Role of oxygen in semiconductor negative electron affinity photocathodes

Source contamination, photovoltage effects, and stimulated election desorption of cesium are factors that determine the initial high performance and longevity of negative electron affinity electron sources. As part of the study of these factors, we investigate the effect of oxygen contamination during the activation process and operation of a traditional GaAs (100) and a novel GaN (0001) emitter. We use synchrotron radiation photoemission spectroscopy, a focused mercury arc discharge lamp, and a helium neon (HeNe) laser to obtain simultaneously elemental analyses of the emitting surface and the corresponding total quantum yield at various stages, respectively, for GaN and GaAs. Our results indicate: (1) carbon uptake does not occur in our carbon free system over time, (2) oxygen uptake is observed for both GaN (0001) and GaAs (100) activated surfaces, a property common to the chemistry of the Cs/O adlayer, (3) the oxygen species appears to change over time and the initial species is assigned to an ion of nondissociated oxygen in the Cs/O activation layer, (4) the chemical changes of the Cs/O adlayer are not accompanied by a significant loss of cesium from the surface, and (5) the onset of decay of the quantum yield begins at a later time for the GaN (0001) emitter in comparison to the GaAs (100) electron source. A chemical model for the activation layer and its transformation over time is developed, consistent with points (1) through (4) in a separate report [F. Machuca et al. (unpublished)]. The complete account of the decay of the quantum yield of both Cs/O activated III-V emitters is compared and discussed in this article. (C) 2002 American Vacuum Society.