Search

A simple prediction of the well-known unification model of active galactic nuclei is that a sample of sources should exhibit an anticorrelation between the solid angle of the dusty torus and of the ionization cone (as the sum of them shall equal 4π); however, this has never been detected. In this work, we analyse the correlation between the [O  iii ] 5007 narrow emission line equivalent width and L IR(λ)/ L bol for a large sample of luminous quasars. For the first time, we detect a clear intrinsic anticorrelation between them, which immediately verifies the torus/ionization-cone geometry in luminous quasars. More interestingly, the anticorrelation significantly weakens with increasing wavelength from ∼2 to 12 |$\mu$| m, and disappears at ∼12 |$\mu$| m. Simulations show that a cool dust component (in addition to the equatorial torus) with its strength positively correlating with the solid angle of the ionization cone is required to explain the observations. This shows that the polar dust seen in nearby active galaxies also exists in luminous quasars, with its contribution to total dust emission increasing with λ (from ∼2 to 12 |$\mu$| m) and reaching between 39 per cent and 62 per cent (model dependent) at rest frame 12 |$\mu$| m. Our findings provide a unique approach to map the otherwise spatially unresolvable inner structure of quasars. [ABSTRACT FROM AUTHOR]

Deep-ultraviolet (DUV) phototransistors have shown great potential applications in UV imaging, artificial intelligence, and wearable optoelectronics. Among a large number of wide bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 is considered as an ideal candidate for DUV photodetector applications. Herein, we report a high responsivity (R) and fully flexible Ta-doped β-Ga2O3 DUV phototransistor which exhibits outstanding optoelectrical properties with a high R of 1.32 × 106 A/W, a large detectivity of 5.68 × 1014 Jones, a great photo-to-dark current ratio of 1.10 × 1010%, a high external quantum efficiency of 6.60 × 108%, and an ultra-fast response time of ~3.50 ms. Besides, the flexible Ta-doped β-Ga2O3 device also displays high reliability and mechanical flexibility that can sustain well after over 1 × 104 bending cycles. Moreover, high-contrast imaging of UV light was obtained on the flexible DUV detector arrays, which can be efficiently trained and recognized by an artificial neural network. Our findings offer a perspective to develop wearable optoelectronics and UV imaging based on high-performance flexible β-Ga2O3 DUV phototransistors, providing an inspiration for the future work in artificial intelligence and bionic robot fields. [ABSTRACT FROM AUTHOR]

Two‐dimensional material (2D) that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications. Defects in 2D materials are critical and fundamental to understand the chemical, physical, and optical properties. Photoluminescence arises in 2D materials owing to various physical phenomena including activator/dopant‐induced luminescence and defect‐related emissions, and so forth. With the advanced transmission electron microscopy (TEM) technologies, such as aberration correction and low voltage technologies, the morphology, chemical compositions and electronic structures of defects in 2D material could be directly characterized at the atomic scale. In this review, we introduce the applications of state‐of‐the‐art TEM technologies on the studies of the role of atomic defects in the photoluminescence characteristics in 2D material. The challenges in spatial and time resolution are also discussed. It is proved that TEM is a powerful tool to pinpoint the relationship between the defects and the photoluminescence characteristics. [ABSTRACT FROM AUTHOR]

Abstract: Single‐crystalline diamond wafers are synthesized, and all‐carbon solar‐blind photodetectors are constructed based on the diamond with interdigitated graphite as electrodes. The graphite electrodes are made by laser‐induced graphitization of the diamond. The peak responsivity of the photodetector is 21.8 A W−1, and detectivity is 1.39 × 1012 cm Hz1/2 W−1 when the bias voltage is 50 V, it is noted that both of these are among the best values ever reported for diamond‐based photodetectors. The photodetectors are employed as the sensing pixels in an imaging system, and a clear image is obtained, which is the first report on solar‐blind imaging of diamond photodetectors. The results reported in this paper provide a facile approach to high‐performance diamond solar‐blind photodetectors and their applications in solar‐blind imaging, thus may address a significant step toward the preparation and application of diamond photodetectors. [ABSTRACT FROM AUTHOR]

HgCdTe electron avalanche photodiodes (e-APDs) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we performed accurate numerical simulations of the current–voltage characteristics and multiplication factor in planar homojunction (p-i-n) HgCdTe APDs. Based on the Okuto–Crowell avalanche model, an efficient physical model has been obtained by concerning the major generation-recombination processes, such as trap-assisted tunneling and band-to-band tunneling (BBT) recombination. Simulated current–voltage characteristics were in good agreement with available data in the literature, which demonstrates the validity of the proposed model. The origins of dark current in high reverse voltages are jointly dominated by BBT and the avalanche mechanism. It is proved to be effective for reducing BBT by improving the uniformity of the electric field distribution across the junction. The electric performance of p-i-n e-APD can be improved by optimizing the APD structure, such as eliminating the sharp corners of junctions, light doping, and the appropriate thickness in multiplication region. Our works provide a good deal of insight into the fundamental carrier transport processes involved in HgCdTe e-APDs. [ABSTRACT FROM AUTHOR]

