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High-performance neuromorphic computing (i.e., brain-like computing) is envisioned to seriously demand optoelectronically integrated artificial neural networks (ANNs) in the future. Optoelectronic synaptic devices are critical building blocks for optoelectronically integrated ANNs. For the large-scale deployment of high-performance neuromorphic computing in the future, it would be advantageous to fabricate optoelectronic synaptic devices by using advanced silicon (Si) technologies. This calls for the development of Si-based optoelectronic synaptic devices. In this work we review the use of Si materials to make optoelectronic synaptic devices, which have either two-terminal or three-terminal structures. A series of important synaptic functionalities have been well mimicked by using these Si-based optoelectronic synaptic devices. We also present the outlook of using Si materials for optoelectronic synaptic devices. [ABSTRACT FROM AUTHOR]

4H Silicon carbide (SiC) is widely recognized as one of the most advanced wide bandgap semiconductors used in the production of high-efficiency power electronic devices. Impurities play a crucial role in achieving the desired electrical properties in 4H-SiC, yet the understanding of impurities in this material remains limited. In this study, first-principles formation-energy calculations were employed to establish a comprehensive database of formation-energy diagrams for impurities in 4H-SiC. This database includes valuable information on site preference, lattice distortion, solubility, and charge transition levels (CTLs) of the impurities. The site preference for each impurity is closely related to factors such as the Fermi energy, chemical potential, and the impurity species itself. To assess the lattice distortion caused by each impurity, a comparison was made between the volume changes before and after doping. Moreover, the solubility of each impurity was determined using the detailed balance theory, thereby enabling a direct measure of the maximum impurity concentration achievable in the material. Based on the CTLs, the impurities in 4H-SiC were classified into four categories: n-type impurities, p-type impurities, amphoteric impurities, and non-electroactive impurities. This comprehensive property database for impurities in 4H-SiC provides valuable insights for tailoring the material properties through controlled doping, thereby ultimately leading to enhanced performance of power electronic devices. [ABSTRACT FROM AUTHOR]

4H Silicon carbide (4H-SiC) is widely recognized as a highly promising material for high-voltage and high-power electronic applications due to its exceptional properties. The performance of devices based on 4H-SiC is often weakened by the presence of carbon-related point defects, particularly carbon vacancies (VC). The defects of VC introduce deep-level traps (e.g., Z1/2 and EH6/7) that deteriorate device functionality. Experimental and theoretical studies on VC have led to some conflicting results about the charge states of VC, especially for the charge state ordering of EH6/7. We now employ ab initio metadynamics (META) to systematically investigate configuration space including the direction and magnitude of bond distortion and identify the most stable structures of VC. Eventually, the charge states of VC in 4H-SiC are identified. The Z1 (EH6) and Z2 (EH7) indicate transitions from acceptor (donor) levels of VC, located on the h and k sublattice sites, respectively. Z1 and Z2 demonstrate negative-U ordering, characterized by U values of −0.16 and −0.37 eV, respectively. Conversely, EH6 and EH7 display positive-U ordering, with U values of 0.16 and 0.08 eV, respectively. The current results provide insights into the properties of VC in 4H-SiC, highlighting the effectiveness of META in the exploration of complex potential energy surfaces associated with point defects in solids. [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]

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]

Abstract: The optical and electrical properties of quantum dots (QDs) have generated great interest in the use of QDs for photovoltaics. The optical coupling of QDs with currently manufactured solar cells is one of the most promising photovoltaic applications of QDs. Here we demonstrate that by spin-coating Si-QD ink at the solar cell surface the efficiency of screen-printed Si solar cells may be improved. The enhancement of solar cell efficiency results from the porous Si-QD film induced increase of light absorption. The solar cell efficiency may be further improved as the efficiency of down-shifting short-wavelength light to red light by Si-QDs increases. [Copyright &y& Elsevier]

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]

We investigate the crack-healing mechanism of 4H silicon carbide (4H-SiC) and reveal the effect of dopants on the crack-healing behavior of 4H-SiC. Vickers indentation tests and thermal annealing are utilized to generate cracks and heal cracks in 4H-SiC, respectively. High-temperature thermal annealing in the air atmosphere is found to be capable of effectively healing indentation-induced cracks and releasing indentation-induced stress in undoped 4H-SiC by the formation and viscous flow of glass phase SiO2. Nitrogen (N) doping is found to assist the atomic diffusion of 4H-SiC. The crack healing of N-doped 4H-SiC is realized by the synergy of host solid diffusion and the padding of glassy SiO2. In contrast, vanadium (V) doping hinders the viscous flow of SiO2 and results in the incomplete healing of cracks in V-doped 4H-SiC. Although the generation of cracks lowers the bending strength of 4H-SiC, the healing of cracks by the padding of glassy SiO2 is found to effectively recover the bending strength of indented 4H-SiC samples. Our work opens a pathway to design thermal processing technologies to heal the cracks and enhance the mechanical properties of 4H-SiC wafers. [ABSTRACT FROM AUTHOR]

