Ligand assisted reprecipitation（LARP） is a widely used method for cesium lead halide perovskite nanocrystals（NCs） synthesis. Nevertheless, the ultrafast kinetics of LARP, as well as the inefficient transport properties and discontinuity of batch reactors, challenge the particle size control and experimental repeatability. To address these issues, an ultrasonic cavitation-enabled microfluidic approach was developed to achieve the continuous synthesis of cesium lead halide perovskite via LARP. It was found that the mixing between the good solvent and antisolvent in the microchannel was greatly enhanced by intensive ultrasonic cavitation. The mixing time could be reduced to below 10 ms under the irradiation of 35 W ultrasound. By modulating the mixing degree, LARP was proved to be a mixing-sensitive process. The effects of ultrasonic power, ultrasonic treatment time, total flow rate, water additive, and reprecipitation temperature on the synthesis of CsPbBr 3 NCs were systematically investigated. As compared to CsPbBr 3 NCs synthesized in the batch reactor, the sample synthesized via the ultrasonic cavitation-enabled microfluidic approach possessed stronger photoluminescence intensity and better repeatability.Moreover, the ultrasonic cavitation-enabled microfluidic approach could also realize the continuous synthesis of cesium lead halide perovskite NCs with different halide compositions to cover a wide visible spectrum（426–661 nm）. The ultrasonic cavitation-enabled microfluidic approach paved the way for the large-scale of high-quality cesium lead halide perovskite NCs.

Herein, a metal-semiconductor-metal (MSM) structure dual-band solar-blind ultraviolet (UV) photodetector based on 60.5-period Al0.5Ga0.5N/AlN superlattices (SLs) is reported. The device exhibits distinct dominant responses in the solar-blind UV region with dual peaks located at 266 nm and 243 nm. The formation of dual-band solar-blind UV photodetection is clarified, which can be originated from the reflection enhancement property of the highly periodic Al0.5Ga0.5N/AlN SLs that restrains the light absorption around 259 nm resulting in weak spectral response in this specific waveband. (C) 2021 Elsevier B.V. All rights reserved.

Herein, the dislocation defects of an aluminum nitride (AlN) template heteroepitaxially grown by metal-organic chemical vapor deposition are exposed by phosphoric acid etching. The effect of dislocation-related V-shaped pits preparation on the strain of AlN epilayer is intensively studied. The results show that the dislocation defect etching leads to a strain gradient elongating along the growth direction of AlN and alleviates its initial tensile strain. Through a secondary epitaxy on the dislocation-etched AlN, it further reveals that the strain evolves into compressive strain state accompanied by surface cracking inhibition. It provides an effective way of stress regulation by dislocation-related etching and can be developed as a mask-free lateral epitaxial growth technique for high-performance AlN template preparation.

In this letter, based on the method of introducing one mesothermal AlN (MT-AlN) interlayer, a high-quality AlN template is firstly obtained by adjusting the growth rate of the high-temperature AlN (HTAlN) epilayers on both sides of the MT-AlN interlayer. Then, the epitaxial growth of polarization-graded AlGaN-based solar-blind ultraviolet photodetector (SUV-PD) structural material is implemented on the pre-grown AlN template by introducing an n-AlxGa1-xN gradient layer. It leads to the elimination of band discontinuity and forms a polarization-graded induced field resulting in a high-performance back-illuminated AlGaN-based SUV-PD. This research provides an important reference for the development of high-performance back-illuminated AlGaN-based SUV-PDs. (C) 2020 Elsevier B.V. All rights reserved.

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were used to measure the band structure for high lattice-mismatched In0.82Ga0.18As/InP. The valence band offset was determined to be 0.43 eV, which is in agreement with the theoretical values based on the previous analysis. Together with a conduction band offset of 0.44 eV, it is indicated that a type-I band structure forms at the In0.82Ga0.18As/InP heterojunction. The precise determination of the band structure of In0.82Ga0.18As/InP is crucial for future device design and performance improvement. Besides, the valence band offset of In0.82Ga0.18As/GaAs was estimated to be 0.24 eV, which also presents a type-I band alignment.

