240 nm AlGaN-based deep ultraviolet micro-LEDs:size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated.Then,the external quantum efficiency(EQE)and light extraction efficiency(LEE)are systematically investigated by comparing size and edge effects.Here,it is revealed that the peak optical output power increases by 81.83%with the size shrinking from 50.0 to 25.0μm.Thereinto,the LEE increases by 26.21%and the LEE enhancement mainly comes from the sidewall light extraction.Most notably,transversemagnetic(TM)mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design.However,when it turns to 12.5μm sized micro-LEDs,the output power is lower than 25.0μm sized ones.The underlying mechanism is that even though protected by SiO2 passivation,the edge effect which leads to current leakage and Shockley-Read-Hall(SRH)recombination deteriorates rapidly with the size further shrinking.Moreover,the ratio of the p-contact area to mesa area is much lower,which deteriorates the p-type current spreading at the mesa edge.These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm,which will pave the way for wide applications of deep ultraviolet(DUV)micro-LEDs.

Characteristics and Mechanisms of Persistent Wet–Cold Events with Different Cold-air Paths in South China

We investigate the characteristics and mechanisms of persistent wet–cold events(PWCEs)with different types of coldair paths.Results show that the cumulative single-station frequency of the PWCEs in the western part of South China is higher than that in the eastern part.The pattern of single-station frequency of the PWCEs are“Yangtze River(YR)uniform”and“east–west inverse”.The YR uniform pattern is the dominant mode,so we focus on this pattern.The cold-air paths for PWCEs of the YR uniform pattern are divided into three types—namely,the west,northwest and north types—among which the west type accounts for the largest proportion.The differences in atmospheric circulation of the PWCEs under the three types of paths are obvious.The thermal inversion layer in the lower troposphere is favorable for precipitation during the PWCEs.The positive water vapor budget for the three types of PWCEs mainly appears at the southern boundary.

Rapid inactivation of human respiratory RNA viruses by deep ultraviolet irradiation from light-emitting diodes on a high-temperatureannealed AlN/Sapphire template

Efficient and eco-friendly disinfection of air-borne human respiratory RNA viruses is pursued in both public environment and portable usage.The AlGaN-based deep ultraviolet(DUV)light-emission diode(LED)has high practical potentials because of its advantages of variable wavelength,rapid sterilization,environmental protection,and miniaturization.Therefore,whether the emission wavelength has effects on the disinfection as well as whether the device is feasible to sterilize various respiratory RNA viruses under portable conditions is crucial.Here,we fabricate AlGaN-based DUV LEDs with different wavelength on high-temperature-annealed(HTA)AlN/Sapphire templates and investigate the inactivation effects for several respiratory RNA viruses.The AlN/AlGaN superlattices are employed between the template and upper n-AlGaN to release the strong compressive stress(SCS),improving the crystal quality and interface roughness.DUV LEDs with the wavelength of 256,265,and 278 nm,corresponding to the light output power of 6.8,9.6,and 12.5 mW,are realized,among which the 256 nm-LED shows the most potent inactivation effect in human respiratory RNA viruses,including SARS-CoV-2,influenza A virus(IAV),and human parainfluenza virus(HPIV),at a similar light power density(LPD)of~0.8 mW/cm2 for 10 s.These results will contribute to the advanced DUV LED application of disinfecting viruses with high potency and broad spectrum in a portable and eco-friendly use.

Spatio-Temporal Characteristics of Winter Cold Events in China from 1960 to 2020

Under the background of global warming, extreme cold events occur frequently. It is important to enhance the understanding of cold air patterns for forecasting cold air and reducing cold air-induced meteorological disasters. The study used the daily minimum temperatures from the National Climate Centre to classify the cold events affecting China into five different grades and the characteristics of different intensity cold events in China during the winter from 1960 to 2020 were analyzed. The results showed that there is little difference in the distribution of the frequency of general cold events from north to south, with duration longer in the north than in the south and an increase in frequency in the north in the last 60 years. The frequency of strong cold events is more in the north of China than in the south of China, and the duration is longer in the south than in the north China, with the frequency decreasing in most parts of the country. In addition to latitude, cold events frequency is closely linked to topography, with basins surrounded by high mountains being difficult to be affected by cold events, especially extreme cold events. In terms of month distribution, December was subject to the highest frequency of cold events and the longest duration of a single cold events process.