Manganese (Mn)-doped silicon nanocrystals (Si NCs) in which Mn is in its most likely substitutional or interstitial location have been investigated. Large differences in magnetic moment are found between Si NCs doped with substitutional Mn and those doped with interstitial Mn. For all the doped Si NCs, quantum confinement affects the electronic states of majority spin more significantly than those of minority spin. As the NC size changes from 1.2 to 1.5 nm, the spin flipping of electronic transition across the NC optical gap occurs for Si NCs doped with substitutional Mn. This leads to significant changes in the optical absorption of Si NCs. It is likely that such spin-flipping also happens for larger (>1.7 nm) Si NCs doped with interstitial Mn. [ABSTRACT FROM AUTHOR]

Basal plane dislocations (BPDs) in 4H-SiC are the main reason for the degradation of bipolar devices. In the experiment, the distribution of 4-fold symmetry BPDs is observed in the etch pit density of the 4H-SiC wafer, which is inconsistent with the phenomenon of the distribution of 6-fold symmetry BPDs because of the hexagonal structure of 4H-SiC. To explain the distribution of 4-fold symmetry BPDs in the grown crystal, 3D modeling of BPDs in 4H-SiC crystals grown by the PVT method has been performed. The results show that the adoption of 4-degree off-axis seed crystals that are widely used in the industry caused the distribution of 4-fold symmetry BPDs. Moreover, the distribution of BPDs is 6-fold symmetry in on-axis grown 4H-SiC single crystals; with the increase of the off-angle, the distribution of BPDs gradually changes from 6-fold to 4-fold symmetry, and the maximum value of the BPD density is at the periphery. In the end, the strategy of growing 4H-SiC crystals with large-size seed crystals has been proposed to reduce the BPD density in the wafer and improve the quality of the substrate. The calculation results shows that the use of 220 mm seed crystals can reduce the BPD density by 15% compared with 200 mm seed crystals. [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]

Top-seeded solution growth (TSSG) is a crucial technique for producing high-quality SiC crystals, with the quality of the crystal being significantly influenced by the temperature, flow, and carbon concentration in the solution. A global numerical simulation has been performed to investigate the effects of solution height and crystal rotation in SiC crystal growth. The simulation results indicate that as the solution height increases, there are two distinct flow patterns: single-vortex pattern and double-vortex pattern. Because of the change in the flow pattern, the flow field beneath the seed crystal and free surface will undergo a significant change. The varying effects of seed crystal rotation are also demonstrated under different solution height scenarios. Increasing the rotational speed of the seed crystal will enhance the stability of the solution flow field and crystal growth rate for the single-vortex pattern. Increasing the rotational speed or solution height appropriately is beneficial for enhancing the crystal growth rate in the double-vortex pattern. Excessively high rotational speeds or solution heights, however, will reduce the rate of crystal growth. [ABSTRACT FROM AUTHOR]

This paper focuses on the generation and transformation of defects associated with subsurface damages (SSDs) in seed crystals during the physical vapor transport (PVT) growth of 4H-SiC crystals. SSDs in the 4H-SiC seed crystal were first revealed and labeled through photo-chemical etching. After 30 min of PVT growth on the 4H-SiC seed crystal, we found that ridge-like substances with a height ranging from 0.2 to 1.5 μm were formed in the region above the SSDs in the 4H-SiC seed crystal. Raman spectra analysis indicated that these ridge-like substances were 4H-SiC under tensile strain. We then continued the PVT growth to obtain a 4H-SiC boule and carried out a wafering process to obtain 4H-SiC single-crystal wafers. For the bottom wafer, above the 4H-SiC seed wafer, the full width at a half maximum (FWHM) of the rocking curves of the region above the SSDs of the seed wafer was higher than that in the other regions, indicating that the SSDs degrade the crystalline quality of 4H-SiC single crystal. The average value of the FWHM of the rocking curves of the top wafer became lower than that of the bottom wafer, indicating that the effect of SSDs gradually recovers. Molten-KOH etching indicated that SSDs in the 4H-SiC seed crystal give rise to the formation of low-angular grain boundaries (LAGBs) that consist of a wall of threading edge dislocations (TEDs) in 4H-SiC single crystals. [ABSTRACT FROM AUTHOR]

