[GaN(Mg)-Cs]:[O-Cs] model for the Negative Electron Affinity GaN (0001) surface

Because of the limitations of the existing Negative Electron Affinity (NEA) photocathode surface emission models, [GaN(Mg)-Cs]:[O-Cs] photoemission model based on dual-dipole model is established, which can explain the photoemission mechanism of the NEA GaN photocathode well. Cs, O adsorption process on the GaN (0001) surface during the activation is discussed, and the GaN(Mg)-Cs dipole layer is found having a unified direction which is conducive for photoelectron escaping, so photocurrent has a significant increase when Cs is introduced. There is not a unified direction for the O-Cs dipole layer. Because of the surface defects, part of the O-Cs dipoles have the direction which is conducive for photoelectron escaping, and the photocurrent has a modest growth after introducing O. Finally, the performance of GaN and GaAs photocathode are compared based on the photoemission model, and the reason of better stability of NEA GaN photocathode is interpreted.