In this paper, comparison between back‐illuminated p‐i‐n AlGaN‐based ultraviolet photodetectors (UV‐PDs) with and without an n‐AlGaN inserted layer is carried out. The results show that the introduction of n‐AlGaN interlayer significantly reduces the dark current of AlGaN‐based UV‐PDs. The mechanism involved is clarified and can be attributed to the role of n‐AlGaN interlayer which depletes to isolate the leakage paths generated by dislocations of AlGaN material. Besides, it also greatly improves the spectral performances of the p‐i‐n AlGaN‐based UV‐PDs, which can be related to the additional built‐in electric fields introduced by n‐AlGaN inserted layer that contribute to separate and transport the photon‐generated carriers.

Revealing the effect of the V-pits on the property of GaN is the key to understand and control the defects and thus to improve the performance in the GaN based devices. Here, the influence of the V-pits on the morphology, optics, structure and strain properties of GaN epilayer was investigated. The GaN films with different V-pits density were obtained by changing the growth parameters of metalorganic chemical vapor deposition. The scanning electron microscope, photoluminescence, high resolution X-ray diffraction and Raman were used to character the influence of V-pits on the GaN film. The results show that the sidewalls of V-pits might have higher bandgap than that of flat areas, which can suppress the shallow donor level related radiation recombination. Moreover, the V-pits can release the stress without apparently destroying the structural property. The results obtained here have importance of direction in controlling and taking advantage of V-pits.

Field emission electron beam (EB) pumped AlGaN-based semiconductors are considered to be a potentially promising way to overcome the technical bottlenecks that restrict the development of AlGaN-based UV luminescence devices and realize efficient UV light sources. However, the required field emission electron sources based on nanomaterials are still inefficient due to their low field emission current density. Herein, a type of UV-light-assisted self-positive-feedback enhanced field emission electron source is proposed to develop a high-efficiency electron source which is promising for application in EB pumped AlGaN-based UV light sources that can also be generalized to deep UV (DUV) luminescence devices. The UV-light-assisted field emission source is composed of an n-GaN metal-semiconductor-metal (MSM) structure photodetector assembled with 1D ZnO nanorods by a self-assembled hydrothermal growth method, which simultaneously possesses attributes of the photoelectric effect and electron emission. The optical, photoelectric, and field emission properties are investigated in detail. The results show that the 1D ZnO nanorods/n-GaN heterostructure photodetector presents an obvious photoconductive effect. It has a peak spectral responsivity of 0.793 A W-1 at a bias voltage of 1.3 V, corresponding to an EQE higher than 267.8%, with an internal photoconductive gain reaching up to 2.51 × 103. As to the field emission properties, its turn-on electric field can be greatly reduced from 3.6 V μm-1 in the dark to 1.36 V μm-1 under UV illumination, and the field emission current density increases from lower than 3 mA cm-2 to as high as 8 mA cm-2 at an electric field of 4.5 V μm-1. The mechanism involved can be attributed to an increase of electron concentration in both the conduction bands and an increase of conduction band bending under UV illumination that reduces the effective potential barrier height of the ZnO nanorods. Through this research, an efficient field emission electron source with a self-enhancing effect is developed by combining the photoelectric effect with the electron emission process.

In this paper, AlN templates with different mesothermal AlN (MT-AlN) interlayer deposition cycles are prepared via metal-organic chemical vapor deposition (MOCVD) at first. The influence of the MT-AlN interlayer on the crystalline quality of AlN templates has been carefully investigated. The results show that the MT-AlN interlayer played the role of providing a strain gradient to release the strain from AlN along the growth direction. The crystalline quality, surface morphology and residual compressive stress of AlN templates can be greatly optimized through introducing one MT-AlN interlayer cycle during the growth of HT-AlN. And then, a PIN structure back-illuminated AlGaN-based solar-blind ultraviolet photodetector (SUV-PD) is further designed, grown and fabricated based on the optimized AlN template. The SUV-PD exhibits a low dark current density, a bandpass spectral responsivity with a peak value of 0.15 A W-1 at 271 nm, corresponding to an EQE as high as 68.8%, and a response speed of 6.5 ns. These results strongly reflect the high quality of AlN templates prepared via inserting a MT-AlN interlayer based on a common growth method.