Semiconducting silicon carbide (4H-SiC) exhibits characteristics of high hardness, notable brittleness, and excellent chemical stability. The commonly employed technique for achieving an ultra-smooth and flat surface is chemical mechanical polishing (CMP), which is utilized to process the 4H-SiC surface. Wet oxidation, as an important process of chemical-mechanical polishing of single-crystal 4H-SiC, directly affects the rate and surface quality of CMP. This paper provides a comprehensive overview of the current research status of wet oxidation of single-crystal 4H-SiC. It discusses the oxidants used in the wet oxidation of 4H-SiC, such as KMnO4, H2O2, K2S2O8. Based on this, it further summarizes commonly employed oxidation-enhancement methods, including photocatalytic-assisted oxidation, electrochemical oxidation, and Fenton reaction. The mechanism of wet oxidation of single-crystal 4H-SiC is analyzed from the aspect of theoretical calculation, and the future research direction of wet oxidation of 4H-SiC is proposed. [ABSTRACT FROM AUTHOR]

High-purity semi-insulating (HPSI) 4H silicon carbide (4H-SiC) single crystals are critical semiconductor materials for fabricating GaN-based high-frequency devices. One of the major challenges for the growth of HPSI 4H-SiC single crystals is the unintentional doping of nitrogen (N) and boron (B). The addition of hydrogen has been supposed to mitigate unintentional doping. However, the underlying mechanism has not been well understood. In this work, the role of hydrogen in the growth of HPSI 4H-SiC single crystals is investigated by first-principles formation-energy calculations. We find that the addition of hydrogen significantly mitigates N doping while hardly affecting B doping. Once hydrogen is added, hydrogen may adsorb at the growing surface of 4H-SiC, leading to surface passivation. Since N can react with hydrogen to form stable NH3 (g), the chemical potential of N is reduced, so that the formation energy of N in 4H-SiC increases. Hence, the critical partial pressure of nitrogen required for the growth of HPSI 4H-SiC single crystals increases by two orders of magnitude. Moreover, we reveal that the adjustment of relative B and N doping concentrations has a substantial impact on the Fermi energy of HPSI 4H-SiC. When the doping concentration of N is higher than that of B, N interacts with carbon vacancies (VC) to pin the Fermi energy at Z1/2. When the doping concentration of B is higher than that of N, the Fermi energy is pinned at EH6/7. This explains that the resistivity of unintentionally doped HPSI 4H-SiC may vary. [ABSTRACT FROM AUTHOR]

As a semiconductor with a wide bandgap, 4H silicon carbide (4H-SiC) has considerable potential for high-temperature and high-power devices. It is widely established that p-type 4H-SiC is formed predominantly by doping Al. Although Al diffusion in 4H-SiC is often negligible at low temperatures due to the tight bonding of Al in 4H-SiC, the diffusion coefficient of Al dramatically rises when the temperature is rather high. While diffusion is the most fundamental physical processes, the diffusion mechanism of Al in 4H-SiC remains unknown. Due to the large atomic radius of Al relative to the host Si/C atoms and the fact that Al occupies the Si lattice in 4H-SiC, the diffusion of Al is typically mediated by point defects such as vacancies and self-interstitials. We now investigate the diffusion of Al in 4H-SiC using first-principles calculations and compare the activation energy of Al diffusion mediated by carbon vacancies (VC) to that of Al diffusion mediated by Si interstitials (Sii). It is found that Al diffusion is actually a Sii-mediated process, in which a nearby Sii first kicks a substitutional Al atom to an interstitial site. The kicked-out Al then spreads via interstitial sites. The diffusion coefficient is calculated, which is comparable to experimental results. [ABSTRACT FROM AUTHOR]

