Investigation of the mechanism of the activation of GaAs negative electron affinity photocathodes

Abstract

This paper has discussed the mechanism of the formation of negative electron affinity (NEA) GaAs photocathodes which are treated with cesium and oxygen. The formation is mainly due to the drop of the electron affinity when the GaAs surface is covered with cesium atoms. This drop is not due to the formation of surface dipoles as usually supposed. The essence is that in cesium processing the surface potential barrier of GaAs is caused by the considerably weak nuclear force field of the surface cesium layer rather than by that of GaAs. The further decrease of the electron affinity by oxygen is due to the formation of O−2 ions below the cesium layer. These O−2 ions would let the excited photoelectrons in the conduction band pass easily into the weak nuclear force field of the cesium layer and get into the vacuum. A surface structure model and the energy diagram are suggested.