The design of novel structural material is an effective way to improve photodetection device performance. In this paper, the fabrication and performance of high In content InGaAs detectors were investigated. Using the two-step growth method, mismatch defect was effectively inhibited even with larger lattice mismatch at the interface. Meanwhile, the spectral response can cover the entire near-infrared region at room temperature. Through experiments and simulation, the optoelectronic properties of detector with different materials in the p-region are explored, elucidating the critical role of cap material in the transport properties of carriers. Compared to the typical InP cap detector, the InAsP cap detector shows better device performance. Also the dark current mechanism is analyzed on the basis of bias–temperature relation, and the result shows that the tunneling current plays a key role at high bias or low temperature. The introduction of a novel InGaAs detector provides a potential application to the development of near-infrared detection.

Large lattice mismatched heterostructures In0.78Ga0.22As/GaAs were grown with a low-temperature (LT) InGaAs buffer. Misfit dislocation arrays were observed at the LT-buffer/GaAs interfaces. Transmission electron microscopy (TEM) was used to investigate the microstructures of the heterointerfaces. The relationship between misfit dislocations and strain relaxation at the interface was analysed. It was found that strain redistribution gives rise to the discrepancy of interfacial structure and misfit strain relaxation for different samples. The experimental results are in favor of the conclusion and confirm the analysis we proposed.

The influence of the polarization charges on the properties of AlGaN‐based heterojunction p–i–n ultraviolet photodetectors was investigated. It is found that the polarization charges at the hetero‐interface can enhance the electric field intensity and result in an increased dark current. On the contrary, the polarization charges can lower the photoresponse because the direction of polarization electric field is opposite to the applied electric field in the light absorption layer.

An experimental approach to evaluate the average energy required by holes for impact ionization in Al0.4Ga0.6N alloys is presented. A series of back-illuminated separate absorption and multiplication structures with different thickness of multiplication layer are employed. With the measured quantum efficiency of devices and a consideration of energy conservation in ionization process, the average energy for electron-hole pair production by energetic holes is obtained and it qualitatively agrees with the bond energy of Al0.4Ga0.6N which is estimated from the Vegard's law.

A surface plasmon (SP) is a fundamental excitation state that exists in metal nanostructures. Over the past several years, the performance of optoelectronic devices has been improved greatly via the SP enhancement effect. In our previous work, the responsivity of GaN ultraviolet detectors was increased by over 30 times when using Ag nanoparticles. However, the physics of the SP enhancement effect has not been established definitely because of the lack of experimental evidence. To reveal the physical origin of this enhancement, Kelvin probe force microscopy (KPFM) was used to observe the SP-induced surface potential reduction in the vicinity of Ag nanoparticles on a GaN epilayer. Under ultraviolet illumination, the localized field enhancement induced by the SP forces the photogenerated electrons to drift close to the Ag nanoparticles, leading to a reduction of the surface potential around the Ag nanoparticles on the GaN epilayer. For an isolated Ag nanoparticle with a diameter of ~200 nm, the distribution of the SP localized field is located within 60 nm of the boundary of the Ag nanoparticle. For a dimer of Ag nanoparticles, the localized field enhancement between the nanoparticles was the strongest. The results presented here provide direct experimental proof of the localized field enhancement. These results not only explain the high performance of GaN detectors observed with the use of Ag nanoparticles but also reveal the physical mechanism of SP enhancement in optoelectronic devices, which will help us further understand and improve the performance of SP-based optoelectronic devices in the future.

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.

