First-principles investigation of Cs-NF3 co-adsorption on GaAs(100)-β2(2 × 4) surface

Highlights: • Cs-NF₃ co-adsorption mechanism on the GaAs(1 0 0)-β2(2 × 4) surface is studied. • Ten different adsorption sites with two oppsite vertical directions are analyzed. • Cs-up adsorption models show lower adsorption energy and stronger dipole moment. • [Cs⁺-NF₃^-] dipole and [Cs⁺-As^-] dipole is formed in Cs-up models and NF₃-up models respectively. The mechanism of Cs-NF₃ adsorption on the GaAs(1 0 0)-β2(2 × 4) reconstructed surface is investigated by using the first-principles method based on density functional theory. Cs-NF₃ adsorption cases at ten different sites named as D, D′, T2, T2′, T3, T3′, T4, T4′, H and H′ with two opposite vertical directions are simulated. The results show that the vertical layered direction can remarkably affect surface properties. Compared with the NF₃-up models, the Cs-up models can possess lower adsorption energy, smaller work function and stronger dipole moment. Meanwhile, it is found that in the Cs-up models, the trench site T4 is the energy-preferred position for its lowest adsorption energy while the topmost As-dimer bridge site D is the most unstable for adsorption for its highest adsorption energy. Besides, the hollow site H′ can achieve the maximum drop of work function. Furthermore, Mulliken charge distribution indicates that the NF₃ molecule gets negatively charged in Cs-up models while that is neutrally charged in NF₃-up models. Combined with the change of atomic geometric structure, it is inferred that the [Cs⁺-NF₃^-] dipole is formed in Cs-up models while only the [Cs⁺-As^-] dipole is formed in NF₃-up models. These results can help to understand Cs-NF₃ co-activation behavior for GaAs photocathode.