4H silicon carbide (4H-SiC) substrate wafers without surface / subsurface damages and low surface roughness are ideal substrates for the development of power electronics and radio frequency (RF) microwave devices, which hold great promise in applications of new energy, rail transportation, smart grid and 5G communication. The processing of 4H-SiC substrate wafers includes slicing, grinding, lapping, polishing and cleaning. However, the surface damages ( SDs) and subsurface damages (SSDs) introduced during the processing of 4H-SiC substrates affects the properties of 4H-SiC substrates and epitaxial layers, and thus the performance and reliability of devices based on 4H-SiC. This paper focuses on the formation and removal mechanisms of SDs / SSDs during the processing of 4H-SiC substrate wafers. Based on the detection method of SDs / SSDs, the morphologies and characterization approaches of SDs / SSDs are reviewed. Finally, three commonly used technologies for the removal SDs / SSDs, along with their technical advantages, development challenges and trends, are briefly discussed. [ABSTRACT FROM AUTHOR]

One of the major challenges of 4H-silicon carbide (4H-SiC) is that the preparation of low resistivity p-type 4H-SiC single crystals lags seriously behind that of low resistivity n-type 4H-SiC single crystals, hindering the development of important 4H-SiC power devices such as n-channel insulated gate bipolar transistors. In particular, the resistivity of p-type 4H-SiC single crystals prepared through the physical vapor transport technique can only be lowered to around 100 mΩ cm. One of the key causes is the incomplete ionization of the p-type dopant Al with an ionization energy ∼0.23 eV. Another factor is the compensating effect. It cannot simply assume nitrogen (N) is the sole compensatory center, since the number of the compensating center is larger than the concentration of N doping. In this work, we systematically investigate the compensation of native defects and self-compensation in Al-doped 4H-SiC. It is found that the positively charged carbon vacancies ( V C 2 + ) are also the dominant compensating centers in Al-doped 4H-SiC. When the Al concentration is in the range of 1016–1019 cm−3, the concentration of holes is lower by one order of magnitude than the Al concentration because of the compensation of V C 2 + . As the Al concentration exceeds 1020 cm−3, the concentration of holes is only in the order of magnitude of 1019 cm−3 owing to the dominant compensation of V C 2 + and supplementary self-compensation of interstitial Al ( Al i 3 + ). We propose that the passivation of V C 2 + as well as quenching is effective to enhance the hole concentration of Al-doped 4H-SiC. [ABSTRACT FROM AUTHOR]

A 4H-SiC single crystal is a substrate material for high-frequency and high-voltage power devices. Dislocation density is a fundamental criterion for evaluating the quality of 4H-SiC single crystals. 4H-SiC single crystals grown by the physical vapor transport (PVT) method generally have high dislocation density. To investigate the effect of nitrogen doping on dislocation proliferation in SiC crystals, the thermal field of the ingot during PVT growth was calculated by COMSOL Multiphysics, and thermal stress was calculated by thermal elastic theory. Finally, the dislocation density of the crystal was calculated based on the Alexander–Haasen model for inhomogeneous nitrogen doping. By comparing the calculation and experimental results, we proposed a possible value of the effective stress to evaluate the effect of a nitrogen dopant on dislocation density, which helps calculate the dislocation density in the n-type SiC. [ABSTRACT FROM AUTHOR]

The widespread use of 4H silicon carbide (4H-SiC) is just around the corner since high-power electronics based on 4H-SiC are increasingly fabricated to enable the low-carbon development of the world. Quantum technologies are also intensively explored by scrutinizing 4H-SiC as a platform for wafer-scale integration of semiconductor and quantum technologies. Given the importance of impurities and defects for any semiconductor, comprehensive and insightful understanding of impurities and defects in 4H-SiC is imperative. In this Perspective, we summarize recent experimental and theoretical advances in researches on impurities and defects in 4H-SiC after briefly reviewing the history of 4H-SiC. Impurity engineering and defect engineering for the realization of the full potential of 4H-SiC are also discussed. Challenges for the study on impurities and defects in 4H-SiC are finally outlined. [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]

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]

Since the advent of atomically flat graphene, two-dimensional (2D) layered materials have gained extensive interest due to their unique properties. The 2D layered materials prepared on epitaxial graphene/silicon carbide (EG/SiC) surface by molecular beam epitaxy (MBE) have high quality, which can be directly applied without further transfer to other substrates. Scanning tunneling microscopy and spectroscopy (STM/STS) with high spatial resolution and high-energy resolution are often used to study the morphologies and electronic structures of 2D layered materials. In this review, recent progress in the preparation of various 2D layered materials that are either monoelemental or transition metal dichalcogenides on EG/SiC surface by MBE and their STM/STS investigations are introduced. [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]