Introducing voids around the interlayer of AlN by high temperature annealing

Introducing voids into AlN layer at a certain height using a simple method is meaningful but challenging.In this work,the AlN/sapphire template with AlN interlayer structure was designed and grown by metal-organic chemical vapor deposition.Then,the AlN template was annealed at 1700℃for an hour to introduce the voids.It was found that voids were formed in the AlN layer after high-temperature annealing and they were mainly distributed around the AlN interlayer.Meanwhile,the dislocation density of the AlN template decreased from 5.26×10^(9)cm^(-2)to 5.10×10^(8)cm^(-2).This work provides a possible method to introduce voids into AlN layer at a designated height,which will benefit the design of AlN-based devices.

A New Method of Significance Testing for Correlation-Coefficient Fields and Its Application

Correlation-coefficient fields are widely used in short-term climate prediction research. The most frequently used significance test method for the correlation-coefficient field was proposed by Livezey, in which the number of significantcorrelation lattice(station) points on the correlation coherence map is used as the statistic. However, the method is based on two assumptions:(1) the spatial distribution of the lattice(station) points is uniform;and(2) there is no correlation between the physical quantities in the correlation-coefficient field. However, in reality, the above two assumptions are not valid.Therefore, we designed a more reasonable method for significance testing of the correlation-coefficient field. Specifically, a new statistic, the significant-correlation area, is introduced to eliminate the inhomogeneity of the grid(station)-point distribution, and an empirical Monte Carlo method is employed to eliminate the spatial correlation of the matrix.Subsequently, the new significance test was used for simultaneous correlation-coefficient fields between intensities of the atmospheric activity center in the Northern Hemisphere and temperature/precipitation in China. The results show that the new method is more reasonable than the Livezey method.

Effects of internal noise on the spiking regularity of a clustered Hodgkin–Huxley neuronal network

Spiking regularity in a clustered Hodgkin–Huxley（HH） neuronal network has been studied in this letter. A stochastic HH neuronal model with channel blocks has been applied as local neuronal model. Effects of the internal channel noise on the spiking regularity are discussed by changing the membrane patch size. We find that when there is no channel blocks in potassium channels, there exist some intermediate membrane patch sizes at which the spiking regularity could reach to a higher level. Spiking regularity increases with the membrane patch size when sodium channels are not blocked. Namely, depending on different channel blocking states, internal channel noise tuned by membrane patch size could have different influence on the spiking regularity of neuronal networks.

Performance of High Indium Content InGaAs p-i-n Detector: A Simulation Study

In this work, we investigate the performance of InGaAs p-i-n photodetectors with cut-off wavelengths near 2.6 μm. The influences of different substrate materials on the optoelectronic properties of InGaAs detector are also compared and discussed. GaAs-based device shows a significant enhancement in detector with a better performance for a InGaAs photodetector compared to InP- based device. In addition, our results show that the device performance is influenced by the conduction band offset. This work proves that InAlAs/InGaAs/GaAs structure is a promising candidate for high performance detector with optimally tuned band gap.

Cryogenian and Ediacaran integrative stratigraphy,biotas,and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas

