In order to research H+ beam radiation on photoelectric performance of GaAs photocathodes used in low-light-level optoelectronic devices, based on Monte-Carlo method, quick calculation of damage, along with effects of Cs and Cs-O activation layer on ion trajectory, performance, ionization of ions and recoils are discussed. From the simulation results, the average stopping range increases with the increase of incident energy, and the dispersion varies with the incident angle, the minimum dispersion at 1 keV is obtained at 60° when Cs-O ratio is 2:1, and the minimum dispersion at 2 keV is obtained at 60° when Cs-O ratio is also 2:1. In addition, the produced vacancies increase with the incident energy while the value is almost not influenced by the incident angle, and the backscattered ions increase as the incident angle increase both in 1 keV and 2 keV cases, and decrease with the incident energy. Also, ionization dominate the H+ ion bombarding process instead of producing vacancies and phonons. With the increase of incident energy, the percentage of ionization of ions increases, while those of phonons of ions and ionization of recoils decrease. However, the corresponding percentages of ions and recoils remain nearly unchanged with the increase of incident angle and the variation of composition of Cs or Cs-O activation layer.
Three types of gallium nitride (GaN) materials grown on sapphire (001) by metal chemical vapor deposition (MOCVD) were design for the study of different doping types on the characteristics of GaN epilayers. Surface photovoltage (SPV) spectroscopy of samples was measured in the photo energy range 2.9≤hv≤4.5eV with different chopping frequency. The similar surface photovoltage signals were obtained under sub-band-gap illumination in the photo energy range 2.9≤hv≤3.4 eV and the origins about signals were determined by comparing the surface photovoltage magnitude and phase spectra. By changing the incident light frequency, it can be found that the surface photovoltage magnitude shows a downward trend in the entire photon energy range, and the surface photovoltage magnitude has a more significant decrease in the sub-band gap region. In additional, a laser with 3.06eV photo energy was used to aid measured the surface photovoltage signals of sample with Si doped in the photo energy range 2.9≤hv≤4.5eV with chopping frequency at 172 Hz, 440 Hz, 1k Hz and 3k Hz. The defect states at the interface of the undoped GaN layer and Al2O3 were proved to be the major contribution to the SPV signals in the sub band gap region and were "slow processes" during the formation of SPV of the GaN samples.
Negative-electron-affinity GaAs-based photocathodes have already found widespread application in modern night vision detectors and vacuum electron sources. Considering the importance of surface micro-area analysis for cathode preparation, a new ultrahigh vacuum interconnection system for photocathode preparation and characterization was developed, wherein the scanning focused X-ray imaging positioning technique combined with the X-ray induced secondary electron image was applied to characterize the surface components in the specified micro region of semiconductor photocathodes. With the aid of the advanced characterization technique, the surface components of micro regions of interest for GaAs cathode samples after cleaning and Cs-O activation were analyzed. The experimental results show that the GaAs cathode samples would be subjected to secondary contamination from the metal sheet of sample holder, accompanied by a small amount of sodium and cesium. The subsequent heat treatment and Cs-O activation can hardly remove the sodium contamination, which can affect the arsenic desorption during heat treatment, hinder the Cs-O adsorption in the activation process, and finally reduce the photoemission performance of the activated cathode. Through the application of the X-ray induced secondary electron image, the optimal cleaning method for GaAs cathode was investigated. This surface characterization technique is of practical value to improving analysis accuracy and optimizing the cathode preparation process.
To achieve negative-electron-affinity state, the atomically clean surface of GaAs-based photocathode is usually activated by cesium and oxygen in the ultrahigh vacuum environment. In view of the required computer-control of evaporation flow rates, the solid oxygen dispenser instead of gaseous oxygen is urgently needed just as the regular cesium dispenser. Accordingly, the solid cesium and oxygen dispensers were applied to activate epitaxial GaAs cathode samples. Two types of solid oxygen dispensers composed of barium peroxide powder and silver oxide powder respectively are employed to improve cathode photoemission performance. The experimental results show that the barium peroxidebased oxygen dispenser can release more oxygen and bring in higher activation photocurrent and spectral response than the silver oxide-based one. The unsatisfactory feature is that the silver oxide-based oxygen dispenser released effectual oxygen gas more slowly than the barium peroxide-based oxygen dispenser. Therefore, an effective activation technique was proposed to ameliorate this unfavorable phenomenon for the silver oxide-based dispenser, which can bring out the desired symmetry of photocurrent curve shape during the Cs/O alternate activation process. The improved activation technique would provide guidance for the optimization of activation craft.
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