Erbium-doped silicon (Er-doped Si) materials hold great potential for advancing Si photonic devices. For Er-doped Si, the efficiency of energy transfer (ηET) from Si to Er3+ is crucial. In order to achieve high ηET, we used nonthermal plasma to synthesize Si quantum dots (QDs) hyperdoped with Er at the concentration of ~1% (i.e., ~5 × 1020 cm−3). The QD surface was subsequently modified by hydrosilylation using 1-dodecene. The Er-hyperdoped Si QDs emitted near-infrared (NIR) light at wavelengths of ~830 and ~1540 nm. An ultrahigh ηET (~93%) was obtained owing to the effective energy transfer from Si QDs to Er3+, which led to the weakening of the NIR emission at ~830 nm and the enhancement of the NIR emission at ~1540 nm. The coupling constant (γ) between Si QDs and Er3+ was comparable to or greater than 1.8 × 10−12 cm3·s−1. The temperature-dependent photoluminescence and excitation rate of Er-hyperdoped Si QDs indicate that strong coupling between Si QDs and Er3+ allows Er3+ to be efficiently excited. [ABSTRACT FROM AUTHOR]

As one of the most important steps in machine processing of 4H-silicon carbide (4H-SiC) wafers, lapping exerts an important impact on the quality of 4H-SiC substrate wafers. In this paper, the effects of diamond abrasive morphology and dispersion medium on the material removal rate and surface parameters of 4H-SiC wafers were investigated. Based on the contact between diamond abrasive and 4H-SiC wafer surface during lapping process, a simple wafer material removal rate model was derived. It is found that the abrasive morphology significantly affects the material removal rate of 4H-SiC substrate wafers, the higher the material removal rate, the smaller the total thickness variation (TTV) is obtained for 4H-SiC substrate wafers. Due to the anisotropy of C-face and Si-face of 4H-SiC, the material removal rate of C-face is higher than that of Siface. In terms of effect of the dispersion medium, the Zeta potential absolute value of the water-based slurry is high, and the abrasive is evenly dispersed. It is beneficial to control the disc temperature during the lapping process due to the large thermal conductivity of water. The Zeta potential absolute value of the glycol-based slurry is small, and the abrasive is prone to agglomeration. As the abrasive cutting depth deepens in lapping process, the material removal rate of 4H-SiC wafer increases, and the maximum value of scratch depth also aggravates accordingly. [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]

Owing to the superior properties of the wide bandgap, high carrier mobility, high thermal conductivity and high stability, 4H silicon carbide (4H-SiC) holds great promise for applications in electrical vehicles, 5G communications, and new-energy systems. Although the industrialization of 150 mm 4H-SiC substrates and epitaxial layers has been successfully achieved, the existence of a high density of dislocations is one of the most severe bottlenecks for advancing the performance and reliability of 4H-SiC based high-power and high-frequency electronics. In this topical review, the classification and basic properties of dislocations in 4H-SiC are introduced. The generation, evolution, and annihilation of dislocations during the single-crystal growth of 4H-SiC boules, the processing of 4H-SiC wafers, as well as the homoepitaxy of 4H-SiC layers are systematically reviewed. The characterization and discrimination of dislocations in 4H-SiC are presented. The effect of dislocations on the electronic and optical properties of 4H-SiC wafers and epitaxial layers, as well as the role of dislocations on the performance and reliability of 4H-SiC based power devices are finally presented. This topical review provides insight into the fundamentals and evolution of dislocations in 4H-SiC, and is expected to provide inspiration for further control of dislocations in 4H-SiC. [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]

Basal plane dislocations (BPDs) are one of the most harmful dislocations in 4H silicon carbide (4H-SiC). Understanding the nucleation of BPDs is the basis of reducing the density of BPDs in 4H-SiC. In this work, we investigate the nucleation mechanism of BPDs, as well as the effect of doping on the nucleation of BPDs in 4H-SiC using nanoindentation. It is found that the shear stress plays a dominant role in the nucleation of BPDs in undoped 4H-SiC. This indicates that the shear component of the thermal stress during the growth of 4H-SiC single crystals and that of the mechanical stress during the processing of 4H-SiC wafers both give rise to the nucleation of BPDs. Nitrogen (N) doping is found to facilitate the nucleation of BPDs and decrease the shear stress required for the nucleation of BPDs. In contrast, vanadium (V) doping hinders the nucleation of BPDs, which promotes the polymorph transition from 4H-SiC to 3C-SiC. [ABSTRACT FROM AUTHOR]