Magnetic graphene-Fe3O4 nanocomposites (G/Fe3O4) were fabricated by a facile and fast one-pot method and used as adsorbent to remove dye for wastewater using Rhodamine B as the adsorbate. Samples with different weight ratios of graphene oxide (GO) to Fe3O4 were prepared. The transmission electron microscopy results exhibit that the sizes of Fe3O4 nanocrystals decrease with the increasing of weight ratio of GO to Fe3O4. The magnetic characterization demonstrates that the saturation magnetization of nanocomposites decreases with the decreasing sizes of Fe3O4 nanocrystals. The investigation of adsorption kinetics and isotherm indicates the adsorption process can be described by Langmuir model and nanocomposites with the smaller sizes of Fe3O4 nanoparticles show better adsorption ability. Furthermore, the adsorbents could be recovered conveniently by magnetic separation and recyclable used after desorption process, and the decline in efficiencies of all samples is not more than 1.5% after five cycling runs. (C) 2015 Elsevier Ltd. All rights reserved.

The tunability of surface plasmon resonance can enable the highest degree of localised surface plasmon enhancement to be achieved, based on the emitting or absorbing wavelength. In this article, tunable dipole surface plasmon resonances of Ag nanoparticles (NPs) are realized by modification of the SiO2 dielectric layer thicknesses. SiO2 layers both beneath and over the Ag NPs affected the resonance wavelengths of local surface plasmons (LSPs). By adjusting the SiO2 thickness beneath the Ag NPs from 5 nm to 20 nm, the dipole surface plasmon resonances shifted from 470 nm to 410 nm. Meanwhile, after sandwiching the Ag NPs by growing SiO2 before NPs fabrication and then overcoating the NPs with various SiO2 thicknesses from 5 nm to 20 nm, the dipole surface plasmon resonances changed from 450 nm to 490 nm. The SiO2 cladding dielectric layer can tune the Ag NP surface charge, leading to a change in the effective permittivity of the surrounding medium, and thus to a blueshift or redshift of the resonance wavelength. Also, the quadrupole plasmon resonances were suppressed by the SiO2 cladding layer because the dielectric SiO2 can suppress level splitting of surface plasmon resonances caused by the Ag NP coupling effect.

High spectral response of self-driven GaN-based ultraviolet detectors with interdigitated finger geometries were realized using interdigitated Schottky and near-ohmic contacts. Ni/GaN/Cr, Ni/GaN/Ag, and Ni/GaN/Ti/Al detectors were designed with zero bias responsivities proportional to the Schottky barrier difference between the interdigitated contacts of 0.037 A/W, 0.083 A/W, and 0.104 A/W, respectively. Voltage-dependent photocurrent was studied, showing high gain under forward bias. Differences between the electron and hole mobility model and the hole trapping model were considered to be the main photocurrent gain mechanism. These detectors operate in photoconductive mode with large photocurrent gain and depletion mode with high speed, and can extend GaN-based metal-semiconductor-metal detector applications.

The point defects and photoluminescence (PL) spectra of gallium nitride (GaN) epilayers with Mg, Zn, and unintentional doping were investigated in this study. The concentration of point defects (Ga vacancy and its related complexes) in the Zn-doped GaN is consistent with that in the Mg-doped GaN, but lower than that in undoped GaN. It is suggested that Zn (Mg) atoms occupy Ga sites and suppress the formation of Ga vacancies. Comparing the blue luminescence (BL) band intensity of GaN:Zn with that of GaN:Mg, a factor of 10 strong PL intensity demonstrates that a moderate incorporation of Zn to GaN is likely to improve the structural quality of GaN. Detailed studies on 2.93 eV BL band for GaN:Zn reveal that the Zn related BL band behaves as a donor-acceptor pairs character. For the acceptor level, isolated Zn(Ga)with the activation energy of 0.386 eV above the valence band is obtained from temperature-dependent PL measurements, whereas the deep donor defect responsible for the 2.93 eV band is deduced to be 164 meV below the conduction band. An O-N-H complex model is suggested to explain the deep donor origin.