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GaN-based pin alpha-particle detectors grown on sapphire substrates have been subjected to 10 MeV electron irradiation over a cumulative dose range of 0 to 200 kGy. The pre- and post-irradiation detectors have been characterized with current–voltage and capacitance–voltage measurements, charge collection efficiency ( CCE ), and alpha-particle pulse-height spectroscopy . The results show that the performance of the detectors underwent significant changes due to enhanced carrier-hopping conductivity through defect states and deep-level traps in the space-charge region induced by the 10 MeV electron irradiation. Such detectors can be used for alpha detection with confidence in an environment of background high energy electrons , up to a dose of about 200 kGy, and the response can degrade rapidly if the dose exceeds 200 kGy. In this work, the maximum CCE was achieved in a detectors irradiated with a cumulative dose of 100 kGy.

The temperature-dependent radiation-detection performance of an alpha-particle detector that was based on a gallium-nitride (GaN)-based pin structure was studied from 290 K to 450 K. Current–voltage–temperature measurements ( I – V – T ) of the reverse bias show the exponential dependence of leakage currents on the voltage and temperature. The current transport mechanism of the GaN-based pin diode from the reverse bias I – V fitting was analyzed. The temperature-dependent pulse-height spectra of the detectors were studied using an 241 Am alpha-particle source at a reverse bias of 10 V, and the peak positions shifted from 534 keV at 290 K to 490 keV at 450 K. The variation of full width at half maximum ( FWHM ) from 282 keV at 290 K to 292 keV at 450 K is almost negligible. The GaN-based pin detectors are highly promising for high-temperature environments up to 450 K.

High-quality VO 2 films have been successfully synthesized on SiO 2 /Si substrates by a simple and efficient method in a tube furnace. The as-synthesized VO 2 films were structurally uniform and single-crystalline. These films show a large resistance change of more than four orders of magnitude with a 5 K width of the hysteresis loop as the temperature is cycled through the phase transition. VO 2 thin-film devices have been fabricated by reactive ion etching (RIE) and deposition processes. The current response of devices under light illumination and bias exhibited a higher sensitivity and a much wider pressure range than that without light illumination. Experimental data also revealed that the current response of a VO 2 film device shows an approximately linear dependence on the logarithm of air pressure. These high-quality VO 2 thin films with excellent properties have potentially promising application in vacuum sensor.

This paper presents modeling results of Mie-type GaAs nanopillar array resonant structures and the design of negative electron affinity photocathodes based on Spicer’s three-step model. For direct-bandgap GaAs with high intrinsic absorption coefficient in the 500 ∼ 850 nm spectral range, photoelectrons were found to be highly localized inside the nanopillars near the top and side surfaces where electrons can be efficiently transported and emitted into vacuum, and the light reflectance can be reduced to ∼1% level at resonance wavelengths. Predictions of spectrally resolved photoemission indicate that these nanophotonics resonators, when properly optimized, can increase the photo-electron emission quantum efficiency at resonance wavelengths to levels limited only by the surface-electron escape probability, significantly outperforming traditional flat wafer photocathodes. Ultrafast photoelectric response is also expected from these nanostructured photocathodes due to the much shorter photoelectron transport distance in nanopillars compared to flat wafers. Given these unique optoelectronic properties, GaAs nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.

A particular type of semiconductor photocathode with a nanostructured surface is shown to increase photoemission yield by more than a factor of three, compared to a flat photocathode. This improvement stems from the excitation of Mie resonances, which serve to reduce reflectivity and enhance light absorption near the surface, where electrons are more efficiently emitted. Nanostructured photocathodes, which can be described as ``electrically bright'' yet ``optically dark'', could benefit many applications, including high-current electron cooling of hadron beams in particle accelerators.

