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Photoemission from p-GaAs(Cs,O) and p-GaN(Cs,O) photocathodes with NEA will be discussed. Photoelectron energy analyzers for determination of longitudinal and transverse energy distributions will be compared. By use of these analyzers, the following results were obtained. Domains of validity for actual models of the (Cs,O) - activation layer of GaAs - photocathode were determined. It was revealed, that the maximal value of QE of p-GaAs(Cs,O) - photocathode is achievable with (Cs,O) – activation layer, which fit with dipole layer model. On the other hand, the maximal value of NEA for this photocathode is achievable with (Cs,O) – layer, which correspond to heterojunction model. Two – step photoelectron escape from NEA-photocathode with their transitional capture within near – surface band bending region is proved. Subsequent escape of photoelectron to the vacuum occurs in the ballistic mode, or is accompanied by the elastic and inelastic scattering processes. Elastic scattering is dominated by the random electric field at the semiconductor – vacuum interface and by the surface roughness. Dominant mechanisms of inelastic scattering of photoelectrons include their interaction with surface optical phonons and with surface plasmons. Both processes of inelastic scattering are accompanied by the scattering of photoelectron's direction.
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