The complex evolutionary history of the Qinghai-Tibetan Plateau and its surrounding areas,including the continental blocks(Indian,Lhasa,South Qiangtang,Tarim,Olongbuluk,Central Qilian,Alxa,North China,Yangtze,Central Iran and Oman)and the orogenic belts between them,has long been the frontier in Earth science research.The Cryogenian and Ediacaran strata are extensively distributed in these blocks.Specifically,relatively complete Cryogenian and Ediacaran successions have been discovered in Oman,Indian,Yangtze,and Tarim blocks,while only the Ediacaran successions have been reported in Iran,the South Qiangtang,Central Qilian,Alxa,and North China blocks.Based on previous studies together with the integration of new materials and advancement obtained through the Second Tibetan Plateau Scientific Expedition and Research,this review aims to synthesize a correlative stratigraphic framework of the representative Cryogenian and Ediacaran sequences from the Qinghai-Tibetan Plateau and its surrounding areas.Furthermore,the Cryogenian and Ediacaran biotas and major geological events in these areas are comprehensively discussed in aspects of current research status.The results indicate that,in general,Ediacaran fossils of each area exhibit distinct features in preservation and assemblage composition,but the typical late Ediacaran fossils Cloudina and Shaanxilithes have been reported from most of these areas.In addition to the two global Cryogenian glaciations,late Ediacaran glaciogenic deposits are extensively recorded in the areas within and around the northern QinghaiTibetan Plateau(including the North China,Alxa,Central Qilian,Olongbuluk,and Tarim blocks,and the North Qilian Accretionary Belt),as well as central and southern Iran.However,further research is required to determine the age,distribution,and origin of these late Ediacaran glaciogenic deposits.Meanwhile,the middle Ediacaran DOUNCE/Shuram Excursion is widely documented in the Qinghai-Tibetan Plateau and its surrounding areas.The available data show that,after the break-up of the Rodinia supercontinent,most of the continental blocks in the areas were located along the northern margin of East Gondwana and a few(such as North China)were located between the Gondwana and Laurentia.In general,the paleogeographic evolution of most of these blocks during the Cryogenian and Ediacaran remains disputatious,necessitating further research to resolve the controversies surrounding their paleogeographic reconstruction models during this critical time interval.

Nonvolatile and reconfigurable two-terminal electro-optic duplex memristor based on Ⅲ-nitride semiconductors

With the fast development of artificial intelligence(AI),Internet of things(IOT),etc,there is an urgent need for the technology that can efficiently recognize,store and process a staggering amount of information.The AlScN material has unique advantages including immense remnant polarization,superior temperature stability and good latticematch to other III-nitrides,making it easy to integrate with the existing advanced III-nitrides material and device technologies.However,due to the large band-gap,strong coercive field,and low photo-generated carrier generation and separation efficiency,it is difficult for AlScN itself to accumulate enough photo-generated carriers at the surface/interface to induce polarization inversion,limiting its application in in-memory sensing and computing.In this work,an electro-optic duplex memristor on a GaN/AlScN hetero-structure based Schottky diode has been realized.This twoterminal memristor shows good electrical and opto-electrical nonvolatility and reconfigurability.For both electrical and opto-electrical modes,the current on/off ratio can reach the magnitude of 104,and the resistance states can be effectively reset,written and long-termly stored.Based on this device,the“IMP”truth table and the logic“False”can be successfully reproduced,indicating the huge potential of the device in the field of in-memory sensing and computing.

Directional surface plasmon polariton scattering by single low-index dielectric nanoparticles:simulation and experiment

Directionally scattered surface plasmon polaritons(SPPs)promote the efficiency of plasmonic devices by limiting the energy within a given spatial domain,which is one of the key issues to plasmonic devices.Benefitting from the magnetic response induced in high-index dielectric nanoparticles,unidirectionally scattered SPPs have been achieved via interference between electric and magnetic resonances excited in the particles.Yet,as the magnetic response in low-index dielectric nanoparticles is too weak,the directionally scattered SPPs are hard to detect.In this work,we demonstrate forward scattered SPPs in single low-index polystyrene(PS)nanospheres.We numerically illustrate the excitation mechanism of plasmonic induced electric and magnetic multipole modes,as well as their contributions to forward SPP scattering of single PS nanospheres.We also simulate the SPP scattering field distribution obtaining a forward-to-backward scattering intensity ratio of 50.26:1 with 1μm PS particle.Then the forward scattered SPPs are experimentally visualized by Fourier transforming the real-space plasmonic imaging to k-space imaging.The forward scattered SPPs from low-index dielectric nanoparticles pave the way for SPP direction manipulation by all types of nanomaterials.

Tunable piezoelectric and ferroelectric responses of Al1-xScxN:The role of atomic arrangement

