Cesium, oxygen coadsorption on AlGaN(0001) surface: experimental research and ab initio calculations

Abstract

Cs, O activation experiment of Al0.24Ga0.76N photocathodes was carried out and the photocurrent variation was recorded. Then first principle calculations were performed to study Cs, O coadsorption on Al0.25Ga0.75N(0001) surface. Three surface models were built, one model represents pure surface, and the other two are defective surface models with Ga or N vacancy on the topmost layer. Cs only adsorption systems were built based on clean surface, and Cs, O coadsorption systems were built based on the three surfaces. Work function, average dipole charge, average dipole length, and dipole moments were calculated. Results show there exist double dipoles in the Cs, O coadsorbed Al0.25Ga0.75N(0001) surfaces. One is Cs-induced dipole [Cs–Al0.25Ga0.75N], which was caused by electrons transferring from Cs adatoms to Al0.24Ga0.75N substrate. This dipole is perpendicular to surface, and it is helpful for electrons escaping to vacuum. The other dipole [Cs–O] is composed of Cs and O adatoms, which is induced by electrons transferring from the Cs adatoms to O adatoms. Cs, O coadsorption results in lower work function on defective Al0.24Ga0.75N(0001) surfaces than the pure one. The reason is that the direction of [Cs–O] dipole on the defective surface is oriented to the substrate. Nevertheless, the direction difference between [Cs–O] dipole and surface without defect is too small to lower work function significantly. This work gives theoretical explanation for photocurrent variation and dipole moments formation of Cs, O coadsorbed AlGaN photocathodes.