Basal plane dislocations (BPDs) are one of the most harmful dislocations in 4H silicon carbide (4H-SiC). Understanding the nucleation of BPDs is the basis of reducing the density of BPDs in 4H-SiC. In this work, we investigate the nucleation mechanism of BPDs, as well as the effect of doping on the nucleation of BPDs in 4H-SiC using nanoindentation. It is found that the shear stress plays a dominant role in the nucleation of BPDs in undoped 4H-SiC. This indicates that the shear component of the thermal stress during the growth of 4H-SiC single crystals and that of the mechanical stress during the processing of 4H-SiC wafers both give rise to the nucleation of BPDs. Nitrogen (N) doping is found to facilitate the nucleation of BPDs and decrease the shear stress required for the nucleation of BPDs. In contrast, vanadium (V) doping hinders the nucleation of BPDs, which promotes the polymorph transition from 4H-SiC to 3C-SiC. [ABSTRACT FROM AUTHOR]

• This paper reports on calculation of dislocation density in 200 mm SiC crystal grown by PVT method. • Numerical study based on a model was carried out for modeling of dislocation density. • Evolution of crystal growth interfaces were captured using a dynamic mesh technique. Physical vapor transport (PVT) is the dominant method for growing 200 mm SiC crystals, and the crystals produced by this method still have dislocations, which affect the performance of the device. In this study, the finite element analysis of 200 mm SiC crystal growth has been conducted to investigate the influencing parameters on the dislocation density. The calculations are based on the model of multiple resistance heating. The transient heat transfer for the crystal growth is first calculated. A dynamic mesh technique is then employed to consider the shape evolution of the crystal during the growth. Finally, the distributions of the internal stress and dislocation density have been calculated based on the Alexander-Haasen model. The comparison among different parameters provides guidance for reducing the thermal stress and dislocation density in the 200 mm SiC crystal growth. [ABSTRACT FROM AUTHOR]

The role of dopants on deformation and mechanical properties of 4H silicon carbide (4H-SiC) is proposed by using nanoindentation. It is found that the hardness, elastic modulus, and fracture toughness of 4H-SiC substrate wafers all decrease on the order of vanadium (V) doping, undoping, and nitrogen (N) doping. For all three types of 4H-SiC, basal plane dislocations (BPDs), threading edge dislocations, and cracks are formed during the nanoindentation. Polymorph transitions from 4H-SiC to amorphous SiC and 3C-SiC are found as the penetration depth of the indent increases from the subsurface to the deeper region. N doping is found to weaken the bond strength of 4H-SiC, which enhances the glide and piling up of BPDs in nanoindentated N-doped 4H-SiC. In contrast, V doping effectively hinders the glide of BPDs, which accumulates a high-stress field and facilitates the polymorph transition from 4H-SiC to 3C-SiC and amorphous SiC. The insight on the effects of dopants on the deformation and mechanical properties of 4H-SiC may help the design of the processing of differently doped 4H-SiC substrate wafers. [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]

The co-doping of silicon nanocrystals (Si NCs) with boron (B) and phosphorus (P) is an important means to tune the optical properties of Si NCs. However, all the previous work only concerns heavy co-doping. In this study, we carry out first-principles study on light co-doping that leads to the incorporation of dopants at the NC surface, rather than inside NCs. The size (diameter) of Si NCs currently investigated is ~2.2 nm. A Si NC without doping is in the form of SiH. It is found that the formation energy of a lightly co-doped Si NC is between those of B- and P-doped Si NCs, hardly being affected by the distance between dopants. Electron localization around P is mainly responsible for the light co-doping-induced reduction of the bandgap of Si NCs. The redshifts of excitation- and emission-energy induced by light co-doping is slightly larger than those induced by B and P doping in most cases. The band-edge radiative recombination rates of undoped, B-doped, P-doped, and lightly co-doped Si NCs have been compared. [ABSTRACT FROM AUTHOR]

Wide bandgap semiconductors are ideal materials for the application of high power density, high frequency and low- loss power electronic devices, owning to the advantages of wide bandgap, high electron saturation velocity, and high breakdown field. Attributed to the high thermal conductivity, high chemical stability and high heat resistance of silicon carbide (SiC), epitaxy of wide bandgap semiconductors on SiC is promising to exploit the advantages of wide bandgap semiconductors and improve the performance of wide bandgap semiconductor devices. Benefiting from the continuous quality-improvement and cost-reduction of SiC substrates, wide bandgap semiconductors grown on SiC substrates are ushering in an explosive growth. Epitaxy of high quality wide bandgap semiconductors on SiC substrates is critical to the performance and reliability of wide bandgap semiconductor devices. This paper summarizes the recent progress obtained on epitaxial growth of SiC, gallium nitride (GaN) and gallium oxide (Ga2O3) on SiC substrates. In addition, the prospective of wide bandgap semiconductors grown on SiC substrates for high-performance electronics are prospected. [ABSTRACT FROM AUTHOR]