In this study,we present first-principles investigations of the atomic structure of Al_(1-x)Sc_(x)N and its influence on its piezoelectric and ferroelectric properties.The unbiased structure searching revealed that Al_(1-x)Sc_(x)N with phase separation feature,where Al N and Sc N form a layered structure with different symmetries,is more stable than the corresponding wurtzite structure.The piezoelectric response of Al_(1-x)Sc_(x)N is strongly dependent on the atomic arrangements;in particular,Al_(0.5)Sc_(0.5)N with a wurtzite structure exhibits a large positive e33of 4.79 C/m^(2),whereas Al_(0.5)Sc_(0.5)N with a phase separation structure exhibits a negative e33of-0.67 C/m^(2).Moreover,the ferroelectric switching of Al_(1-x)Sc_(x)N demonstrated two distinct pathways for the wurtzite and phase separation structures,and the spontaneous polarization thus calculated exhibits entirely different values.Accordingly,we demonstrated that Al_(1-x)Sc_(x)N with a phase separation structure exhibits a low polarization switching barrier of 0.15 e V/f.u.and a large spontaneous polarization of-0.77 C/m^(2);thus,it can serve as a novel Al_(1-x)Sc_(x)N-based ferroelectric material.As the dipoles in Al_(1-x)Sc_(x)N with a phase separation structure are localized in the AlN region,they are individually switchable at no domain wall energy cost and are stable against extrinsic effects.

Electrically pumped optomechanical beam GaN-LED accelerometer based on the quantum-confined Stark effect

Micro-nano optomechanical accelerometers are widely used in automobile,aerospace,and other industrial applications.Here,we fabricate mechanical sensing components based on an electrically pumped GaN light-emitting diode(LED)with a beam structure.The relationship between the blueshift of the electroluminescence(EL)spectra and the deformation of the GaN beam structure based on the quantum-confined Stark effect(QCSE)of the InGaN quantum well(QW)structure is studied by introducing an extra mass block.Under the equivalent acceleration condition,in addition to the elastic deformation of GaN-LED,a direct relationship exists between the LED’s spectral shift and the acceleration’s magnitude.The extra mass block(gravitational force:7.55×10^(-11)N)induced blueshift of the EL spectra is obtained and shows driven current dependency.A polymer sphere(PS;gravitational force:3.427×10^(-12)N)is placed at the center of the beam GaN-LED,and a blueshift of 0.061 nm is observed in the EL spectrum under the injection current of 0.5 mA.The maximum sensitivity of the acceleration is measured to be 0.02 m∕s^(2),and the maximum measurable acceleration is calculated to be 1.8×10^(6)m∕s^(2).It indicates the simultaneous realization of high sensitivity and a broad acceleration measurement range.This work is significant for several applications,including light force measurement and inertial navigation systems with high integration ability.

Quantum engineering of non-equilibrium efficient p-doping in ultra-wide band-gap nitrides

Ultra-wide band-gap nitrides have huge potential in micro-and optoelectronics due to their tunable wide band-gap,high breakdown field and energy density,excellent chemical and thermal stability.However,their application has been severely hindered by the low p-doping efficiency,which is ascribed to the ultrahigh acceptor activation energy originated from the low valance band maximum.Here,a valance band modulation mode is proposed and a quantum engineering doping method is conducted to achieve high-efficient p-type ultra-wide band-gap nitrides,in which GaN quantum-dots are buried in nitride matrix to produce a new band edge and thus to tune the dopant activation energy.By non-equilibrium doping techniques,quantum engineering doped AIGaN:Mg with Al content of 60%is successfully fabricated.The Mg activation energy has been reduced to about 21 meV,and the hole concentration reaches higher than 10^(18)cm^(-3)at room temperature.Also,similar activation energies are obtained in AIGaN with other Al contents such as 50%and 70%;indicating the universality of the quantum engineering doping method.Moreover,deep-ultraviolet light-emission diodes are fabricated and the improved performance further demonstrates the validity and merit of the method.With the quantum material growth techniques developing,this method would be prevalently available and tremendously stimulate the promotion of ultra-wide band-gap semiconductor-based devices.

Room-temperature continuous-wave electrically pumped InGaN/GaN quantum well blue laser diode directly grown on Si

Current laser-based display and lighting applications are invariably using blue laser diodes(LDs)grown on freestanding GaN substrates,which are costly and smaller in size compared with other substrate materials.1–3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride(InGaN/GaN)multiple quantum well(MQW)structure can substantially reduce the cost of blue LDs and boost their applications.To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure,a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/AlxGa1−xN buffer layers between GaN and Si substrate.Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers(QBs)and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer.A continuous-wave(CW)electrically pumped InGaN/GaN quantum well(QW)blue(450 nm)LD grown on Si was successfully demonstrated at room temperature(RT)with a threshold current density of 7.8 kA/cm^(2).

