in hafnia-based systems seems to be correlated with oxygen vacancy dynamics, but the coupling of this and ferroelectric response is rarely studied. Here it is shown that Hf0.5Zr0.5O2 can be antiferroionic, with antiferroelectric behaviour coupled to surface electrochemistry.

Intermittent, equilibrium dynamics of brownmillerite/perovskite domains in an epitaxial SrCoO _x on LSAT substrate are revealed using in situ Bragg X-ray photon correlation spectroscopy. The dynamics appear to be thermally activated and can be tuned via epitaxial strain, consistent with results from density functional theory and phase field simulations.

Abstract

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoO _x with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemical applications.

The groundbreaking discovery of Piezo channels reveals how mechanical force initiates the nerve impulses that help to perceive and adapt to the world. This work is an attempt to mimic Piezo channels. A revolutionary two-terminal transistor gated only by a tiny force <1 µN instead of voltage, achieving over three orders of magnitude ON/OFF ratio is developed.

Abstract

Silicon-based field effect transistors have underpinned the information revolution in the last 60 years, and there is a strong desire for new materials, devices, and architectures that can help sustain the computing power in the age of big data and artificial intelligence. Inspired by the Piezo channels, a mechanically gated transistor abandoning electric gating altogether, achieving an ON/OFF ratio over three orders of magnitude under a mechanical force of hundreds of nN is developed. The two-terminal device utilizes flexoelectric polarization induced by strain gradient, which modulates the carrier concentration in a Van der Waals structure significantly, and it mimics Piezo channels for artificial tactile perception. This simple device concept can be easily adapted to a wide range of semiconducting materials, helping promote the fusion between mechanics and electronics in a similar way as mechanobiology.

Anti-ambipolar heterojunctions hold promising potential for next-generation integrated circuit chips and telecommunication technologies, enabled by their high data processing efficiency, lower power consumption, and simplified circuit design. This review summarizes the historical and recent advances in anti-ambipolar heterojunctions, highlighting their significance in materials, devices, circuits, and broad scale research.

Abstract

Anti-ambipolar heterojunctions are vital in constructing high-frequency oscillators, fast switches, and multivalued logic (MVL) devices, which hold promising potential for next-generation integrated circuit chips and telecommunication technologies. Thanks to the strategic material design and device integration, anti-ambipolar heterojunctions have demonstrated unparalleled device and circuit performance that surpasses other semiconducting material systems. This review aims to provide a comprehensive summary of the achievements in the field of anti-ambipolar heterojunctions. First, the fundamental operating mechanisms of anti-ambipolar devices are discussed. After that, potential materials used in anti-ambipolar devices are discussed with particular attention to 2D-based, 1D-based, and organic-based heterojunctions. Next, the primary device applications employing anti-ambipolar heterojunctions, including anti-ambipolar transistors (AATs), photodetectors, frequency doublers, and synaptic devices, are summarized. Furthermore, alongside the advancements in individual devices, the practical integration of these devices at the circuit level, including topics such as MVL circuits, complex logic gates, and spiking neuron circuits, is also discussed. Lastly, the present key challenges and future research directions concerning anti-ambipolar heterojunctions and their applications are also emphasized.