Relationship of annealing time and intrinsic defects of unintentionally doped 4H-SiC

With annealing temperature kept at 1573 K, the effects of annealing time on stability of the intrinsic defects in epitaxial unintentionally doped 4H-SiC prepared by low pressure chemical vapour deposition have been studied by electron spin resonance （ESR） and low temperature photoluminescence. This paper reports the results shown that annealing time has an important effect on the intrinsic defects in unintentionally doped 4H-SiC when annealing temperature kept at 1573 K. When the annealing time is less than 30 min, the intensity of ESR and photoluminescence is increasing with annealing time prolonged, and reaches the maximum when annealing time is 30 min. Then the intensity of ESR and photoluminescence is rapidly decreased with the longer annealing time, and much less than that of as-grown 4H-SiC when annealing time is 60 min, which should be related with the interaction among the intrinsic defects during the annealing process.

Local Tuning of the Surface Potential in Silicon Carriers by Ion Beam Induced Intrinsic Defects

The immobilization of biomaterials on a carrier is the first step for many different applications in life science and medicine. The usage of surface-near electrostatic forces is one possible approach to guide the charged biomaterials to a specific location on the carrier. In this study, we investigate the effect of intrinsic defects on the surface potential of silicon carriers in the dark and under illumination by means of Kelvin probe force microscopy. The intrinsic defects were introduced into the carrier by local, stripe-patterned ion implantation of silicon ions with a fluence of 3 × 1013 Si ions/cm2 and 3 × 1015 Si ions/cm2 into a p-type silicon wafer with a dopant concentration of 9 × 1015 B/cm3. The patterned implantation allows a direct comparison between the surface potential of the silicon host against the surface potential of implanted stripes. The depth of the implanted silicon ions in the target and the concentration of displaced silicon atoms was simulated using the Stopping and Range of Ions in Matter (SRIM) software. The low fluence implantation shows a negligible effect on the measured Kelvin bias in the dark, whereas the large fluence implantation leads to an increased Kelvin bias, i.e. to a smaller surface work function according to the contact potential difference model. Illumination causes a reduced surface band bending and surface potential in the non-implanted regions. The change of the Kelvin bias in the implanted regions under illumination provides insight into the mobility and lifetime of photo-generated electron-hole pairs. Finally, the effect of annealing on the intrinsic defect density is discussed and compared with atomic force microscopy measurements on the 2nd harmonic. In addition, by using the Baumgart, Helm, Schmidt interpretation of the measured Kelvin bias, the dopant concentration after implantation is estimated.

ESR characters of intrinsic defects in epitaxial semi-insulating 4H-SiC illuminated by Xe light

The intrinsic defects in epitaxial semi-insulating 4H-SiC prepared by low pressure chemical vapor deposition （LPCVD） are studied by electron spin resonance （ESR） with different illumination times. The results show that the intrinsic defects in as-grown 4H-SiC consist of carbon vacancy （Vc） and complex-compounds-related Vc. There are two other apexes presented in the ESR spectra after illumination by Xe light, which are likely to be Vsi and VcCsi. Illumination time changes the relative density of intrinsic defects in 4H-SiC; the relative density of intrinsic defects reaches a maximum when the illumination time is 2.5 min, and the ratio of Vc to complex compounds is minimized simultaneously. It can be deduced that some Vsi may be transformed to the complex-compounds-related Vc because of the illumination.

First-principles calculation on the concentration of intrinsic defects in 4H-SiC

Based on the first-principles pseudopotentials and the plane wave energy band method,the supercells of perfect crystal 4H-SiC and those with intrinsic defects VC,VSi,VC-C and VC-Si were calculated.Ignoring the atomic relaxations,the results show that the formation energy of intrinsic defects is ranked,from low to high,as VC,VC-C,VSi to VSi-Si at 0 K.The equilibrium concentration of each intrinsic defect can be deduced from the formation energy of each intrinsic defect.The concentration ranks,from high to low,as VC,VC-C,VSi,VSi-Si,which is in accordance with the ESR and PL results.The stabilizing process of metastable defects VSi converting to VC-C was explained by formation energy.

