Search

Single‐crystal silicon carbide (SiC) is an important semiconductor material for the fabrication of power and radio frequency (RF) devices. The major technique for growing single‐crystal SiC is the so‐called physical vapor transport (PVT) method, in which not only the thermal field but also the fluid‐flow field and the distribution of gas species can be hardly measured directly. In this study, a multi‐component flow model is proposed that includes the inside and outside of a growth chamber and a joint between the seed crystal holder and crucible which allows exchanges of the gas species. The joint is simulated as a thin porous graphite sheet. The Hertz‐Knudsen equation is used to describe the sublimation and deposition. The convection and diffusion are described by the Navier–Stokes equations and mixture‐averaged diffusion model, in which the Stefan flow is taken into account. The numerical simulations are conducted by the finite element method (FEM) with a multi‐physics coupled model, which is able to predict the fluid flow field, species distribution field, crystal growth rate, and evolution of the molar concentration of dopant gas. Using this model, the effects of several experimental conditions on the transport of gas species and the growth rate of single‐crystal SiC are analyzed. [ABSTRACT FROM AUTHOR]

Tunable emission wavelength and excellent biocompatibility have positioned silicon quantum dots (Si QDs) as important optoelectronic materials in areas like displays, lighting, and biomedical imaging. However, the challenges of low luminescence efficiency impede the utilization of Si QDs, restraining the advancement of Si QDs‐based light‐emitting diode (LED) devices. This study primarily concentrates on optimizing the surface structure of Si QDs and refining the Si QDs‐based LED device structures. A strategy involving active oxidation followed by hydrofluoric etching and hydrosilylation is proposed to enhance the optical characteristics of Si QDs. A variety of characterization methods are employed to evaluate the impact of the active oxidation on the photoluminescence and surface structures of Si QDs. This approach ultimately achieves an impressive enhancement in the photoluminescence quantum yield (PL QY) of Si QDs from 6.7% to 60.3%. Furthermore, the atmospherically oxidized Si QDs with the highest PL QY are selected as the emitting‐layer material to fabricate LEDs with the structure of ITO/PEDOT:PSS/TFB/Si QDs/ZnMgO/Ag. The introduction of ZnMgO effectively balances charge injection in the Si‐QDs‐based LEDs. As a result, the devices achieve electroluminescence with an external quantum efficiency of 13.2%. [ABSTRACT FROM AUTHOR]

Artificial synapse devices are dedicated to overcoming the von Neumann bottleneck. Adopting light signals in visual information processing and computing is vital for developing next‐generation artificial neuromorphic systems. A strategy to construct all‐optically controlled artificial synaptic devices based on full oxides with amorphous ZnAlSnO/SnO heterojunction in a two‐terminal planar configuration is proposed. All synaptic behaviors are operated in the visible optical pathway, with excitatory synapse under red (635 nm) light and inhibitory synapse under green (532 nm) and blue (405 nm) lights. Based on the different inhibitory effects, two modes of long‐term depression (LTD) and RESET processes can be implemented through green and blue lights, respectively. The energy consumption of an event can be as low as 0.75 pJ. A three‐layer perceptron model is designed to classify 28 × 28‐pixel handwritten digital images and performed supervised learning using a backpropagation algorithm, demonstrating the bio‐visually inspired neuromorphic computing with a training accuracy of 92.74%. The all‐optically controlled artificial synapses with write/erasure behaviors in visible RGB region and rational microelectronic process, as presented in this work, are essential in developing future artificial neuromorphic systems and highlight the huge potential of next‐generation computer systems. [ABSTRACT FROM AUTHOR]

