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Basal plane dislocations (BPDs) are a key factor influencing the advancement of the 4H-SiC semiconductor. In this paper, the effects of shear forces on the nucleation and movement of BPDs are revealed by employing molecular dynamics simulations. The stress–strain curves of 4H-SiC subjected to different shear forces at different temperatures are obtained. It is found that the decrease in mechanical properties of 4H-SiC is mainly due to the occurrence of dislocation. This study also delves into the complexities of dislocation entanglement and slip, unraveling the impact on the mechanical properties of 4H-SiC. Moreover, the causes of dislocation within the crystal lattice were clarified from a microscopic atomic vantage point, shedding light on the intricate mechanisms involving chemical bond rupture and regeneration. These findings not only enrich our understanding of BPDs nucleation but also provide invaluable insights for mitigating BPDs in 4H-SiC. [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]

Basal and prismatic slips induced by thermoelastic stresses during the growth of 4H-SiC are investigated by using the finite element method (FEM) and considering factors such as the crystal diameter, temperature, and off-axis angle. It is found that with the increase of the crystal diameter from 6 to 8 inches, the prismatic slip more likely occurs, leading to a higher density of basal plane dislocations (BPDs). However, the basal slip hardly changes. The temperature difference, rather than the growth temperature, is the primary factor contributing to the increase in the slip stresses. The stresses of the basal slip are significantly affected by a small off-axis angle, whereas those of the prismatic slip are not unaffected until the off-axis angle reaches 30 degrees. The mechanism for the decrease of the density of BPDs along the growth direction in an 8 inch 4H-SiC crystal is elucidated. We verify that the prismatic slip in an 8 inch 4H-SiC crystal contributes more to the BPD formation than the basal slip. [ABSTRACT FROM AUTHOR]

Neuromorphic computing offers significant advantages in addressing data redundancy and enhancing system energy efficiency. Although extensive research has been conducted on pulsed neural networks and bionic sensors, the development of artificial electronic afferent neurons for low-energy information transfer remains limited. This study introduces an artificial afferent neuron comprising a vanadium dioxide (VO2) device, capacitor and resistor. The VO2 devices exhibit stable electrically induced metal-insulator transition (MIT). Leveraging the MIT of this device, we develop an artificial afferent neuron to transform constant or sinusoidal analog signals into pulsed voltage signals. The output frequency increases with the increase of the input voltage, mimicking the faster pulse outputs of biological afferent neurons in response to stronger stimuli. [ABSTRACT FROM AUTHOR]

The primary challenge for preparing SiC crystals during the PVT process is the fractures, especially with the increase in diameter (≥8 inches). In this study, the elastic–plastic behaviors of the azimuthal component (σφφ) of primary stress causing SiC crystal fractures have been investigated to research the fracture mechanism. It is concluded that the plastic deformations caused by prismatic plane slips contribute positively to σφφ. Besides, the magnitude of the plastic component of σφφ is determined by the magnitude of resolved shear stresses (RSS) on each prismatic slip system over the same period, according to the Alexander–Haasen (AH) model. Further, the evolution of the magnitude of RSS (‖RSS‖) during the cooling process has been simulated, and the effects of the experimental conditions on the elastic–plastic behaviors of σφφ have been investigated qualitatively. Control experiments of SiC crystal growth have been carried out simultaneously, and our conclusions have been verified by the experimental results. Lastly, the maximum ‖RSS‖ can be a criterion for the fracture of SiC crystal, giving a critical value near 30 MPa under our experimental conditions. The analysis of the relation between the growth conditions and fractures guides the growth of perfect 8-inch SiC crystals. [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]

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]

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]

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]

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]

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]

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]