Exploration on structural stability, electronic and optical properties of Cs-activated and Cs/O-activated AlGaN thin film and nanowire photocathode surface.

To explore effects of surface activation on AlGaN-based photocathode, this paper analyzes in detail the structural stability, charge transfer, band structure, density of states, absorption coefficient and reflectivity of Cs-activated and Cs/O-activated AlGaN thin films and nanowires by using first-principles. Our results reveal that adsorption energy of AlGaN thin films and nanowires adsorbed by Cs will gradually increase as Cs coverage increases, and structural stability will be weakened. Cs-adsorbed thin film surfaces are more stable than nanowire when Cs coverage is same. Cs/O co-adsorbed AlGaN systems are more stable under high Cs coverage. And Cs/O co-adsorbed AlGaN possess the most stable structure when the ratio of Cs to O is 2:1. Band structure and density of states imply that Cs and O adsorption introduce new energy levels, which are derived from s, p orbitals of Cs and s orbitals of O, respectively. Furthermore, only when the Cs/O ratio is 2:1, the work function of AlGaN thin film is lower than that of Cs-only adsorption, which is conducive to electron escape and improving quantum efficiency. Results of optical properties show that Cs activation and Cs/O activation can greatly improve the optical performance of AlGaN.