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The (Cs,O)-activation procedure for p-GaAs(Cs,O)-photocathodes was studied with the aim of demarcating the domains of validity for the two practical models of the (Cs,O)-activation layer: The dipole layer (DL) model and the heterojunction (HJ) model. To do this, the photocathode was activated far beyond the normal maximum of quantum efficiency, and several photocathode parameters were measured periodically during this process. In doing so, the data obtained enabled us to determine the domains of validity for the DL- and HJ-models, to define more precisely the characteristic parameters of the photocathode within both of these domains and thus to reveal the peculiarities of the influence of the (Cs,O)-layer on the photoelectron escape probability.

The review of the studies of GaAs and GaN surfaces performed by the authors at the Institute of Semiconductor Physics (Novosibirsk) are presented. The results of these studies are used for further elucidation of the physics of photoemission from the surfaces with negative electron affinity (NEA) and improving the performance of NEA-photocathodes. In particular, the requirements to III-V semiconductor surfaces, which are necessary for the preparation of NEA photocathodes with ultimate parameters, are analyzed. To meet these requirements, the experimental procedures aimed at preparing atomically clean and flat surfaces with certain reconstructions, which are subsequently activated to the state of NEA in ultra-high vacuum by coadsorption of cesium and oxygen, are developed. The surface morphological, structural, and electronic phenomena occurring on various stages of these preparation procedures are studied, and the relation of these phenomena to the physical limits of NEA-photocathode performance are discussed.