The electron emission model of a negative electron affinity graded-bandgap AlGaAs/GaAs electron-injection cathode was developed from two-dimensional continuity equations. The emission current was obtained from a simulation of the model, and the emission current efficiency and emission current per unit length were calculated. Based on the simulation results and preparation conditions, the range of optimum parameters for the cathode structure were determined. The ranges of optimum thickness for the p-AlGaAs and the graded-bandgap p-AlGaAs layers were 0.05–0.15 μm and 0.1–0.3 μm, respectively. The optimum width of the base electrode ranged from 1 to 4 μm, and the optimum molar ratios of Al in the p-AlGaAs and the n-AlGaAs layers were 0.2–0.3 and 0.4–0.5, respectively. This abrupt PN heterojunction inhibited the hole current and increased the emission current efficiency, with a maximum value of 25.3%. According to the emission current per unit length, the optimum range of width of emission unit surface was 6 to 10 μm, and the peak emission current per unit length reached 43.2 μA/μm.

Herein, a novel solid-based up-conversion fluorescence resonance energy transfer (FRET) sensor was developed using rhodamine B hydrazide, which provided a selective fluorescence response and suitable affinity towards Cu2+ ions over other biologically relevant metal ions because the Cu2+ ion could promote the hydrolysis of α-amino acid esters of rhodamine B hydrazide and yield the Cu·α-amino acid chelate. This solid-based detection system is more convenient for the detection of Cu2+ based on color change and emission spectra instead of the complicated and tedious measurements than other up-conversion sensors and up-conversion luminescent nanoparticles used as an excitation source; moreover, the proposed system shows high selectivity, minimum photo-damage to living organisms, and high chemical stability.

A photoemission model of graded band-gap AlGaAs/GaAs wire NEA photocathode is developed based on the numerical solution of coupled Poisson and continuity equations. The emission current density and integral sensitivity of graded band-gap AlGaAs/GaAs wire photocathode as a function of incident light wavelength, Al composition range, and wire length, are simulated according to the model. The simulation results show that, compared with the GaAs (Al composition 0) wire photocathode, the peak integral sensitivities for the photocathodes with wire width of 1 µm and linearly graded Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4 increase by 29.5%, 38.5%, 42.1%, and 43.8%, respectively. The optimum wire lengths are 4.7, 5.9, 7.1, and 8.4 µm for the wire photocathodes with Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

Schottky diode nuclear radiation detectors based on semi-insulating gallium-arsenide (GaAs) wafers were fabricated. The minimum leakage current and current density of the detectors were 7.8 nA and 2.5 × 10−7 A/cm2, respectively, at a reverse bias of 100 V. The Schottky GaAs detectors were subjected to 10 MeV electron irradiation with a radiation dose of 0 to 200 kGy. Pre- and post-irradiation detectors were characterized by current–voltage, charge collection efficiency (CCE), and alpha-particle pulse-height spectroscopy. The detection efficiency of all detectors improved after irradiation, which may be attributed to defects induced by high-energy electron irradiation increasing the inhomogeneity of semi-insulating GaAs wafers. However, the energy resolution and CCE of the post-irradiation detectors decreased. After electron irradiation, the post-irradiation detectors were annealed at 200 °C for 5 min in nitrogen. The CCE of the annealed detectors improved, which suggests that the damages caused by high-dose electron irradiation can be repaired partially by annealing at 200 °C.

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs reflection-mode photocathodes was determined using two-dimensional Poisson and continuity equations through numerical method. Based on the MTF model, we calculated the theoretical MTF of graded and uniform band-gap reflection-mode photocathodes. We then analyzed the effects of Al composition, wavelength of incident photon, and thicknesses of AlGaAs and GaAs layer on the resolution. Calculation results show that graded band-gap structures can increase the resolution of reflection-mode photocathodes. When the spatial frequency is 800 lp/mm and wavelength is 600 nm, the resolution of graded band-gap photocathodes generally increases by 15.4–29.6%. The resolution improvement of graded band-gap photocathodes is attributed to the fact that the built-in electric field in graded band-gap photocathodes reduces the lateral diffusion distance of photoelectrons.

