Identification of subsurface damage of 4H-SiC wafers by combining photo-chemical etching and molten-alkali etching

In this work,we propose to reveal the subsurface damage(SSD)of 4H-SiC wafers by photo-chemical etching and identify the nature of SSD by molten-alkali etching.Under UV illumination,SSD acts as a photoluminescence-black defect.The selective photo-chemical etching reveals SSD as the ridge-like defect.It is found that the ridge-like SSD is still crystalline 4H-SiC with lattice distortion.The molten-KOH etching of the 4H-SiC wafer with ridge-like SSD transforms the ridge-like SSD into groove lines,which are typical features of scratches.This means that the underlying scratches under mechanical stress give rise to the formation of SSD in 4H-SiC wafers.SSD is incorporated into 4H-SiC wafers during the lapping,rather than the chemical mechanical polishing(CMP).

Discrimination of dislocations in 4H-SiC by inclination angles of molten-alkali etched pits

Discrimination of dislocations is critical to the statistics of dislocation densities in 4H silicon carbide(4H-SiC),which are routinely used to evaluate the quality of 4H-SiC single crystals and homoepitaxial layers.In this work,we show that the inclination angles of the etch pits of molten-alkali etched 4H-SiC can be adopted to discriminate threading screw dislocations(TSDs),threading edge dislocations(TEDs)and basal plane dislocations(BPDs)in 4H-SiC.In n-type 4H-SiC,the inclination angles of the etch pits of TSDs,TEDs and BPDs in molten-alkali etched 4H-SiC are in the ranges of 27°−35°,8°−15°and 2°−4°,respectively.In semi-insulating 4H-SiC,the inclination angles of the etch pits of TSDs and TEDs are in the ranges of 31°−34°and 21°−24°,respectively.The inclination angles of dislocation-related etch pits are independent of the etching duration,which facilitates the discrimination and statistic of dislocations in 4H-SiC.More significantly,the inclination angle of a threading mixed dislocations(TMDs)is found to consist of characteristic angles of both TEDs and TSDs.This enables to distinguish TMDs from TSDs in 4H-SiC.

Assessing the effect of hydrogen on the electronic properties of 4H-SiC

As a common impurity in 4 H silicon carbide(4 H-Si C),hydrogen(H)may play a role in tuning the electronic properties of 4 H-Si C.In this work,we systemically explore the effect of H on the electronic properties of both n-type and p-type4 H-Si C.The passivation of H on intrinsic defects such as carbon vacancies(V_(Si) )and silicon vacancies(V_(Si)) in 4 H-Si C is also evaluated.We find that interstitial H at the bonding center of the Si-C bond(H_(i)^(bc)) and interstitial H at the tetrahedral center of Si(H_(i)^(bc)) dominate the defect configurations of H in p-type and n-type 4 H-Si C,respectively.In n-type 4 H-Si C,the compensation of HSi-te iis found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4 H-Si C.The compensation of Hbc iis negligible compared to that of V_(Si)on the p-type doping of Al-doped 4 H-Si C.We further examine whether H can passivate VCand improve the carrier lifetime in 4 H-Si C.It turns out that nonequilibrium passivation of VCby H is effective to eliminate the defect states of V_(Si),which enhances the carrier lifetime of moderately doped 4 H-Si C.Regarding the quantum-qubit applications of 4 H-Si C,we find that H can readily passivate V_(Si)during the creation of V_(Si)centers.Thermal annealing is needed to decompose the resulting V_(Si)-n H(n=1-4)complexes and promote the uniformity of the photoluminescence of V_(Si)arrays in 4 H-Si C.The current work may inspire the impurity engineering of H in 4 H-Si C.

A new algorithm based on C-V characteristics to extract the epitaxy layer parameters for power devices with the consideration of termination

Doping concentration and thickness of an epitaxy layer are the most essential parameters for power devices.The conventional algorithm extracts these two parameters by calculating the doping profile from its capacitance-voltage(C-V)characteristics.Such an algorithm treats the device as a parallel-plane junction and ignores the influence of the terminations.The epitaxy layer doping concentration tends to be overestimated and the thickness underestimated.In order to obtain the epitaxy layer parameters with higher accuracy,a new algorithm applicable for devices with field limited ring(FLR)terminations is proposed in this paper.This new algorithm is also based on the C-V characteristics and considers the extension manner of the depletion region under the FLR termination.Such an extension manner depends on the design parameters of the FLR termination and is studied in detail by simulation and modeling.The analytical expressions of the device C-V characteristics and the effective doping profile are derived.More accurate epitaxy layer parameters can be extracted by fitting the effective doping profile expression to the C-V doping profile calculated from the C-V characteristics.The relationship between the horizontal extension width and the vertical depth of the depletion region is also acquired.The credibility of the new algorithm is verified by experiments.The applicability of our new algorithm to FLR/field plate combining terminations is also discussed.Our new algorithm acts as a powerful tool for analyses and improvements of power devices.