SiC is considered as one of the more promising semiconductor materials because of its advantages such as wide band gap, high critical breakdown field strength, high thermal conductivity and high carrier saturation mobility. In recent years, physical vapor transport (PVT) method has made great breakthroughs in the preparation of large size and high quality SiC single crystal substrates, further promoting the application of SiC in the field of high voltage, high frequency and high temperature electronic devices. SiC powder is the raw material of PVT method to grow SiC single crystal. The purity of powder will directly affect the impurity content of SiC single crystal, so as to affect the electrical properties of SiC single crystal. The growth of high quality semi-insulating SiC single crystal is directly limited by the content of N element in the SiC powder. Therefore, the synthesis of high-purity SiC powder is the key to the growth of high-quality SiC single crystal by PVT method. This paper mainly introduces the synthesis methods and research status of high purity SiC powder, the advantages and disadvantages of gas-phase and solid-phase synthesis of high purity SiC powders are reviewed, and the development direction of high purity SiC powders synthesis in the future is put forward. [ABSTRACT FROM AUTHOR]

A low-temperature preparation process is significantly important for scalable and flexible devices. However, the serious interface defects between the normally used titanium dioxide (TiO2) electron transport layer (ETL) obtained via a low-temperature method and perovskite suppress the further improvement of perovskite solar cells (PSCs). Here, we develop a facile low-temperature chemical bath method to prepare a TiO2 ETL with tantalum (Ta) and niobium (Nb) co-doping. Systematic investigations indicate that Ta/Nb co-doping could increase the conduction band level of TiO2 and could decrease the trap-state density, boosting electron injection efficiency and reducing the charge recombination between the perovskite/ETL interface. A superior power conversion efficiency of 19.44% can be achieved by a planar PSC with a Ta/Nb co-doped TiO2 ETL, which is much higher than that of pristine TiO2 (17.60%). Our achievements in this work provide new insights on low-temperature fabrication of low-cost and highly efficient PSCs. [ABSTRACT FROM AUTHOR]

Large polymer residues introduced by the graphene transfer process is still a major obstacle limiting the integration of chemical vapor deposition (CVD)-grown graphene into next-generation electronic and photoelectronic devices. Here we use cera alba, a natural and environmental-friendly material that derives from honeycomb, as the supporting layer for ultraclean graphene transfer. The transferred graphene has a low surface roughness with a surface height fluctuation within 5 nm and an only 80.08% average sheet resistance of the polymethyl methacrylate (PMMA)-transferred graphene. Further, the ultraclean graphene is used as electrodes for the PbI2-based UV photodetector and enables a 135% improvement on responsivity. The cera alba assisted transfer method reported here could achieve clean and damage-free graphene transfer, promoting the application of CVD-grown two-dimensional (2D) materials in large-area thin-film electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

As the most fundamental material for microelectronics, silicon (Si) has bourgeoned in the past more than half a century. However, given the indirect bandgap of Si, the use of Si in optoelectronics is relatively limited due to its mediocre optical absorption and rather poor optical emission. During many years of efforts for extending the use of Si in optoelectronics Si nanocrystals (NCs) that are one type of the most important Si nanostructures have attracted significant attention owing to their remarkable electronic and optical properties. Si NCs are actually crystalline Si nanoparticles, which may be called Si quantum dots if their size is smaller than ∼10 nm. With the manipulation of the size, surface and doping of Si NCs great tunability for the light emission from Si NCs with the quantum yield of more than 60% has been realized. Based on the efficient light emission from Si NCs high-performance Si-NC light-emitting devices have been demonstrated. In the meantime, the efficient light emission from Si NCs has also been utilized for synaptic simulations in neuromorphic computing and down-shifting in photovoltaics. Broadband optical absorption ranging from the ultraviolet to mid-infrared has been recently obtained for Si NCs mainly by taking advantage of doping. This has enabled the use of Si NCs in novel solar cells, photodetectors and optoelectronic synaptic devices. The continuous improvement of the electronic and optical properties of Si NCs has made Si NCs unfading Si materials for optoelectronics, contributing to the development of Si-based optoelectronic integration. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) has important application prospects in power and radio frequency (RF) devices. However, preparing large-diameter, high-quality SiC crystals is challenging. The major technique for growing large-diameter SiC crystals is the physical vapor transport (PVT) method and the key point for adjusting conditions is to find an optimal thermal field. In this study, we propose a resistance heating method called the "3 separation heater method" with 3 heaters separated by the graphite foam insulations so that 3 important parts (the SiC source powder area, SiC seed area, and seed holder) of the growth cell can be heated independently through thermal radiation from each separated heater. Further, the insulation separators play an important role as a switch of the heat flux. The numerical studies of the thermal field design for this model are conducted with the help of the numerical simulation software COMSOL Multiphysics and its appropriate parameters are explored. At last, the relationship between the parameters and crystal growth conditions, such as the radial temperature difference (RTD) and edge temperature gradient (ETG), is expressed by the back-propagation (BP) neural network and the optimal parameter values for crystal growth conditions have been confirmed using the non-dominated sorting genetic algorithm (NSGA-II). • A resistance heating method called the "3 separation heater method" is proposed. • The effects of the optimizing parameters on the thermal field are analyzed. • The relationships between the parameters and targets are mapped by ANN algorithm. • The optimal thermal field and parameters are obtained by NSGA-II algorithm. [ABSTRACT FROM AUTHOR]