Intrinsic defects of nonprecious metal electrocatalysts for energy conversion: Synthesis, advanced characterization, and fundamentals

With the depletion of fossil fuels and environmental pollution, energy storage and conversion have become the focus of current research. Water splitting and fuel cell technologies have made outstanding contributions to energy conversion. However, the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have slow kinetics, which limit the capacity of fuel cells. It is of great significance to develop catalysts for the OER and ORR and continuously improve their catalytic performance. Many studies have shown that intrinsic defects, especially vacancies (anion and cation vacancies), can effectively improve the efficiency of electrochemical energy storage and conversion. The introduction of intrinsic defects can generally expose more active sites, enhance conductivity, adjust the electronic state, and promote ion diffusion, thereby enhancing the catalytic performance. This review comprehensively summarizes the latest developments regarding the effects of intrinsic defects on the performance of non-noble metal electrocatalysts. According to the type of intrinsic defect, this article reviews in detail the regulation mechanism, preparation methods and advanced characterization techniques of intrinsic defects in different materials (oxides, non-oxides, etc.). Then, the current difficulties and future development of intrinsic defect regulation are analyzed and discussed thoroughly. Finally, the prospect of intrinsic defects in the field of electrochemical energy storage is further explored.

Intrinsic defects and the influences on electrical transport properties in quaternary diamond-like compounds:Cd_(2)Cu_(3)In_(3)Te_(8)as an example

Over the years,the fact that the quaternary diamond-like thermoelectric materials show much lower carrier mobilities than ternary compounds remains mysterious.In this work,by adopting first-principles defect chemistry and electrical transport calculations,the fundamental origin of the difference on carrier mobility between quaternary and ternary diamond-like compounds is addressed,exemplified by Cd_(2)Cu_(3)In_(3)Te_(8).The results of defect chemistry show that the main intrinsic defects in quaternary compound Cd_(2)Cu_(3)In_(3)Te_(8) are substitutional defects,i.e.,CdIn and CdCu,differing from the copper vacancy defect in ternary Cu-based compound such as CuInTe_(2).The low defect formation energies in Cd_(2)Cu_(3)In_(3)Te_(8) result in high defect concentrations,which is caused by the similar atomic radii and electronegativities between CdeIn and CdeCu.Further calculations show that the low-energy defects are mainly located around the valence band maximum in Cd_(2)Cu_(3)In_(3)Te_(8).The electrical transport calculations,considering both the acoustic phonon scattering and ionized impurity scattering,demonstrate that mainly due to the higher concentration of the ionized defects,the mobility of the quaternary Cd_(2)Cu_(3)In_(3)Te_(8) is much lower than that of ternary CuInTe2.Our work sheds light on the intrinsic defects in quaternary diamond-like compounds and their influence on charge transport.

Defect Engineering on Carbon‑Based Catalysts for Electrocatalytic CO2 Reduction

Electrocatalytic carbon dioxide(CO2)reduction(ECR)has become one of the main methods to close the broken carbon cycle and temporarily store renewable energy,but there are still some problems such as poor stability,low activity,and selectivity.While the most promising strategy to improve ECR activity is to develop electrocatalysts with low cost,high activity,and long-term stability.Recently,defective carbon-based nanomaterials have attracted extensive attention due to the unbalanced electron distribution and electronic structural distortion caused by the defects on the carbon materials.Here,the present review mainly summarizes the latest research progress of the construction of the diverse types of defects(intrinsic carbon defects,heteroatom doping defects,metal atomic sites,and edges detects)for carbon materials in ECR,and unveil the structure-activity relationship and its catalytic mechanism.The current challenges and opportunities faced by high-performance carbon materials in ECR are discussed,as well as possible future solutions.It can be believed that this review can provide some inspiration for the future of development of high-performance ECR catalysts.