Phosphors with photonic crystal (PC) structures may demonstrate modulated optical properties and have potential applications in various optical devices. In this study, we represent the fabrication of ZnWO4 and ZnWO4 : Eu3+ inverse opal photonic crystal (IOPC) scintillators based on polymethyl methacrylate (PMMA) templates. Typical modified photoluminescent properties in PCs were observed such as the suppression of emissions near the edge of photonic stop bands (PSBs) and prolonged decay time constants. Moreover, it is very interesting to observe that the luminescent quantum efficiency of the IOPC samples was remarkably enhanced compared to the corresponding ground powder references (REFs) and the quantum efficiency of ZnWO4 IOPCs was as high as 70%, which was almost the optimum among various ZnWO4-based phosphors reported before. Furthermore, the energy transfer (ET) efficiency from tungstate groups to Eu3+ was determined based on luminescent dynamics, which indicated that the efficiency in the IOPCs was considerably improved compared to that in the REFs owing to the suppression of the spontaneous emission rate for the tungstate groups. It was also demonstrated that ZnWO4 : Eu3+ IOPCs could be used as white light phosphors.

Temperature-dependent material parameters and device performances of Ga x In 1− x As 1− y Sb y TPV cells applied in low temperature (800–1200 °C) radiators are simulated using the PC-1D. As is well known, the optimum bandgap ( E g ) decreases towards lower radiator temperatures. So far, the lowest achievable E g of Ga x In 1− x As 1− y Sb y at 300 K is 0.5 eV. We mainly considering the Ga 0.8 In 0.2 As 0.18 Sb 0.82 ( E g = 0.5 eV) TPV cell. The effects of doping concentration and recombination mechanisms of the emitter layer on photovoltaic conversion efficiencies ( η cel ) are analyzed in detail, and η cel can be improved by optimizing doping concentration and suppressing carrier recombination. The effects of GaSb window layer on η cel are also presented. It shows the type-II energy-band alignment GaSb(window)/GaInAsSb(emitter) heterostructure affect η cel mainly through V oc . For the first time, the effects of operating temperatures on device performances are analyzed based on temperature-dependent material parameters, and the temperature coefficients of the device performances are presented.

We reported a method combining the gas–liquid interface method and inductively coupled plasma (ICP) to fabricate highly-ordered nanowire arrays on a 2 inch GaAs substrate. A large-area and highly-ordered SiO2 nanosphere monolayer was fabricated and the GaAs nanowire array was etched by ICP. The flow rates and ratios of the etching gases were studied. Finally, highly-ordered GaAs nanowire arrays with high quality surface morphology were obtained with the volumetric flow rates at 9 sccm and 21 sccm for BCl3 and N2 gases, respectively. Reflectance of the samples was found to be consistently below 10%, the lowest being 3%.

By analyzing the main recombination mechanisms in GaInAsSb materials, the dependences of the dark current density and open circuit voltage in n+-p GaInAsSb thermophotovoltaic cells on the recombination parameters, carrier concentration and cell thickness are calculated. The results show that the dark current mainly comes from p-region, and it is related with the surface and Auger recombinations in low and high carrier concentration ranges, respectively. The surface and Auger recombinations can be suppressed by reducing the surface recombination velocity and carrier concentration, respectively. The dark current density can be suppressed by optimizing material parameters and device surface passivation technique. So the high open circuit voltage can be obtained for GaInAsSb thermophotovoltaic cells.

In this paper, a numerical simulation on the open-circuit voltage (VOC) of the P-GaSb window/P-Ga0.8In0.2As0.18Sb0.82 emitter/N-Ga0.8In0.2As0.18Sb0.82 base/N-GaSb structure thermophotovoltaic (TPV) cell is performed and an analysis of the effects of device parameters on VOC is presented. The simulations are carried out with the fixed spectral control filter and for the radiator temperature of Trad = 950 °C, cell temperature of Tdio = 27 °C, the radiation photons are injected from the front P-region. The thick P-Ga0.8In0.2As0.18Sb0.82 emitter with the longer minority carrier diffusion length is the main optical absorption region. The simulated results are compared with the available experimental data, and a good agreement is obtained. The effects of the layer thickness, carrier concentration, injection level and main recombination mechanisms (e.g. the radiative, Auger, bulk Shockley–Read–Hall (SRH) and surface recombination) of the P-Ga0.8In0.2As0.18Sb0.82 emitter and N-Ga0.8In0.2As0.18Sb0.82 base on VOC are analyzed. It indicates that the parameters of the emitter region have stronger effect than that of the base region on VOC. Dependence of VOC on the material parameters of P-GaSb window layer is also analyzed, both the carrier concentration and thickness of P-GaSb window layer have effect on VOC. Moreover, adding a back surface reflector (BSR) to the TPV cell can increase VOC.