High‐efficiency and low‐loss processing is the mainstay to reduce the cost and deepen the application of 4H silicon carbide (4H‐SiC) wafers in high‐power and high‐frequency electronics. In this study, the high‐yield slicing of 4H‐SiC wafers is realized by combining femtosecond laser irradiation and bandgap‐selective photo‐electrochemical (PEC) exfoliation. By combining light‐absorption measurements, micro‐Raman, and micro‐photoluminescence characterizations, it is found that the damage layer formed inside 4H‐SiC after femtosecond‐laser irradiation consists of amorphous silicon and amorphous carbon. This indicates that the femtosecond‐laser irradiation leads to phase separation in 4H‐SiC. The bandgap of the damage layer is 0.4 eV. Taking advantage of the different bandgap energies of the damage layer and the perfect 4H‐SiC region, the damage layer is removed from the perfect region of 4H‐SiC by using bandgap‐selective PEC etching. During the PEC etching, light‐generated holes can selectively oxidize and corrode the damaged layer with the assistance of the HF solution, and leave the upper and lower perfect 4H‐SiC layers being intact. The current work contributes to the development of the high‐yield and high‐throughput femtosecond laser slicing of 4H‐SiC wafers. [ABSTRACT FROM AUTHOR]

4H silicon carbide (4H‐SiC) holds great promise for high‐power and high‐frequency electronics, in which high‐quality 4H‐SiC wafers with both global and local planarization are cornerstones. Chemical–mechanical polishing (CMP) is the key processing technology in the planarization of 4H‐SiC wafers. Enhancing the performance of CMP is critical to improving the surface quality and reducing the processing cost of 4H‐SiC wafers. In this review, the superior properties of 4H‐SiC and the processing of 4H‐SiC wafers are introduced. The development of CMP with chemical, mechanical, and chemical–mechanical synergistic approaches to improve the performance of CMP is systematically reviewed. The basic principle and processing system of each improvement approach are presented. By comparing the material removal rate of CMP and the surface roughness of CMP‐treated 4H‐SiC wafers, the prospect on the chemical, mechanical, and chemical–mechanical synergistic improvement approaches is finally provided. [ABSTRACT FROM AUTHOR]

Scintillators are essential for high‐energy radiation detection in a variety of potential applications. However, due to complex fabrication processes and nanocrystal homogeneity, conventional scintillators are challenging to meet the need for cost‐effective, environmentally friendly, and flexible X‐ray detection. Here, monodisperse nanocrystals (NCs) with small grain size and colloidal stability are obtained by adjusting the doping concentration of Zn2+ ions and controlling the morphology uniformity of Cs3Cu2I5 NCs. The photoluminescence quantum yield (PLQY) for the optimal doping concentration is as high as 92.8%, which is a 28.5% improvement compared to nondoped NCs. Density functional theory calculations reveal that the Zn2+ dopant inclines to occupy Cu sites and the I‐rich condition suppresses the formation of I vacancy, enriching the excited electron density at the band‐edge to enhance the self‐trapped exciton emission. Moreover, high luminescence performance and flexible X‐ray scintillator films are prepared using Zn2+‐doped Cs3Cu2I5 NCs, with a spatial resolution of up to 15.7 lp mm–1. This work provides an effective strategy for the development of environmentally friendly, low‐cost, and efficient blue‐emitting 0D all‐inorganic metal halides, as well as shows their great potential for high‐performance flexible lead‐free and low‐toxicity X‐ray detector applications. [ABSTRACT FROM AUTHOR]

Neuromorphic computing promises to overcome the Von Neumann bottleneck of traditional computers. Optoelectronic synaptic devices are greatly desired given their potential applications in neuromorphic computing. In this work, optoelectronic synaptic devices with long‐term memory are fabricated. The devices are based on a GaN/AlGaN/AlN/GaN heterojunction and a SiNx charge trapping layer. Synaptic functionalities including excitatory postsynaptic current, paired pulse facilitation, and transition from short‐term memory to long‐term memory are realized. The retention time of long‐term memory may be more than 10 years, demonstrating the reliable charge storage of the devices. Logic functions are also realized by synergizing the photogating and electrical gating of the devices. The responsivity and specific detectivity are 2.64 × 105 A W−1 and 1.79 × 1016 Jones at the optical power density of 1.4 mW cm−2, respectively. In addition, the devices have a reconfigurable switch of 1000 cycles with working temperature up to 200 °C. [ABSTRACT FROM AUTHOR]