We have investigated terahertz (THz) resonant absorption spectra in grating-gate GaN high electron mobility transistors. The results indicate that both the symmetrical plasmon mode and the asymmetrical plasmon mode play an important role in the strong absorption of THz waves. The excitation process and dynamic response of these plasmons are investigated in detail. Our results also indicate that the asymmetrical plasmon is induced by the surface polarization field of the electrodes and the resonant strength of this plasmon is reduced significantly by the decay of the polarization field. Variations in the resonant strength of the plasmonic peaks are consistent with the surface resonant layer model showing that the method we used can be utilized for the study of coupling between THz radiation and plasmons in the channel. In order to achieve wider tunability, more advanced device structures can be explored such as a device that contains double channel layers, in which complicated plasmons can be excited. [ABSTRACT FROM PUBLISHER]

The direct current characteristics of AlGaN/GaN double-channel HEMTs (DC-HEMTs) are investigated by using 2-D numerical simulations. The output characteristics have been predicted with the drift-diffusion, thermodynamic, hydrodynamic, and hot-electron models, respectively. The prediction by the hydrodynamic model is in good agreement with the experiment. It is demonstrated that the hot-electron effect makes a negligible contribution to the negative differential conductance (NDC) of an AlGaN/GaN DC-HEMT; instead, the NDC effect is caused by the self-heating effect. The transfer and transconductance characteristics of an AlGaN/GaN DC-HEMT are also discussed in detail. Finally, a new \In0.18\Al0.82\N /GaN/AlGaN/GaN DC-HEMT structure is proposed for optimizing AlGaN/GaN DC-HEMTs. [ABSTRACT FROM PUBLISHER]

Blue InGaN/GaN multiple quantum well light-emitting diodes (LEDs) with the conventional AlGaN and AlGaN-GaN-AlGaN (AGA) electron blocking layer (EBL) are investigated numerically. The simulation results show that the LEDs with the AGA EBL exhibit much higher output power and smaller efficiency droop at high current as compared to those with the conventional EBL due to the enhancement of the electron confinement and improvement of the hole injection from p-type region, which are induced by the strong electrostatic fields and tunneling effect in the AGA EBL. [ABSTRACT FROM AUTHOR]

Si 3 N 4 ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25 °C to 800 °C within X-band (8.2–12.4 GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si 3 N 4 ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si 3 N 4 ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si 3 N 4 grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si 3 N 4 ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si 3 N 4 ceramics will be promising candidates as microwave absorbing materials for high-temperature applications. [ABSTRACT FROM AUTHOR]

Home-made SiC nanofibers were introduced into Si 3 N 4 matrix for enhanced electromagnetic interference (EMI) shielding properties. The microstructural characterization revealed that SiC nanofibers modified Si 3 N 4 ceramic is composed of SiC conductive networks made of uniformly distributed SiC nanofibers, micro-pores and insulating Si 3 N 4 matrix. Moreover, EMI shielding effectiveness (SE T ) for SiC nanofibers modified Si 3 N 4 ceramic is dependent on SiC nanofibers content. When the SiC nanofibers content is increased from 0 to 8 wt%, the EMI SE T of as-prepared ceramics increased from 3.43–4.15 dB to 5.58–9.31 dB. Due to the low electrical conductivity of as-prepared ceramics, multi-reflection caused by the micro-pores and nanofibers networks is mainly responsible for the EMI shielding. The study indicates that introduction of SiC nanofibers would be a favorable solution for Si 3 N 4 based thermostructural material with an improved EMI shieding. [ABSTRACT FROM AUTHOR]

Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 °C. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β -SiC, present a withe-like morphology with diameter of 20–50 nm and aspect ratio of 100–150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than −10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach −19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than −2.1 dB within the whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials. [ABSTRACT FROM AUTHOR]

An alternative-grating gated AlGaN/GaN field-effect transistor (FET) is proposed by considering the slit regions to be covered by a highly doped semiconductor acting as supplemental gates. The plasmonic resonant absorption spectra are studied at THz frequencies using the FDTD method. The 2DEGs, under supplemental gates, modulated by a positive voltage, can make the excitation of the higher order plasmon modes under metallic fingers more efficient in comparison to ungated regions in common slit-grating gate transistors. Moreover, the supplemental gates can confine the electric field of dipole oscillation between metallic gate fingers under THz radiation. The competition of the near-field enhancement and screening effect of the supplemental gate fingers results in the intensity of the higher order plasmon resonances being maximized at increased doping concentration. Our results demonstrate the possibility of significant improvement in the excitation of plasmon resonances in FETs for THz detection.

The doping-dependent photoconductive properties of individual GaAs nanowires have been studied by conductive atomic force microscopy. Linear responsivity against the bias voltage is observed for moderate n-doped GaAs wires with a Schottky contact under illumination, while that of the undoped ones exhibits a saturated response. The carrier lifetime of a single nanowire can be obtained by simulating the characteristic photoelectric behavior. Consistent with the photoluminescence results, the significant drop of minority hole lifetime, from several hundred to subpicoseconds induced by n-type doping, leads to the distinct photoconductive features. Moreover, by comparing with the photoelectric behavior of AlGaAs shelled nanowires, the equivalent recombination rate of carriers at the surface is assessed to be >1 × 10(12) s(-1) for 2 × 10(17)cm(-3) n-doped bare nanowires, nearly 30 times higher than that of the doping-related bulk effects. This work suggests that intentional doping in nanowires could change the charge status of the surface states and impose significant impact on the electrical and photoelectrical performances of semiconductor nanostructures.