Comparison of activation behavior of Cs-O and Cs-NF3-adsorbed GaAs(1 0 0)-β2(2 × 4) surface: From DFT simulation to experiment

Abstract

To clarify the performance differences between Cs-O and Cs-NF3-activated GaAs photocathodes, the changes in adsorption characteristics with Cs coverage for the Cs-O and Cs-NF3-adsorbed GaAs(100)-β2(2 × 4) surfaces were investigated by first-principles calculation based on density function theory. The simulation results show that under the same Cs coverage, the Cs-NF3-adsorbed GaAs surface is more stable than the Cs-only and Cs-O-adsorbed surfaces. In the case of small Cs coverage, the Cs-O-adsorbed GaAs surface exhibits a lower work function than Cs-NF3 does. When the Cs coverage reaches one monolayer, however, the Cs-NF3 adsorption model possesses a stronger dipole moment resulting in an even lower work function. According to theoretical prediction guidance, the Cs-O and Cs-NF3 activation experiments adopting the excessive Cs deposition recipe were performed for preparing GaAs photocathodes. The experimental results show that the Cs-NF3-activated GaAs photocathode has a higher quantum efficiency in the whole waveband and better emission stability under white light illumination than the Cs-O-activated photocathode, which agrees with the differences in work function and adsorption energy between the two adsorption model types. It is concluded that the Cs-NF3 activation recipe using excessive Cs supply is preferred to enhance photoemission performance of GaAs photocathode.