• 100 mm 4H-SiC crystal is grown by tiling four seed-crystal pieces. • The tiling method may help speed up the increase of the diameter of SiC crystals. • The polytype of the obtained crystal at the gaps are not affected by tiling. • The possible reason of dislocation distribution in different crystal orientation is discussed. Increasing the diameter and reducing the dislocation density of 4H silicon carbide (4H-SiC) single crystals are the development tendency of 4H-SiC single-crystal substrate to reduce the cost of 4H-SiC based devices. In this work, we use the growth of a 100 mm 4H-SiC single crystal as an example to demonstrate that the tiling approach is attractive to enlarge the diameter of physical-vapor-transport (PVT)-grown 4H-SiC single crystals by a single growth period. By titling four pieces of 4H-SiC seed crystals, a 100 mm 4H-SiC boule without crystal deterioration is obtained by the PVT growth method. The shape of the growth interface is convex and complete after the PVT growth, indicating the tiling gaps between neighboring seed-crystal pieces are healed after the PVT growth of 25-mm thick 4H-SiC. Wafers are obtained after sequential slicing, lapping, and chemical mechanical polishing (CMP). The high-resolution X-ray diffraction (HRXRD) rocking curve measurements of the wafer at the latest growth stage indicate that the 4H-SiC crystal is basically healed, with mesoscopic boundaries being created at the central area of the wafer. Raman spectra indicate that the healing polymorph across the tiling gaps is 4H-SiC. We find that threading edge dislocations (TEDs) dominate the dislocation types in the 4H-SiC substrate wafers at the initial, middle, and latest growth stages. As the PVT growth proceeds, the density of dislocations in 4H-SiC wafers significantly decreases, as a result of the crystalline healing. Our work opens a pathway to enlarge the size of 4H-SiC single crystals in a single PVT growth period, which paves the way for the high-efficiency growth of large-size 4H-SiC single-crystals. [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]

Abstract: Variable-energy positron annihilation spectroscopy has been applied to the study of defects in TiO2/p +-Si structures, in the as-grown state and after annealing in vacuum and in hydrogen, to investigate whether annealing (and film thickness) resulted in an increase of vacancy-type defects in the oxide films. It was found that the concentration of such defects remained unchanged after vacuum annealing for all films studied, but after H2 annealing more than doubled for 150nm-thick films, and increased by an order of magnitude for 100nm-thick films. The nature of the vacancies was examined further by measuring high-precision annihilation lines and comparing them with a reference Si spectrum. The changes observed in the ratio spectra associated with oxygen electrons suggest that the defects are oxygen vacancies, which have been shown to enhance electroluminescence from TiO2/p +-Si heterostructure-based devices. [Copyright &y& Elsevier]

Abstract: The antireflection of porous-pyramids structured silicon surface has been studied. The porous surface is formed by stain etching in HF/Fe(NO3)3 aqueous solution after textured in KOH/IPA solution. Reflectivity measurements show an overall reflectance of 4.2% for porous-pyramids textured silicon surface in the range from 400 to 900nm. An optimal etching time of 30min is obtained when both reflectivity and photo-generated carriers lifetime are considered. This technique may be probably used in the texturization process for high-efficiency silicon solar cells. [ABSTRACT FROM AUTHOR]