Neuromorphic visual systems based on optoelectronic synaptic devices have been recently studied to simulate the retina and visual cortex of a human being. Now it is shown that an array of optoelectronic synaptic devices based on the two‐terminal structure of Si/perovskite/Au may mimic the functionalities of lateral geniculate nucleus (LGN) cells. Benefiting from the photovoltaic effect, the devices can work under a self‐powered mode. Diverse synaptic functionalities such as postsynaptic current, paired‐pulse facilitation/depression, spike duration‐dependent plasticity, spike number‐dependent plasticity, and spike rate‐dependent plasticity have been simulated. By modulating the electric bias of the devices in the array the simulation of the positional and orientational recognition of the LGN cells is demonstrated. [ABSTRACT FROM AUTHOR]

Ratiometric near‐infrared (NIR) fluorescence holds great promise for important applications such as temperature sensing, food safety detection, and biological imaging owing to its self‐calibration and contactless measurements in the NIR region. For ratiometric NIR fluorescence, the suppression of optical reabsorption and signal interference is crucial. In this work, freestanding erbium (Er)‐hyperdoped silicon quantum dots (Si QDs) with UV absorption and NIR emission are synthesized via nonthermal plasma, effectively avoiding optical reabsorption. Er with the valence of +3 is found to be mainly located in the subsurface region of Er‐hyperdoped Si QDs, which emit NIR light at the wavelengths of 830 and 1540 nm. The distinct difference between the two NIR wavelengths (Δλ = 710 nm) well impedes signal interference in ratiometric NIR fluorescence. It is shown that Er‐hyperdoped Si QDs may be a powerful platform of ratiometric NIR fluorescence by exemplarily demonstrating their temperature‐sensing capability in a wide temperature range (297–477 K) with high relative sensitivity (3.05% K−1), high‐temperature resolution (<0.018 K), and high repeatability (>98%). The temperature sensing of Er‐hyperdoped Si QDs may be further employed to construct logic gates, enabling in‐sensor computing. [ABSTRACT FROM AUTHOR]

Environmentally friendly metal halides have emerged as emitters in lighting and X‐ray imaging applications. However, the conventional scintillator fabrication process involves high‐temperature sintering or powder grinding, resulting in large bulk crystals or agglomerates, which hamper flexible device integration and processability. Here, large‐area flexible scintillation screen films containing (C24H20P)2MnBr4 nanocrystals are prepared in situ, which prevents the generation of agglomerated powders or large bulk crystals, and the preparation duration and cost are reduced. The film exhibits good mechanical stability and homogeneity, and the photoluminescence quantum yield is over 85%. Moreover, the film presents excellent scintillation performance with detection limit as low as 0.608 μGyair s−1 and 14.5 lp mm−1 X‐ray imaging resolution. This excellent performance and simple preparation method are expected to favor industrial mass production. [ABSTRACT FROM AUTHOR]

Optoelectronic synaptic devices that mimic biological synapses are critical building blocks of artificial neural networks (ANN) based on optoelectronic integration. Here it is shown that an optoelectronic synaptic device based on the hybrid structure of silicon nanocrystals (Si NCs) and poly(3‐hexylthiophene) (P3HT) can work with dual modes, exhibiting versatile synaptic plasticity. In the three‐terminal mode, the device is a synaptic transistor, which has wavelength‐selective synaptic plasticity due to potential wells enabled by the Si NCs/P3HT hybrid structure. In the two‐terminal mode, it is a synaptic metal‐oxide‐semiconductor (MOS) device, which is capable of mimicking spike‐rate‐dependent plasticity (SRDP) and metaplasticity with optical stimulation. Based on the wavelength‐selective synaptic plasticity a light‐stimulated ANN is proposed to recognize handwritten digits with an accuracy of around 90.4%. In addition, the SRDP and metaplasticity may be well used for the simulation of edge detection of images, facilitating real‐time image processing. [ABSTRACT FROM AUTHOR]