In this paper, a simulation and analysis on the short-circuit current density ( J sc ) of the P-GaSb window/P-Ga x In 1 −x As 1 −y Sb y emitter/N-Ga x In 1 −x As 1− y Sb y base/N-GaSb substrate structure is performed. The simulations are carried out with a fixed spectral control filter at a radiator temperature ( T rad ) of 950 °C, diode temperature ( T dio ) of 27 °C and diode bandgap ( E g ) of 0.5 eV. The radiation photons are injected from the front P-side. Expressions for minority carrier mobility and absorption coefficient of Ga x In 1− x As 1− y Sb y semiconductors are derived from Caughey–Thomas and Adachi’s model, respectively. The P-Ga x In 1− x As 1− y Sb y emitter with a much longer diffusion length is adopted as the main optical absorption region and the N-Ga x In 1− x As 1− y Sb y base region contribute little to J sc . The effect of P-GaSb window and P-Ga x In 1− x As 1− y Sb y emitter region parameters on J sc is mainly analyzed. Dependence of J sc on thickness and carrier concentration of the window are analyzed; these two parameters need to be properly selected to improve J sc . Contributions from the main carrier recombination mechanisms in the emitter region are considered; J sc can be improved by suppressing the carrier recombination rate. Dependence of J sc on the carrier concentration and layer thickness of the emitter P-region are also analyzed; these two parameters have strong effect on J sc . Moreover, adding a back surface reflector (BSR) to the diode can improve J sc . The simulated results are compared with the available experimental data and are found to be in good agreement. These theoretical simulations help us to better understand the electro-optical behavior of Ga x In 1− x As 1− y Sb y TPV diode and can be utilized for performance enhancement through optimization of the device structure.

The resolution model of graded doping and graded composition reflection-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, the theoretical modulation transfer functions (MTFs) of different structure reflection-mode photocathodes were calculated, and the effects of doping concentration, Al composition, AlGaAs and GaAs layer thickness on the resolution of cathodes were analyzed. The simulation results show that both graded composition and graded doping structures can increase the resolution of photocathode, and the effect of graded composition structure is more pronounced. The resolution improvement is attributed to the built-in electric field induced by a graded composition or doping structure. The simulation results also show that the MTFs of cathodes are affected by the AlGaAs and GaAs layer thickness.

Two types of negative electron affinity gallium arsenide (GaAs) wire array photocathodes were fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material. High density GaAs wire arrays with high periodicity and good morphology were verified using scanning electron microscopy, and photoluminescence spectra confirmed the wire arrays had good crystalline quality. Reflection spectra showed that circular GaAs wire arrays had superior light trapping compared with square ones. However, after Cs/O activation, the square GaAs wire array photocathodes showed enhanced spectral response. The integral sensitivity of the square wire array photocathodes was approximately 2.8 times that of the circular arrays.

The degree of polarization of photoemitted electrons extracted from bulk unstrained GaAs photocathodes is usually considerably less than the theoretical maximum value of 50%, as a result of depolarization mechanisms that originate within the photocathode material and at the vacuum surface interface. This paper provides a comprehensive review of depolarization mechanisms and presents a systematic experimental evaluation of polarization sensitivities to temperature, dopant density, quantum efficiency, and crystal orientation. The highest measured polarization was ∼50%, consistent with the maximum theoretical value, obtained from a photocathode sample with relatively low dopant concentration and cooled to 77 K. In general, measurements indicate electron spin polarization can be enhanced at the expense of photoelectron yield (or quantum efficiency).

