For the InxGa1-xAs photocathode with GaAs substrate, the range of the infrared spectrum response is extended with the In component increase, but the bandgap decreases, and the corresponding surface escape probability and the electron diffusion length will be greatly reduced. In order to overcome these difficulties, a method of dividing the emission layer into multi sublayers is proposed, namely, In component was gradually increased with the gradual increase of sublayer. Three types of reflective InGaAs/GaAs photocathodes with varied component emission layer were epitaxial grown by MOCVD, in which one sample is a thin emission layer InGaAs/GaAs photocathode, and the other two samples are the conventional thickness emission layer InGaAs/GaAs photocathodes and their emission layers are divided into three 0.6μm thickness sublayers and a 0.04 μm thickness varied component layer is introduced between sublayers in one of them, aims to form a gradual change structure between the sublayer interfaces. Then, the influences of different emission layer structures on the InGaAs/GaAs photocathode performance are investigated through comparing and analyzing the measured spectral response curves. Results show that the critical thickness of the surface epitaxial layer can be effectively improved by using the sublayer interface with varied component, and the test data after activation experiments indicate that the photocathode with multiple emission sublayers and varied component layer between them has the best spectral response, meanwhile, through fitting spectral response curves, we found the increase of its surface escape probability and electron diffusion length, which leads to the higher integral sensitivity and long-wave response.
InGaAsP photocathodes show great potential for near-infrared applications, particularly at 1.06 μm. In order to study the influence of Zn doping on the optoelectronic properties of In0.875Ga0.125As0.25P0.75, models of In0.875Ga0.125As0.25P0.75, In0.84375Ga0.125Zn0.03125As0.25P0.75 and In0.875Ga0.09375Zn0.03125As0.25P0.75 were constructed. The band structure, formation energy, Mulliken population and optical properties of the Zn doping crystals were calculated from first-principles. Results show that Zn doping reduces the stability of In0.875Ga0.125As0.25P0.75, and In atom is more inclined to be replaced by Zn atom due to In0.84375Ga0.125Zn0.03125As0.25P0.75 has the lower formation energy than In0.875Ga0.09375Zn0.03125As0.25P0.75. The covalency of In-As, In–P and GaP bonds is enhanced due to the Zn atom replacing In atom, but Zn atom replaces Ga atom only increases the covalency of Ga-P bond. After Zn doping, the Fermi levels of In0.84375Ga0.125Zn0.03125As0.25P0.75 and In0.875Ga0.09375Zn0.03125As0.25P0.75 appear in the valence band, indicating that they show p-type properties. Zn atom replacing In atom increases the bandgap, but Zn atom replacing Ga atom is just opposite. Compared to In0.875Ga0.125As0.25P0.75, doping Zn improves the carrier concentrations, causing the increase of valence band holes which are closely related to conductive behavior of Zn doped crystals. The optical properties of In0.84375Ga0.125Zn0.03125As0.25P0.75 and In0.875Ga0.09375Zn0.03125As0.25P0.75 are almost the same. The substitution of Zn atom for In or Ga atom in In0.875Ga0.125As0.25P0.75 improves the electron transition and increases the absorption coefficient in low energy side, but the metal reflective region shifts to lower energy side.
GaAs is an important short-wave near-infrared photocathode material. In this paper, the first-principles plane wave pseudopotential method based on the density functional theory framework is used to study the influence mechanism of external electric field on the electronic structure of GaAs. Applying an electric field in different directions to GaAs shows that the (011) electric field direction has the strongest effect on opening the GaAs energy gap. Then, the electric fields of different strength are applied along the (011) direction. The results show that the energy gap of GaAs is 0.937eV when no electric field is applied. With increasing the electric field strength in the (011) direction, the energy gap of GaAs decreases gradually, when the electric field strength reaches 1eV/Å/e, the energy gap of GaAs is almost zero. Notice that in the conduction band region where the total density of state of GaAs gradually shifts to Fermi surface and the Span gradually decrease with increasing the electric field strength, while valence band is the opposite of the conduction band.
Based on the properties of InGaAs photocathode, the critical thickness of epitaxial layer is calculated, the structure of InGaAs/InP photocathode is designed, and the In0.53Ga0.47As/InP semiconductor material samples are epitaxially grown by MOCVD. We use the ultra-high vacuum preparation technology in cathode growth. After chemical cleaning, utilizing the GaAs photocathode multi-information measurement system which prepared by our laboratory, the InGaAs/InP photocathode samples are thermally purified at 650°C, 550°C and 400°C, respectively. Finally, the thermal purification results of InGaAs/InP photocathode materials are obtained through the surface analysis which carried out by XPS. At the same time, the spectral response curves at different thermal purification temperatures are given out. The research data will contribute to the further development of InGaAs/InP photocathode in the field of near-infrared low light level detection.
Combining forest fire monitoring system with fire extinguishing unmanned aerial vehicle (UAV) formation , a system design scheme for forest fire security and protection system is also presented in this paper, at the same time , this system combined with machine vision technology and GIS positioning system , and the fusion of multi-date information to guarantee the accurate and precise positioning of the fire events, etc . At the first, the fire information will be real-time processing by airborne imaging devices, then be transmitted to the control center and be analyzed , the fire area, the number of the required fire extinguishing UAV and UAV formation information will be calculated and the take-off command will be sent . During the UAV formation flight process, the geographic data and formation flight data of UAV will be transmitted in real time and the perceived wind speed data will be transmitted back to the control center when the UAV formation arrives at the scene of fire , then , after the correction of the date , the command of sending fire extinguishing material will be sent by control center to put out forest fires, ultimately avoid the fire spread, minimize losses, and enhance the security level of forest fire disaster lash-up processing capability. Therefore , this forest fire security system have a good application prospect.
According to the characteristics of forest fire spread fast, difficult to save, in order to determine the forest fire danger rating quickly and strive for fire time, it is important to calculate the fire area. Considering that there is a close relationship between the gray image of infrared image with temperature, the gray value of infrared image corresponds to high temperature object will be large. At the same time fire occurs, the temperature of the flame is generally higher than the temperature of the surrounding environment , infrared image can be used to exclude a lot of non-fire interference source and provides a simple and convenient criterion for the identification and segmentation of flame area, so this paper presents a calculation method of fire area based on infrared image. The method take full advantage of the significant characteristics of the obtained forest fire infrared image, the coordinate relationship between the image and the camera is established and the camera is calibrated. Then extract the edge of the forest fire spread, utilizing sobel operator rough location, and then grayscale image interpolation, cubic spline interpolation function so that the target to achieve sub-pixel level grayscale images after interpolation, the use of the maximum variance between the threshold is determined to achieve sub-pixel edge detection. The edge of the forest fire spread is extracted, with images used to calculate the forest fire burned area, with the error of measuring result calculated. The results show that the maximum error is controlled within 4.5%. Therefore, the forest fire area calculation methods can be applied to the fire control and post- disaster assessment of forest fire. The method is feasible and convenient calculation and will be of great significance to the forest-fire prevention.
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