2D transition metal chalcogenides (TMCs) have attracted tremendous interest from both the scientific and technological communities due to their variety of properties and superior tunability through layer number, composition, and interface engineering. Wafer‐scale production of 2D TMCs is critical to the industrial applications of these materials. Extensive efforts have been bestowed to the large‐area growth of 2D TMCs through various approaches. In this review, recent advances in obtaining large‐area 2D TMCs by different methods such as chemical vapor deposition (CVD), metal–organic CVD, and physical vapor deposition are first highlighted and their advantages and disadvantages are also evaluated. Then, strategies for the control of the grains, morphology, layer number, and phase to achieve controllable and uniform thicknesses and large crystal domains for 2D TMCs are discussed. Applications of large‐area 2D TMCs in electronics, optoelectronics, spintronics, etc., are also introduced. Finally, ideas and prospects for the future developments of wafer‐scale 2D TMCs are provided. [ABSTRACT FROM AUTHOR]

Neuromorphic computing can potentially solve the von Neumann bottleneck of current mainstream computing because it excels at self‐adaptive learning and highly parallel computing and consumes much less energy. Synaptic devices that mimic biological synapses are critical building blocks for neuromorphic computing. Inspired by recent progress in optogenetics and visual sensing, light has been increasingly incorporated into synaptic devices. This paves the way to optoelectronic synaptic devices with a series of advantages such as wide bandwidth, negligible resistance–capacitance (RC) delay and power loss, and global regulation of multiple synaptic devices. Herein, the basic functionalities of synaptic devices are introduced. All kinds of optoelectronic synaptic devices are then discussed by categorizing them into optically stimulated synaptic devices, optically assisted synaptic devices, and synaptic devices with optical output. Existing practical scenarios for the application of optoelectronic synaptic devices are also presented. Finally, perspectives on the development of optoelectronic synaptic devices in the future are outlined. [ABSTRACT FROM AUTHOR]

Heterostructures built from 2D materials and organic semiconductors offer a unique platform for addressing many fundamental physics and construction of functional devices. Interfaces play a crucial role in tailoring the heterostructure properties. Here, density functional theory computations are performed to explore the interfacial properties of heterostructures made of group VI transition metal dichalcogenides (TMD) and organic molecules such as perylene tetracarboxylic dianhydride (PTCDA) and pentacene. First principle calculations predict that the organic pentacene layer exhibits covalent interfacing with MoSe2 and WSe2, while the interface of other studied TMD/organic heterostructures form van der Waals (vdW) interfaces. Owing to the different molecular geometry of PTCDA and pentacene in their respective heterostructures, the work function can be modulated of the order of 1.0 eV in comparison with pure monolayer MX2 in MX2/pentacene (M = Mo, W; X = S, Se) heterostructures, while the change of work function in MX2/PTCDA (M = Mo, W; X = S, Se) is negligible (order of 0.1 eV) in comparison with pure monolayer MX2. This study will be helpful to design high‐performance optoelectronic devices based on TMDs and organic semiconductors. [ABSTRACT FROM AUTHOR]

Ultrathin Si nanomembranes (SiNM) have attracted wide attention due to their potential applications in various fields, including flexible electronics and image sensors for spherical monocentric lens. However, due to silicon's relative high Young modulus, it is quite difficult to perfectly integrate the Si based circuits into these flexible and spherical systems. In this study, we report the reduction in modulus of SiNM when the thickness of SiNM is down to sub-2 nm. The fabrication steps include the Si thinning process by oxidation of the silicon on insulator (SOI) wafers and the suspended process by semiconductor technology. Direct mechanical measurement by atomic force microscope in contact model shows that the Young's modulus of a 1.6-nm-thick SiNMs decreases to 4 Gpa. It is further confirmed by the first-principle theory simulation. It demonstrates that the surface effects play a significant role in SiNM's mechanical properties. The results in this study are significant for the applications of SiNMs on the flexible electronics. [ABSTRACT FROM AUTHOR]

