Exploration on structural stability, electronic and optical properties of Cs-activated and Cs/O-activated Al_0.5Ga_0.5N thin film and nanowire photocathode surface

Abstract

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 Al0.5Ga0.5N thin films and nanowires by using first-principles. Our results reveal that adsorption energy of Al0.5Ga0.5N 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 Al0.5Ga0.5N systems are more stable under high Cs coverage. And Cs/O co-adsorbed Al0.5Ga0.5N 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 Al0.5Ga0.5N 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 Al0.5Ga0.5N.

Comparison Study of Structural Stability and Optoelectronic Properties of Al0.5Ga0.5N Thin Film and Nanowire Photocathode Surfaces under Different Activation Methods

“…Since the atom number in the outermost layer of the thin film surface and nanowire surface is 4, the Cs atoms adsorbed increased from 1 to 4, and the corresponding Cs coverage increased from 0.25 monolayer (ML) to 1 ML . Also, the Cs adsorption position at different coverage levels originated from our previous study . The adsorption energy is calculated by Formula , and Table presents the results.…”

“…25 Also, the Cs adsorption position at different coverage levels originated from our previous study. 26 The adsorption energy is calculated by Formula 3, and Table 1 presents the results. For Al 0.5 Ga 0.5 N surfaces activated by C S /NF 3 , the adsorption energies increase as Cs coverage magnifies, indicating that the Cs/NF 3 -activated structure with high Cs coverage is unstable.…”