The influence of surface barriers on the photoemission characteristics of AlGaN photocathodes

In order to research the photoemission characteristics of AlGaN photocathodes with different surface barrier structures, AlGaN photocathodes with varying Al composition were grown by MOCVD. The atomic composition of the photocathode surface was measured by Ar+ ion sputtering and XPS. We find that the C atoms only exist at the surface, but O atoms are present up to 6 nm from its surface. The photocathodes were cleaned and activated with different processing methods, and the result show that the quantum efficiency value of Cs/O activated AlGaN photocathodes were about 42% higher than that of Cs-only activated AlGaN photocathode. Through thermal cleaning at 850 °C and Cs/O activation, the maximum quantum efficiency of the AlGaN photocathode was 49.39% at 240 nm. Through simulation of the electronic energy distribution, we determined that the surface barrier I structure formed by [AlGaN: Mg-Cs] dipoles at the photocathode surface was mainly affecting the surface electrons escapes probability and determining the photocathode quantum efficiency. The surface barrier II structure formed by [Cs-O] dipoles at the photocathode surface determined the photocathode performance, and also affected the electronic energy distribution concentration.