Study of Cs adsorption on Mg-doped β-Ga2O3(100) surface: A first-principles investigation

β-Ga₂O₃ is an ideal solar-blind ultraviolet (UV) photocathode material due to its ultra-wide bandgap, high electron mobility and excellent stability. However, its p-type surface sensitization mechanism remains unclear. In this work, we employ first-principles calculations to systematically investigate the effect of different Cs adsorption sites and coverages on the surface stability, electronic structure, and optical properties of the Mg-doped β-Ga₂O₃(100) surface. These findings demonstrate that for single Cs atom adsorption, the B₂ site (bridge site between Mg atom and Ga atom) exhibits the highest adsorption stability. As the Cs coverage gradually increases, the adsorption energies increase while remaining negative, providing strong thermodynamic evidence for the stability of the adsorption process. Regarding the electronic structure characteristics, the work function initially decreases and then increases, reaching its minimum value at 0.5 monolayer (ML) Cs coverage. In-depth analysis reveals that with increasing Cs coverage, the conduction band minimum (CBM) and the valence band maximum (VBM) exhibit a downward energy shift. When the coverage exceeds 0.5 ML, the Fermi level enters the conduction band, and the surface transforms into an n-type state. In terms of optical properties, Cs adsorption significantly enhances the photodetection capability of the Mg-doped β-Ga₂O₃(100) surface photocathode in the solar-blind UV, improving the material's response efficiency to optical signals within the solar-blind UV band.