The doping of semiconductor nanocrystals (NCs) is crucial for the optimization of the performance of devices based on them. In contrast to recent progress on the doping of compound semiconductor NCs and silicon NCs, the doping of germanium (Ge) NCs has lagged behind. Here it is shown that Ge NCs can be doped with phosphorus (P) during synthesis by a nonthermal plasma. It is found that there are more P atoms in the NC near-surface region than in the NC core. P doping modifies the surface state of Ge NCs. Compressive strain can be incuced in Ge NCs by P which can explain the P-doping-enhanced oxidation resistance of Ge NCs. Stable dispersions of P-doped Ge NCs in acetonitrile can be cast to produce films for field-effect transistors (FETs). FET analysis shows that the electrical conductivity and electron mobility of a Ge-NC film increase with the increase of the P doping level, although the electrical activation efficiency of P in the Ge-NC film is low. Finally, atomic layer deposition of aluminum oxide at the surface of P-doped Ge NCs is shown to improve the performance of the FETs. [ABSTRACT FROM AUTHOR]

Optimizing the light-emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light-emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2-10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum-confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum-confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light-emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out. [ABSTRACT FROM AUTHOR]

Hyperdoping silicon nanocrystals (Si NCs) to a concentration exceeding the solubility limit of a dopant may enable their novel applications. Here, the successful hyperdoping of Si NCs with boron (B) and phosphorus (P) is demonstrated, which are the most important dopants for Si. Despite the hyperdoping, the diamond structure of Si NCs is hardly modified. There are both electrically active B and P in hyperdoped Si NCs. It is proposed that the hyperdoping is made possible mainly by the kinetics in the nonthermal plasma synthesis of Si NCs. Collision between Si NCs and B or P atoms and the binding energy of B or P at the NC surface are critical to the understanding on the differences in the doping efficiency and dopant distribution between B and P. B-hyperdoping-induced tensile stress needs to be taken into account in the investigation on the doping and oxidation of Si NCs. [ABSTRACT FROM AUTHOR]

The dispersion of silicon quantum dots (Si QDs) in water has not been established as well as that in organic solvents. It is now demonstrated that the excellent dispersion of Si QDs in water with photoluminescence (PL) quantum yields (QYs) comparable to those for hydrophobic Si QDs can be realized by combining the processes of hydrosilylation and self-assembly. Hydrogen-passivated Si QDs are initially hydrosilylated with 1-dodecence. The toluene solution of the resulting dodecyl-passivated Si QDs is mixed with the water solution of the amphiphilic polymer of Pluronic F127 to form an emulsion. Dodecyl-passivated Si QDs are encapsulated in the micelles self-assembled from F127 in the emulsion. The size of the Si-QD-containing micelles may be tuned in the range from 10 to 100 nm. Although self-assembly in the emulsion causes the PL QY of Si QDs to decrease, after a few days of storage in ambient conditions, Si QDs encapsulated in the water-dispersible micelles exhibit recovered PL QYs of ≈24% at the PL wavelength of ≈680 nm. The intensity of the PL from Si QDs encapsulated in the water-dispersible micelles is >90% of the original value after 60 min ultraviolet illumination, indicating excellent photostability. [ABSTRACT FROM AUTHOR]