Hyperdoped silicon:Processing,properties,and devices

Hyperdoping that introduces impurities with concentrations exceeding their equilibrium solubility has been attract-ing great interest since the tuning of semiconductor properties increasingly relies on extreme measures.In this review we fo-cus on hyperdoped silicon(Si)by introducing methods used for the hyperdoping of Si such as ion implantation and laser dop-ing,discussing the electrical and optical properties of hyperdoped bulk Si,Si nanocrystals,Si nanowires and Si films,and present-ing the use of hyperdoped Si for devices like infrared photodetectors and solar cells.The perspectives of the development of hy-perdoped Si are also provided.

Protection of isolated and active regions in AlGaN/GaN HEMTs using selective laser annealing

AlGaN/GaN high-electron-mobility transistors with Au-free ohmic contacts are fabricated by selective laser annealing and conventional rapid thermal annealing.The current transport mechanism of ohmic contacts is investigated.High-temperature annealing can be avoided in the isolated region and the active region by selective laser annealing.The implanted isolation leakage current is maintained 10^(-6) mA/mm even at 1000 V after selective laser annealing.On the contrary,high-temperature annealing will cause obvious degradation of the isolation.The morphology of AlGaN surface is measured by atomic force microscope.No noticeable change of the AlGaN surface morphology after selective laser annealing,while the root-mean-square roughness value markedly increases after rapid thermal annealing.The smaller frequency dispersion of capacitance-voltage characteristics confirms the lower density of surface states after selective laser annealing.Thus,dynamic on-resistance is effectively suppressed.

Regulating surface electron structure of PtNi nanoalloy via boron doping for high‐current‐density Li‐O2 batteries with low overpotential and long‐life cyclability

The realization of high‐efficiency,reversible,stable,and safe Li‐O2 batteries is severely hindered by the large overpotential and side reactions,especially at high rate conditions.Therefore,rational design of cathode catalysts with high activity and stability is crucial to overcome the terrible issues at high current density.Herein,we report a surface engineering strategy to adjust the surface electron structure of boron(B)‐doped PtNi nanoalloy on carbon nanotubes(PtNiB@CNTs)as an efficient bifunctional cathodic catalyst for high‐rate and long‐life Li‐O2 batteries.Notably,the Li‐O2 batteries assembled with as‐prepared PtNiB@CNT catalyst exhibit ultrahigh discharge capacity of 20510 mA·h/g and extremely low overpotential of 0.48 V at a high current density of 1000 mA/g,both of which outperform the most reported Pt‐based catalysts recently.Meanwhile,our Li‐O2 batteries offer excellent rate capability and ultra‐long cycling life of up to 210 cycles at 1000 mA/g under a fixed capacity of 1000 mA·h/g,which is two times longer than those of Pt@CNTs and PtNi@CNTs.Furthermore,it is revealed that surface engineering of PtNi nanoalloy via B doping can efficiently tailor the electron structure of nanoalloy and optimize the adsorption of oxygen species,consequently delivering excellent Li‐O2 battery performance.Therefore,this strategy of regulating the nanoalloy by doping nonmetallic elements will pave an avenue for the design of high‐performance catalysts for metal‐oxygen batteries.

Enantioselective Reductive Cross-Coupling of Aryl/Alkenyl Bromides with Benzylic Chlorides via Photoredox/Biimidazoline Nickel Dual Catalysis

The asymmetric reductive arylation and alkenylation of benzylic chloride under photoredox/nickel dual catalysis using chiral bimidazoline(Bilm)ligand is reported to access 1,1-diaryl alkanes and aryl allylic compounds with good yield as well as stereo-and enantioselectivities.This protocol uses more commercially available and less expensive C(sp^(2))-Br as the electrophile coupling partner.A primary result using alkenyl chloride and alkyl chloride is also reported.Various functional groups are tolerated and the applications of this method are investigated by late-stage functionalization and gram-scale reaction.

