Investigation of the cesium activation of GaN photocathodes by low-energy electron microscopy

Low-energy electron microscopy (LEEM) was performed on $p$-$\mathrm{Ga}\mathrm{N}$ samples during in situ cesium deposition. LEEM images of electron reflectivity recorded as a function of the incident electron energy at different $\mathrm{Cs}$ coverages allowed to spatially resolve the evolution of the local work function (WF) during the activation process. While the average WF drops by more than 3 eV, the local WF remains quite uniform across the surface throughout the activation process. Maximum fluctuations of less than 0.2 eV were observed in the WF maps for $\mathrm{Cs}$ coverage of a fraction of a monolayer. These fluctuations are mainly related to the surface topography, in particular, to the atomic steps’ structure, which replicates the substrate miscut. Apart from these weak spatial fluctuations, no $\mathrm{Cs}$ clusters that would induce strong local WF contrast were observed at the scale of the 20-nm resolution of the measurements. These observations agree with the simple model of semiconductor activation to negative electron affinity that describes the formation of a dipole layer as responsible for the lowering of the WF. Additionally, at complete $\mathrm{Cs}$ coverage, the WF becomes fully homogeneous over the surface, smoothing out features originating from defects and topography.