Metal halide perovskite single crystals (SCs) have emerged as promising candidates for high‐performance X‐ray detectors. However, mitigating the adverse effects of thermal and interfacial stress during SC growth remains a significant challenge. In this study, the solution growth of high‐quality perovskite SCs using silicone containers through a constant‐temperature autonomous crystallization process is presented. Unlike conventional hard glass vessels, soft silicone containers significantly reduce the negative impact of thermal and interfacial stress on SC growth. Additionally, the microporous nature of silicone containers permits solvent leakage, facilitating autonomous SC growth at constant temperatures. The hydrophilic surface of the silicone further increases the interfacial nucleation barrier and enhances mass transfer, promoting the rapid growth of larger SCs. This multifaceted approach results in MAPbBr3 SCs with an exceptionally low defect density of 2.15×108 cm−3 and an optimal full‐width‐at‐half‐maximum of X‐ray diffraction rocking curves at 19.04 arcsec, consequently leading to an ultralow X‐ray detection limit of 850 pGyair s−1 for the X‐ray detectors. The superior quality of the SCs, combined with a low‐temperature flip‐chip bonding process, enables the integration of the crystals with pixelated arrays on a printed circuit board for both single‐photon detection and low‐dose X‐ray imaging. [ABSTRACT FROM AUTHOR]

Limited calculations show that monolayer (ML) chromium dichalcogenide (CrS2) has a direct bandgap and valley polarization but with a smaller bandgap than ML MoS2 and with distinct piezoelectric and ferromagnetic properties. It is highly desirable to determine an appropriate metal contact for novel two‐dimensional (2D) CrS2‐based devices. By using density functional theory (DFT), the interface between ML CrS2 and commonly used metals, including s‐electron and d‐electron metals, is studied systematically by evaluating the binding energy, Schottky barrier, orbital overlap, and tunneling barrier at the interfaces. The d‐electron metals show higher binding energy with the ML CrS2 than the s‐electron metals, which is due to the different occupancy and position of the d‐band of the metals. A strong Fermi level pinning is found in the metal–CrS2 contacts. Both n‐type and quasi p‐type phenomena for CrS2 with respect to pristine CrS2 can be produced at the CrS2 contacts with the metals. The higher overlap states between the CrS2 and Ti result in a higher minimum electron density at the Schottky interface, suggesting that Ti is the best contact among the investigated metals for use in CrS2‐based devices for efficient electron injection. The DFT results provide a guideline that is invaluable for experimentally designing novel 2D CrS2 semiconductor devices. [ABSTRACT FROM AUTHOR]

Abstract: 2D material‐based electronic devices like graphene–silicon photodetectors (Gr–Si PDs) have attracted much attention of researchers in the past few years. Due to the nature of Schottky junction, Gr–Si PDs have ultrafast response. However, responsivity of Gr–Si PDs is very low, which hinders their practical application. Low work function of Gr and poor absorbance of Si are mainly responsible for this problem. Here, a novel approach for coupling of Gr–Si PDs with 2D fractal platinum nanoparticles (Pt NPs) is demonstrated to enhance the responsivity and speed up the response of Gr–Si PDs at the same time. 2D morphology of fractal Pt NPs helps to overcome the coffee‐ring effect. Fully covered fractal Pt NPs remarkably improve the absorption of Gr–Si PDs by plasmonic effect. Responsivity of Gr–Si PDs thus is remarkably enhanced to 26 A W−1. Meanwhile, work function of Gr is improved by the physical doping of high‐work‐function Pt NPs. Therefore, the Schottky barrier of Gr–Si junction is increased, resulting in faster response. Improvement in the built‐in electric field reduces the background noise of Gr–Si PDs as well. These results indicate toward a simple and novel approach for fabricating high and fast response Gr–Si PDs. [ABSTRACT FROM AUTHOR]

Excellent dispersion of silicon quantum dots (Si QDs) in water is realized by combining the processes of hydrosilylation and self‐assembly. As described by X. D. Pi and co‐workers on page 751, the size of self‐assembled Si QD micelles can be tuned from 10 to 100 nm. This image schematically shows red‐light‐emitting Si QDs contained in the water‐dispersible micelles under ultraviolet illumination. [ABSTRACT FROM AUTHOR]