Narrow cone emission from negative electron affinity photocathodes

The need for high brightness (e.g., 106Acm−2str−1 at 10keV) electron sources is well established. Negative electron affinity (NEA) semiconductor structures have been studied for this application for several years. It has been argued that one advantage of these materials, particularly GaAs, is that the low effective mass, m*, of electrons in the semiconductor would lead to a narrow cone of emission, and hence increased brightness for a given current density, as a result of Snell’s Law. However this advantage would only apply if the electrons were emitted directly from the Γ-valley (with low m*) rather than indirectly, that is via scattering into the L-valley. We present here a direct experimental confirmation of this narrow cone of emission (semiangle 15°) and describe a quantitative model to account for the results. In photocathodes based on group III-nitride materials, the cone is predicted to be about twice as wide because of the larger values of m*.