The AlGaAs photocathode can be used in the field of underwater optical communication because of its fast response speed and adjustable spectral response range. In order to solve the problem that the low light absorption of the AlGaAs emission layer limits the improvement of its quantum efficiency, the distributed Bragg reflector (DBR) structure is used to reflect the light at a specific wavelength back to the emission layer to further increase the absorption rate, thus improving the response capability of the photocathode at 532 nm. The spectral response model of the AlGaAs photocathode with DBR structure is obtained by solving one-dimensional continuity equation. The optical model of the AlGaAs photocathode with enhanced response at 532 nm is established by the finite-difference time-domain method. The effects of the sublayer periodic pairs, the sublayer material and the thickness of emission layer and buffer layer on the absorption rate of emission layer are analyzed. The light absorption distributions of AlGaAs photocathode with and without DBR structure are compared, and the influence mechanism of DBR structure on the blue-green light absorption capacity of AlGaAs photocathode emission layer is clarified, which can provide a theoretical basis for designing its structural parameters. The results show that the DBR structure with a periodic pair of 20 and Al<sub>0.7</sub>Ga<sub>0.3</sub>As/AlAs has the best reflection effect on 532 nm light. Based on the DBR structure, when the thickness of the emission layer and buffer layer are 495 nm and 50 nm, respectively, the emission layer has the best absorption rate of 532 nm light. Furthermore, two kinds of AlGaAs photocathodes with and without DBR structure are prepared by the metal-organic chemical vapor deposition technology, and the reflectivity and profile structure of the grown samples are characterized. Then the Cs/O activation experiments are performed to compare the spectral response curves. It is found that the spectral response of the AlGaAs photocathode sample with DBR structure at 532 nm wavelength is about twice that of the sample without DBR structure.
Abstract Surface photovoltage (SPV) in p + -GaAs/p-GaAlAs/p-GaAs has been studied by establishing a multilayer model and measuring the SPV at room temperature. The model mainly considers surface recombination velocity, interface recombination velocity and the space charge region (SCR) at the surface of p + -GaAs. The SPV of the multilayer structure is shown to originate predominantly from the minority carrier diffusion, which caused photovoltage between the surface and bottom. Subsequently, the minority carrier diffusion lengths in p + -GaAs and in p-GaAs are obtained from fitting experimental data to the theoretical model. At the same time, the minority carrier diffusion length in p-GaAs is obtained by illuminating the backside (illuminating on p-GaAs) of the p + -GaAs/p-GaAlAs/p-GaAs. The p + -GaAs in p + -GaAs/p-GaAlAs/p-GaAs structure with different thickness are measured to show the variation of SPS with different thickness, but the experimental parameters are not affected. In multi-layer structure, the SPV contributed by different layers has a great difference with different dark saturation current density.
Using the first-principles method based on the density functional theory (DFT), the work function of seven different GaN (0001) (1×1) surface models is calculated. The calculation results show that the optimal ratio of Cs to O for activation is between 3∶1 and 4∶1. Then, Cs/O activation and stability testing experiments on reflection-mode negative electron affinity GaN photocathodes are performed. The surface model [GaN (Mg): Cs] Cs-O after being activated with cesium and oxygen is used. The experiment results illustrate that the adsorption of O contained in the residual gas increases the surface potential barrier and the reduction of the effective dipole quantity is the basic cause of the quantum efficiency decay.
To characterize the degree of damage to the GaAs photocathode surface caused by H+ ion back bombardment in the electron-bombarded complementary metal–oxide–semiconductor (EBCMOS), Stopping Range of Ions in Matter software based on the Monte Carlo method was used to investigate the effect of H+ ions with different incident energies on the surface of Cs-O (Cs-F) activated GaAs photocathode. During the simulations, different Cs/O (Cs/F) ratios ranging from 1:1 to 4:1 were considered. The sputtering rates, backscattering electrons, and longitudinal and lateral displacements along with vacancies/ions were investigated. According to the analysis of sputtering rates and vacancies, the optimal Cs/O ratio and Cs/F ratio are 3:1 and 4:1, respectively. With the increase in the incident energy, the backscattering rates decrease, the peak value of the H+ ion distribution decreases, while the corresponding peak position increases, and the peak value of the vacancy distribution increases first and then decreases, while the corresponding peak position increases. In addition, the projected ranges, and lateral and longitudinal displacements increase with the increase in incident energies, while the projected ranges may far exceed the straggle lengths and make the ion trajectory become more and more concentrated in the high incident energy region. This work helps to understand the degeneration mechanism of the GaAs photocathode operating in EBCMOS.
In order to improve the cathode performance, an exponential-doping structure is applied to the preparation of transmission-mode GaAs photocathodes, in which a forward-directed induced electric field is formed to facilitate photoexcited electrons movement toward surface. To verify the actual effect of the exponential-doping structure, two types of transmission-mode GaAs photocathode samples were grown by molecular beam epitaxy, in which one is exponentially doped and the other is uniformly doped. For the exponential-doping sample, the beryllium doping concentration in the active-layer decreased quasi-exponentially from bulk to surface ranging from 1*1019 cm"3 to 1*1018 cm"3. Following the chemical cleaning and heat cleaning, the activation experiments of the cathode module of 18 mm in diameter was performed in an ultrahigh vacuum chamber (base pressure <; 10" Pa) using the improved activation technique. After activation, the cathode module was transferred to the seal vacuum chamber and indium sealed into the intensifier tube, then the spectral response curve of the sealed image tube was measured by the spectral response measuring system. The results show that the first photocurrent peak of the exponential-doping sample was lower than that of the uniform-doping one during the initial stage of Cs exposure activation in the high-low temperature activation process. However, with the increase of activation time, the exponential-doping sample gradually exceeded the uniform-doping one in the photocurrent peak. Besides, the spectral response measuring results show that the exponential-doping photocathode exhibits a higher spectral response with a distinct inflexion in the long-wavelength threshold region. Through the spectral response curve fit, the fitted electron diffusion length of the exponential-doping sample is larger than the uniform-doping one. All results indicate that the exponential-doping photocathodes can obtain higher cathode performance and better photoemission capability.
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