Effect of As Interstitial Diffusionon on the Properties of Undoped Semi-insulating LECGaAs

Annealing was carried out at 950 and 1120 degreesC under various As pressure for undoped (ND) semi-insulating (SI) LECGaAs. The effects of annealing on native defects and electrical properties were investigated. Experimental results indicate that, after an annealing at 950 degreesC for 14 h under low As pressure, the Hall mobility decreases and the resistivity increases dramatically for the samples. These changes in electrical properties are due to the generation of intrinsic acceptor defects, and the generation of the intrinsic acceptor defects originates from the outdiffusion of As interstitial at high temperature. The generation of the intrinsic defects and these changes in electrical properties can be suppressed by increasing the applied As pressure during annealing. The concentration of the main donor defect E12 (AsGaVGa) can be decreased by about one order of magnitude by an evacuated annealing at 1120 degreesC for 2-8 h followed by a fast cooling. The decrease in E12 concentration can also be suppressed by increasing the As pressure during annealing.

Laser irradiation constructing all-in-one defective graphenepolyimide separator for effective restraint of lithium dendrites and shuttle effect

The commercialization of lithium-sulfur(Li-S)batteries faces several bottlenecks,and the major two of which are the shuttle effect of polysulfides and the wild growth of Li dendrites,responsible for fast capacity decay and severe safety issues.As an essential component of Li-S batteries,the structure and properties of the separators are closely related to the above problems,and the exploration of multifunctional separators is highly sought-after.Herein,an integrated separator composited of defective graphene and polyimide(DG-PI)was innovatively fabricated by electrospinning combined with the laser-induced carbonization strategy.The all-in-one compact architecture with well-interconnected channels shows superior mechanical and thermal stability and wettability.More importantly,the PI nanofibers containing N–/O–functional groups can induce the uniform deposition of lithium on the anode surface,while the DG framework with abundant pentagonal/heptagonal rings and vacancies can strongly trap polysulfides and accelerate polysulfide transformation on the cathode side.The strong chemical interaction between the insulative PI layer and the conductive DG layer modulates the surface charge distribution of each other,leading to more prominent contributions to restraining lithium dendrites and shuttle effect.Therefore,the Li-S batteries based on the integrated DG-PI separators afford an excellent performance in protecting lithium anode(stable cycles of 200 h at 5 mA·cm^(−2))and good cycling stability with a low capacity decay of 0.05%per cycle after 700 cycles at 1 C.This work offers a new design concept of multifunctional Li-S battery separators and broadens the application scope of laser micro-nano fabrication technology.

Origin of varistor properties of tungsten trioxide (WO_3) ceramics

To study the physical origin of the non-ohmic behavior of WO3 ceramics,the effects of heat treatment in different atmospheres on WO3 varistors were investigated.Experiments showed that there was a dependence of the nonlinear coefficient on thermal treatment under different atmospheres.Thermal treatments in argon and oxygen atmospheres at 900℃proved this dependence,and indicated that the nonlinear coefficient got significantly lower when the samples were thermally treated under argon atmosphere.Subsequent exposure to oxygen atmosphere at the same temperature led to the restoration of electrical properties.The result shows that the physical origin of the non-ohmic behavior of WO3 ceramics is oxygen on the grain surfaces adsorbed by intrinsic defects.

Grain boundary layer behavior in ZnO/Si heterostructure

The grain boundary layer behavior in ZnO/Si heterostucture is investigated. The current-voltage （I-V） curves, deep level transient spectra （DLTS） and capacitance-voltage （C-V） curves are measured. The transport currents ofZnO/Si heterojunction are dominated by grain boundary layer as high densities ofinterfacial states existed. The interesting phenomenon that the crossing of in I-V curves of ZnO/Si heterojunction at various measurement temperatures and the decrease of its effective barrier height with the decrement of temperature are in contradiction with the ideal heterojunction thermal emission model is observed. The details will be discussed in the following.