Research on Cs activation mechanism for Ga0.5Al0.5As(0Â 0Â 1) and GaN(0Â 0Â 0Â 1) surface

Based on first-principle density functional theory (DFT), plane wave with ultrasoft pseudopotential method was used to calculate and compare the Cs activation mechanism for Ga0.5Al0.5As(0 0 1) surface and GaN(0 0 0 1) surface. In this work, eight possible Cs adsorption sites are chosen for the Ga0.5Al0.5As(0 0 1) surface while five high-symmetry sites are considered in the calculation model of GaN(0 0 0 1) surface. Results show that Cs adsorption lowers the surface work function and benefits to get the most stable adsorption sites. Then dipole moment with different Cs coverage on two surfaces is investigated. The dipole moment decreases with the increase of Cs coverage and GaN(0 0 0 1) surface changes more obviously than Ga0.5Al0.5As(0 0 1) surface. The repulsion between Cs atomic dipole-dipole is enhanced and it causes depolarization and work function rising again. Finally, an activation experiment is performed to verify the result of our calculations, GaN photocathodes gets the minimum work function earlier than Ga0.5Al0.5As photocathodes.