Comparison Study of Structural Stability and Optoelectronic Properties of Al_0.5Ga_0.5N Thin Film and Nanowire Photocathode Surfaces under Different Activation Methods

Abstract

Different surface activation modes affect the performance and quantum efficiency of AlGaN-based photocathodes. According to first-principles-based calculations, we investigate the structural stability, charge transfer, band structure, density of states, dielectric function, absorption coefficient, and reflectivity of Al 0.5 Ga 0.5 N thin film and nanowire surfaces that are pure, Cs-activated, Cs/O-activated, Cs/NF 3 -activated, and Cs/ NF 3 /Li-activated in detail. Our results imply that the Al 0.5 Ga 0.5 N thin film surface is more easily activated by Cs/NF 3 , while the Al 0.5 Ga 0.5 N nanowire surface is more easily activated by Cs, and the activation process of the thin film is easier. After Cs activation, the band structure of the Al 0.5 Ga 0.5 N surface moves downward, and the Fermi level passes through the conduction band. The system has n-type conductivity, which also leads to a reduction of work function. When the activation process continues with the addition of O or NF 3 , conduction band minimum moves to a high-energy position, further reducing the work function, which is conducive to electron overflow and improving quantum efficiency. In addition, surface activation introduces new energy levels, mainly due to the hybridization of the s and p orbitals of activated atoms. The results of the optical properties imply that surface activation significantly enhances the optical performance of Al 0.5 Ga 0.5 N photocathodes.