Theoretical study of cesium and oxygen activation processes on GaN (0001) surface

The structure properties and work function of reflection-mode GaN photocathodes are investigated by using first-principles calculation within density-function theory (DFT). Seven different GaN (0001) (1×1) surface models are used in this paper to simulate cesium and oxygen activation process. Before starting the surface models calculations, clean bulk GaN surface is first optimized, when compared with the models before optimization, the correctness of the calculations method is verified. Next, change of work function of different surface models caused by adsorption is calculated and analyzed, the results show that over-cesiuminizd atmosphere is not only benefit for work function declining but also conductive to the formation of negative electron affinity (NEA), the optimal ratio of Cs to O for activation is between 3:1 and 4:1, excess Cs or O modules increase the work function and undermine the photocathode. Then a series of experiments are performed to verify the calculation results. First of all, NEA GaN photocathode activation and evaluation system is established, “yo-yo” activation method is verified. Then, an activation experiment is performed on high quality p-type Mg-doped reflection-mode GaN substrate grown by metal organic chemical vapor deposition (MOCVD), the photocurrent is controlled and monitored by multi-information on-line monitoring system, the results are consistent with our calculation models. Finally, combined with the data provided by Stanford University, schematic energy band variation of the GaN photocathode after only-Cs and Cs/O activations is given. Our result opens the possibility to engineer the activation properties of GaN photocathode.