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Activation of p-type III-V semiconductors with cesium and oxygen has been widely used to prepare negative electron affinity (NEA) photocathodes. However, the nature of the chemical species on the surface after the activation is not well understood. In this study, InP NEA photocathodes activated with cesium and oxygen are studied using synchrotron radiation photoelectron spectroscopy, also called photoemission. Based on the O 1s core level as well as the valence band spectra, Cs peroxide and Cs superoxide are identified on the InP surface. Transformation from Cs peroxide to Cs superoxide is observed after the activation, and is probably the major reason for the decay of the quantum yield of the photocathode. The oxidation of the InP substrate is also observed with elapse of time, adding to the decay of the quantum yield.
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Sun et al. (2007) studied this question.
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