Construction of van der Waals substrates for largely mismatched heteroepitaxy systems using first principles

The high density of defects induced by large strains in largely mismatched heteroepitaxy systems, such as AlN and GaN, because of the large lattice and thermal mismatch between substrates and epilayers is the bottleneck problem. Graphene-assisted van der Waals(vdW) heteroepitaxy offers a new opportunity to resolve this problem. However, it suffers from the difficulty of nucleation. Here we theoretically assess the effects of five 2D materials for vdW heteroepitaxy of AlN and GaN, including graphene, hBN, MoS2, gC3N, and gC3N4, and provide physical insights using first-principle calculations. MoS2 and gC3N exhibit significant potential to overcome the shortcomings of graphene owing to their appropriate binding strengths and Al(or Ga)diffusion barriers. Moreover, the interface behavior between the epilayers and the substrates are carefully analyzed. Our findings are helpful not only for obtaining high-quality AlN and GaN films but also for developing new criterions to discover effective 2D materials for vdW heteroepitaxy.

Polarization-enhanced AlGaN solar-blind ultraviolet detectors

AlGaN solar-blind ultraviolet detectors have great potential in many fields,although their performance has not fully meet the requirements until now.Here,we proposed an approach to utilize the inherent polarization effect of AlGaN to improve the detector performance.AlGaN heterostructures were designed to enhance the polarization field in the absorption layer,and a high built-in field and a high electron mobility conduction channel were formed.As a result,a high-performance solar-blind ultraviolet detector with a peak responsivity of 1.42 A/W at 10 V was achieved,being 50 times higher than that of the nonpolarization-enhanced one.Moreover,an electron reservoir structure was proposed to further improve the performance.A higher peak responsivity of 3.1 A/W at30 V was achieved because the electron reservoir structure could modulate the electron concentration in the conduction channel.The investigation presented here provided feasible approaches to improve the performance of the AlGaN detector by taking advantage of its inherent property.

Full-duplex light communication with a monolithic multicomponent system

A monolithic multicomponent system is proposed and implemented on a III-nitride-on-silicon platform,whereby two multiple-quantum-well diodes(MQW-diodes)are interconnected by a suspended waveguide.Both MQW-diodes have an identical low-In-content InGaN/Al0.10Ga0.90N MQW structure and are produced by the same fabrication process flow.When appropriately biased,both MQW-diodes operate under a simultaneous emission-detection mode and function as a transmitter and a receiver at the same time,forming an in-plane full-duplex light communication system.Real-time full-duplex audio communication is experimentally demonstrated using the monolithic multicomponent system in combination with an external circuit.

Review on the Progress of AlGaN-based Ultraviolet Light-Emitting Diodes

AlGaN-based materials have exhibited considerable potential for fabricating ultraviolet(UV)light-emitting diodes(LEDs)owing to their direct,wide,and adjustable energy bandgap.AlGaN-based devices have extensive appli-cability owing to their stable physico-chemical properties.With decades of research effort,significant progress has been achieved in enhancing the working efficiency of AlGaN-based LEDs by optimizing the crystalline qual-ity,doping efficiency,and device design.In this review,methods to obtain high-quality AlGaN-based materials,achieve high doping efficiency,and design UV-LED structures are summarized and discussed.Finally,the issues that need to be addressed in AlGaN-based UV-LED devices are highlighted.

Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors

With the increasing demand for high integration and multi-color photodetection for both military and civilian applications, the research of multi-wavelength detectors has become a new research hotspot. However, current research has been mainly in visible dual-or multi-wavelength detectors, while integration of both visible light and ultraviolet(UV) dual-wavelength detectors has rarely been studied. In this work, large-scale and high-quality monolayer MoS2 was grown by the chemical vapor deposition method on transparent free-standing GaN substrate. Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors was demonstrated using common semiconductor fabrication technologies such as photolithography, argon plasma etching, and metal deposition. High performance of a 280 nm and 405 nm dual-wavelength photodetector was realized.The responsivity of the UV detector reached 172.12 A/W, while that of the visible detector reached 17.5 A/W.Meanwhile, both photodetectors achieved high photocurrent gain, high external quantum efficiency, high normalized detection rate, and low noise equivalent power. Our study extends the future application of dual-wavelength detectors for image sensing and optical communication.