Insights into performance enhancement mechanism of GaAs(100) photocathode by Cs/Li/O and Cs/Li/NF3 co-deposition

To verify the effects of additional Li introduction on the Cs-O and Cs-NF3 activation processes of GaAs(100) photocathodes, Cs-Li-O and Cs-Li-NF3 co -adsorption models of GaAs(100) beta 2(2 x 4) surfaces with different Cs/Li ratios are investigated, on the basis of pristine Cs-O and Cs-NF3 adsorption models. Residual gases including CO2, H2O, CO and H2 are introduced to the Cs-Li-O and Cs-Li-NF3 co -adsorption models to compare the damaging effects of typical residual gases. Simulation results show that the optimal Cs/Li ratio is 3:1, wherein 6Cs-2Li-1NF3 surface model exhibits smaller work function and 6Cs-2Li-1O surface model has better immunity to residual gases. Meanwhile, 6Cs-2Li-1O surface has better resistance abilities to CO2, CO and H2 than 6Cs-1O surface, while the resistances of 6Cs-2Li-1NF3 surface to H2O, CO and H2 are worse than those of 6Cs-1NF3 surface. From the perspective of charge transfer, a reverse dipole moment [Cs/Li-gas] and two positive dipole moments, i.e., [Cs/Li-surface] and [Cs/Li-O/NF3] are formed. Therefore, it is inferred that, the Cs-Li-O activation recipe, in comparison to the Cs-O activation recipe, can improve the photoemission stability of GaAs photocathodes, whereas the Cs-Li-NF3 activation recipe can increase quantum efficiency but is more sensitive to residual gases.