小鼠早期胚胎发育中组蛋白修饰H2AK119ub1与H3K27me3的全基因组解偶联

多梳家族蛋白(Polycomb group proteins)是发育过程中的关键染色质调控因子.组蛋白H2AK119位点的单泛素化修饰(H2AK119ub1,H2Aub)和组蛋白H3K27位点的三甲基化修饰(H3K27me3)分别是由多梳抑制复合体1(Polycomb Repressive Complex,PRC1)和2(PRC2)催化.由于PRC1和PRC2可以相互招募,H2Aub和H3K27me3这两种组蛋白修饰在基因组上表现出很大程度的重合与偶联.然而,它们是否能够独立存在并发挥功能仍然没有定论.本研究新开发了一种超灵敏微量染色质免疫共沉淀测序技术CATCH-Seq,并利用该技术发现H2Aub和H3K27me3在小鼠早期胚胎发育过程中呈现出全基因组范围的解偶联现象,即经典的多梳家族蛋白靶基因上仅有H2Aub的富集,而非经典基因印记区域仅有H3K27me3的富集.这些结果表明,H2Aub能够在早期胚胎发育阶段缺失H3K27me3的情况下负责沉默未来具有二价启动子的基因,但并不直接参与H3K27me3依赖的非经典印记基因的沉默.该工作揭示了H2Aub和H3K27me3这两种组蛋白修饰在哺乳动物早期胚胎发育中相互区别且独立的分布和功能.

Perovskite-Enhanced Silicon-Nanocrystal Optoelectronic Synaptic Devices for the Simulation of Biased and Correlated Random-Walk Learning

Silicon-(Si-)based optoelectronic synaptic devices mimicking biological synaptic functionalities may be critical to the development of large-scale integrated optoelectronic artificial neural networks.As a type of important Si materials,Si nanocrystals(NCs)have been successfully employed to fabricate optoelectronic synaptic devices.In this work,organometal halide perovskite with excellent optical asborption is employed to improve the performance of optically stimulated Si-NC-based optoelectronic synaptic devices.The improvement is evidenced by the increased optical sensitivity and decreased electrical energy consumption of the devices.It is found that the current simulation of biological synaptic plasticity is essentially enabled by photogating,which is based on the heterojuction between Si NCs and organometal halide perovskite.By using the synaptic plasticity,we have simulated the well-known biased and correlated random-walk(BCRW)learning.

Neural symbolic reasoning with knowledge graphs:Knowledge extraction,relational reasoning,and inconsistency checking

Knowledge graphs(KGs)express relationships between entity pairs,and many real-life problems can be formulated as knowledge graph reasoning(KGR).Conventional approaches to KGR have achieved promising performance but still have some drawbacks.On the one hand,most KGR methods focus only on one phase of the KG lifecycle,such as KG completion or refinement,while ignoring reasoning over other stages,such as KG extraction.On the other hand,traditional KGR methods,broadly categorized as symbolic and neural,are unable to balance both scalability and interpretability.To resolve these two problems,we take a more comprehensive perspective of KGR with regard to the whole KG lifecycle,including KG extraction,completion,and refinement,which correspond to three subtasks:knowledge extraction,relational reasoning,and inconsistency checking.In addition,we propose the implementation of KGR using a novel neural symbolic framework,with regard to both scalability and interpretability.Experimental results demonstrate that our proposed methods outperform traditional neural symbolic models.

Pathological Networks Involving Dysmorphic Neurons in Type ⅡFocal Cortical Dysplasia

Focal cortical dysplasia(FCD)is one of the most common causes of drug-resistant epilepsy.Dysmorphic neurons are the major histopathological feature of typeⅡFCD,but their role in seizure genesis in FCD is unclear.Here we performed whole-cell patch-clamp recording and morphological reconstruction of cortical principal neurons in postsurgical brain tissue from drug-resistant epilepsy patients.Quantitative analyses revealed distinct morphological and electrophysiological characteristics of the upper layer dysmorphic neurons in typeⅡFCD,including an enlarged soma,aberrant dendritic arbors,increased current injection for rheobase action potential firing,and reduced action potential firing frequency.Intriguingly,the upper layer dysmorphic neurons received decreased glutamatergic and increased GABAergic synaptic inputs that were coupled with upregulation of the Na^(+)-K^(+)-Cl^(−)cotransporter.In addition,we found a depolarizing shift of the GABA reversal potential in the CamKⅡ-cre::PTENflox/flox mouse model of drug-resistant epilepsy,suggesting that enhanced GABAergic inputs might depolarize dysmorphic neurons.Thus,imbalance of synaptic excitation and inhibition of dysmorphic neurons may contribute to seizure genesis in typeⅡFCD.

Synaptic devices based on semiconductor nanocrystals

To meet a growing demand for information processing,brain-inspired neuromorphic devices have been intensively studied in recent years.As an important type of neuromorphic device,synaptic devices have attracted strong attention.Among all the kinds of materials explored for the fabrication of synaptic devices,semiconductor nanocrystals(NCs)have become one of the preferred choices due to their excellent electronic and optical properties.In this review,we first introduce the research background of synaptic devices based on semiconductor NCs and briefly present the basic properties of semiconductor NCs.Recent developments in the field of synaptic devices based on semiconductor NCs are then discussed according to the materials employed in the active layers of the devices.Finally,we discuss existing problems and challenges of synaptic devices based on semiconductor NCs.