Revisiting heat treatment and surface activation of GaAs photocathodes: In situ studies using scanning tunneling microscopy and photoelectron spectroscopy

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as ${\mathrm{Cs}}_2\mathrm{Te}$ , with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with ${\mathrm{Cs}}_2\mathrm{Te}$ activation on GaAs used heat cleaning temperatures around $400{}^{°}\mathrm{C}$ to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around $580{}^{°}\mathrm{C}$ , which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for $580{}^{°}\mathrm{C}$ heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a $580{}^{°}\mathrm{C}$ heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as ${\mathrm{Cs}}_2\mathrm{O}$ or ${\mathrm{Cs}}_{11}{\mathrm{O}}_3$ .