Scanning photocurrent microscopy is a powerful tool for investigating charge transfer and internal fields, which strongly influence carrier statics and dynamics in semiconductor nanowires. We performed comprehensive numerical modeling of the carrier dynamics of graded-composition and graded-doping AlGaAs nanowires to achieve a greater understanding of these nanowires. The simulation results indicated that the built-in electric field changes the shape of the scanning photocurrent microscopy profiles, which helped us to judge the dopant level, Al composition range and doping type of the material. The simulation results also assess the potential of the scanning photocurrent techniques in graded-doping and graded-composition nanowire properties.

Negative electron affinity GaAs wire-array photocathodes have been fabricated by reactive ion etching and inductively coupled plasma etching of bulk GaAs material followed by Cs-O activation. Scanning electron microscope has revealed that the thus obtained high-density GaAs wire arrays had high periodicity, large height, and good morphology. Photoluminescence spectra indicated the wire arrays were of good crystalline quality and free from any obvious damage. Compared to the original GaAs wafer, the photoluminescence peak positions of the wire arrays were somewhat red-shifted, which may be attributed to the temperature effect and strain relaxation. The wire-array structures showed significantly reduced light reflection compared with the original wafer due to the excellent light-trapping effect. Cs-O activation experiments of the GaAs wire arrays have been performed to reveal the effect of incident angle on quantum efficiency. The results show that maximum quantum efficiency was obtained at about 30°. Given these unique electrical and optical properties, a GaAs wire-array photocathode is an attractive alternative to its planar-structured counterpart.

The effects of AlGaAs/GaAs layer thickness and Al composition range on the spectral response and integral sensitivities of reflection-mode graded band-gap AlGaAs/GaAs photocathodes have been investigated and simulated. The experimental results demonstrate that the spectral response over the wavelength region of interest for graded band-gap photocathodes is greater than that for uniform band-gap cathodes, and the increase in long-wavelength response is more pronounced. These results can be attributed to the built-in electric field in the graded band-gap AlGaAs layer. We established a spectral response model of graded band-gap photocathodes based on the numerical solution of coupled Poisson and continuity equations. According to the model, we calculated the theoretical spectral response and sensitivities of graded band-gap cathodes, and found the optimum Al(x)Ga(1-x)As layer thicknesses are 6, 10, 16, and 22 μm for the reflection-mode cathodes with linearly graded Al composition x ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

Gallium arsenide (GaAs) nanowire array (NWA) photocathodes have unique electrical and optical properties. Based on studies about photon absorption, band structure, and electron transport properties of GaAs nanowire, a photoemission model for GaAs NWA photocathodes is established. According to the model, we simulate and analyze the photocurrent, spectral response, and absorption properties of ordered GaAs NWA photocathodes. The results present a very interesting phenomenon; the photocurrent and spectral response peak at incident angles of 20° and 30°, respectively. These special properties of NWA cathodes differentiate them from their thin film counterparts. We also analyze the effects of nanowire length and diameter on the photocurrent of NWA cathodes, and find the optimum height of the nanowires is 10 μm. This study shows that NWAs exhibit higher absorbance and excellent charge transport. Thus, GaAs NWA photocathodes are excellent candidates for electron sources.

The resolution model of a graded doping and graded composition transmission-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, we calculate the theoretical modulation transfer function (MTF) of different graded doping and graded composition structures. The simulation results show that both graded composition and graded doping structures can increase the resolution of the photocathode. The exponentially doping and linear composition photocathode has the maximum resolution among the possible graded doping and graded composition photocathodes. The resolution improvement is attributed to the built-in electric field induced by a graded composition or graded doping structure. The simulation results also show that the MTFs of AlGaAs/GaAs cathodes increase as the AlGaAs layer thickness decreases, or the incident light wavelength increases.

By calculating the energy distributions of electrons reaching the photocathode surface and solving the Schrödinger equation for an electron tunneling through the surface potential barrier, we have obtained an equation to calculate the energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes based on a two-minima diffusion model. According to the equation, we studied the effects of incident photon energies, diffusion lengths, and surface potential barrier on the electron energy distributions. The equation was also used to fit the measured electron energy distribution curves and the cathode performance parameters were obtained from the fitting. The Γ and L peaks in the theoretical curves are in agreement with the peaks in the experimental curves. The fitted barrier thickness 1.7 Å exactly reflects the GaAs-Cs dipole layer thickness.