Photoemission spectroscopy study of indium phosphide NEA photocathodes: Activation, decay mechanism, and energy and angular distribution

The high quantum efficiency (QE) of InP-based photocathode is realized by depositing Cs and oxygen molecules on the surface of heavily doped p-type semiconductors, where they form a thin activation layer. The atomic structure of this Cs/0 activation layer is, however, not well understood, and the properties of photoelectrons from InP-based cathodes also require careful study. In this study, three aspects of InP photocathodes were studied: (1) the atomic arrangement of Cs oxides in the activation layer; (2) the decay mechanism of InP photocathodes in a standard ultra high vacuum (UHV) system; and (3) the energy and angular distribution of photoelectrons from InP photocathodes.;The atomic arrangement of Cs oxides was investigated by using Angle Dependent Photoemission Spectroscopy (ADPES). Two distinct peaks in the O1s core level and valence band spectra led us to the discovery of two molecular oxygen species incorporated in the thin activation layer: Cs peroxides (O2 2-) and Cs superoxides (O2-). The different angular dependences of these oxides in the photoemission spectra result from the different vertical locations of oxygen molecules in each Cs oxide, and the lateral distribution model of Cs peroxides and Cs superoxides was suggested based on this angular dependence and the estimated thickness of the Cs/O layer (∼7A).;The QE of InP photocathodes in a standard UHV system decreases with time, while commercial sealed photocathode tubes last for years without decay. This QE decay is due to the chemical transformation of Cs peroxides to Cs superoxides and subsequent substrate oxidation, as deduced from observations of the peak evolutions in the valence band, O1s core level, and the In4d core level photoemission spectra. This transformation is thermodynamically favorable when residual oxygen is available, and the decrease of Cs peroxides is in agreement with the lateral distribution model.;Finally, we studied the energy and angular distribution of photoelectrons from InP photocathodes. These properties have practical importance in terms of the resolution and focusing capability of photocathodes. Two different kinds of photoelectrons were observed in energy distribution curve (EDC) measurements. They are thermalized electrons in Gamma valley and electrons transferred into L valley, and we observed the increase of L valley electrons when a higher photon energy is used. The L valley electrons have a larger angular distribution than the Gamma valley electrons, and this can be explained by the larger effective mass of the L valley relative to the Gamma valley.