Optical EngineeringGradient-doping negative electron affinity GaAs photocathodes
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A variety of gradient-doping reflection-mode GaAs photocathode materials are designed and are prepared into negative electron affinity (NEA) GaAs photocathodes by (Cs,O) activation technique for the first time. These gradient-doping photocathodes are grown by molecular beam epitaxy (MBE), in which from GaAs bulk to surface doping concentration is distributed gradiently from high to low. The activation experimental results show that for gradient-doping GaAs photocathodes that are grown over p-type GaAs (100) wafer, where the epitaxial layer doping concentration range is 1019 to 1018 cm–3 and the epitaxial layer thickness is 1 µm, can achieve high integral sensitivity. The highest integral sensitivity 1798 µA/lm is achieved for gradient-doping GaAs photocathodes, which is much higher than that of a common uniform-doping GaAs photocathode under identical cleaning and activation condition. The inherent mechanism responsible for the fact that a gradient-doping GaAs photocathode can obtain higher quantum efficiency is also discussed.