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Negative electron affinity GaAs photocathode is made from p-type doped photocathode, and the p-type GaAs photocathode is commonly doped by Be or Zn atoms. To investigate the doping differences of the two dopants, the undoped GaAs(100)β(2 × 4) reconstruction surface model and doped models with Zn or Be doping at position I, II, III, IV, V are built, and the related performance parameters are calculated and compared. The first-principle method is used to calculate the geometric structure, formation energy, band structure, density of state, work function and Mulliken population of the surface models, it is found that Be is more suitable for forming interstitial doping and Zn is better to form substitution doping, and position II is the best doping position for Zn substitution doping. Then the experiment is carried on the two doped GaAs photocathodes grown by MBE and MOCVD, the experimental result shows the Zn-doped GaAs photocathode grown by MOCVD has better spectral response and quantum efficiency, which verifies the correctness of theoretical result. [ABSTRACT FROM AUTHOR]

The quantum efficiency of the blue–green transmission-mode GaAlAs photocathode has been studied. Two transmission-mode GaAlAs photocathodes with different structures are prepared, and the quantum efficiency curves are measured. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of photocathodes such as the electron diffusion length, the back interface recombination velocity, and the surface electron escape probability. The effects of the Al compositions, the thickness of emission layer, and the electron diffusion length on quantum efficiency are investigated. The results show that both of transmission-mode GaAlAs photocathodes are sensitive to the blue–green light. The peak quantum efficiency of one photocathode appears at about 565 nm, while that of another photocathode appears at about 535 nm. The Al composition of emission layer plays a major role on the peak position of quantum efficiency of transmission-mode GaAlAs photocathode. Besides, the thickness of emission layer and the Al composition of window layer also have a large influence on the quantum efficiency. [ABSTRACT FROM AUTHOR]

Abstract: The conventional quantum efficiency formula for the reflection-mode GaAs photocathode is not applicable to the photocathode with a thin emission layer. The revised quantum efficiency formula has been solved from one-dimension continuity equations, in which the electrons generated in the GaAlAs buffer layer are considered. According to the revised formula, we analyze the impact of some relational performance parameters on the quantum efficiency, such as the thickness of emission layer, the interface recombination velocity etc. Besides, we use the revised and conventional formula to fit the experimental curves of two photocathodes which have the different-thickness emission layers respectively. The results show that compared with the conventional quantum efficiency formula, the revised formula is more suitable to the reflection-mode GaAs photcathode with a thin emission layer, and the GaAlAs buffer layer takes part in the photoemission. [Copyright &y& Elsevier]

Abstract: Metal–insulator–semiconductor (MIS) configuration has been used to measure the surface photovoltage (SPV) spectroscopy of p-GaAs substrate and p-Al x Ga1−x As/p-GaAs structure. The p-AlGaAs layer was on p-type GaAs substrate grown by metal organic chemical vapor deposition. An ideality factor was used to investigate the relationship of the measured SPV signals to the “real” SPV signals. Dependence of the surface photovoltage on incident photon intensity for band-to-band excitation was adopted for the calculation of ideality factor. The ideality factor of MIS configuration was found to be 0.008 for our sample with both sides polished in air ambient. In order to calculate the parameters of p-Al x Ga1−x As/p-GaAs structure, the minority carrier diffusion length in the GaAs substrate was determined from a linear plot of inverse SPV vs. inverse absorption coefficient by intercepting the line with the x-axis. Other parameters of p-Al x Ga1−x As/p-GaAs were studied through the simulation of surface photovoltage. [Copyright &y& Elsevier]

For the applications in vacuum photodetectors and photoinjectors, a crucial limiting factor for conventional GaAs photocathodes is the limited lifetime, depending on the Cs–O activation layer vulnerable to the harmful residual gases. In order to develop a type of GaAs-based photocathode with good stability and repeatability, Cs/O activation and multiple recesiation experiments under the same preparation condition were performed on reflection-mode exponential-doped GaAs and GaAlAs photocathodes grown by metalorganic vapor phase epitaxy, and quantum efficiency and photocurrent decay were measured after activation and recesiation. The experimental results show that the photoemission characteristics on cathode degradation and repeatability are different between GaAs and GaAlAs photocathodes. In an unsatisfactory vacuum system, the operational lifetime for GaAlAs photocathode is nearly twice longer than that for GaAs photocathode after Cs/O activation under a high intensity illumination. After multiple recesiations, the quantum efficiency and operational lifetime for GaAlAs photocathode remain nearly unchanged, while those for GaAs photocathode become lower and lower with the increase of recesiation cycles, which reflects the superiority in stability and repeatability for GaAlAs photocathode in contrast to GaAs photocathode operating in the poor vacuum environment. [ABSTRACT FROM AUTHOR]

Abstract: Resolution properties of reflection-mode exponential-doping and uniform-doping GaAs photocathodes were calculated and comparatively analyzed by using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation. The calculated results show that compared with the uniform-doping GaAs photocathode, the exponential-doping structure can improve significantly not only the resolution, but also the quantum efficiency of the photocathode. This improvement differs from the approach for achieving high resolution by reducing the emitting layer thickness and the electron diffusion length or by increasing the recombination velocity of back-interface , which results in low quantum efficiency. Furthermore, the improvement of resolution and quantum efficiency of the reflection-mode exponential-doping GaAs photocathode is due the effect of the built-in electric field on electron transport and on lateral diffusion. [Copyright &y& Elsevier]

Abstract: In view of the effect of doping structure on the surface activation behavior, the Cs–O activation experiments under the same preparation condition were performed on the exponential-doping and uniform-doping GaAs photocathodes. The activation results show that, compared with the uniform-doping photocathode, the exponential-doping photocathode presents a lower photocurrent peak after the initial Cs activation and a higher photocurrent peak after the Cs–O alternate activation. Based on the double dipole model, the parameters of surface potential barrier are obtained from the calculation of longitudinal energy distribution of emitted electrons according to the ratio of photocurrent peak after Cs–O activation to that after Cs activation. The discrepancy in the photocurrent evolution during the entire activation process is related to the different variations in surface potential barrier profile. [Copyright &y& Elsevier]

Abstract: The properties of GaAs photocathodes mainly depend on the material design and activation technique. In early researches, high-low temperature two-step activation has been proved to get more quantum efficiency than high-temperature single-step activation. But the variations of surface barriers for two activation techniques have not been well studied, thus the best activation temperature, best Cs–O ratio and best activation time for two-step activation technique have not been well found. Because the surface photovoltage spectroscopy (SPS) before activation is only in connection with the body parameters for GaAs photocathode such as electron diffusion length and the spectral response current (SRC) after activation is in connection with not only body parameters but also surface barriers, thus the surface escape probability (SEP) can be well fitted through the comparative research between SPS before activation and SEP after activation. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barrier I and II can be well fitted through the curves of SEP. The fitting results were well proved and analyzed by quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (ADXPS) which can also study the surface chemical compositions, atomic concentration percentage and layer thickness for GaAs photocathodes. This comparative research method for fitting parameters of surface barriers through SPS before activation and SRC after activation shows a better real-time in system method for the researches of activation techniques. [Copyright &y& Elsevier]