The desorption of oxygen and carbon contamination are a key issue on improving the quantum efficiency of negative electron affinity GaAs-based photocathode during the preparation process. In this article, O-bonded and C-bonded absorption are executed in the calculation of pristine (100)-oriented GaAs photocathode of planar structure and nanowire structure. By analyzing the absorption energy, work function and dipole moments of different adsorption models, it is found that the adsorption of impurity atoms changed atomic and electronic structure of GaAs(100) pristine surface and affected the stability. The findings suggest that, oxygen impurities are more difficult to remove than carbon impurities due to more negative absorption energies especially in the surface layer. However, C-absorbed models may have bigger work function values than O-absorbed models in the most cases, which are not beneficial to the photoemission, and the phenomenon can be verified by the calculation results of surface dipole moments.
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.
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