Comparative Research on Cs Activation Mechanism for Al0.5Ga0.5As (001) and Al0.25Ga0.75N (0001) Surface

To investigate and distinguish the difference of Cs activation mechanism on Al_0.5Ga_0.5As (001) and Al_0.25Ga_0.75N (0001) surface, plane wave with an ultrasoft pseudopotential method based on the first-principle density functional theory is used. Surface adsorption energy, work function, and surface dipole moments with different Cs coverage are calculated and compared. Eight possible Cs adsorption sites are chosen for the Al_0.5Ga_0.5As (001) surface, while seven high-symmetry sites are considered in the calculation model of Al_0.25Ga_0.75N (0001) surface. Results show that when only one Cs atom adsorbed on two surfaces, the adsorption energies are all negative and the adsorption sites are all stable sites. The lowest work function and the most stable adsorption sites with different Cs coverage are obtained and discussed. When different amounts of Cs atoms adsorbed on two surfaces, the work function values of Al_0.25Ga_0.75N (0001) surface are all smaller than that of Al_0.5Ga_0.5As (001) surface. The work function of Al_0.5Ga_0.5As (001) surface decreases with the increase of Cs coverage when the Cs coverage changes from 0 to 0.75 ML, while Al_0.25Ga_0.75N (0001) surface achieves the smallest work function when the Cs coverage is 0.5 ML. Finally, photocurrent curves during Cs activation are recorded. The photocurrent of AlGaAs sample reaches at a peak at the time of 29 min when the Cs coverage rise at 0.75 ML, while the photocurrent of AlGaN sample achieves a peak at the time of 30 min when the Cs coverage rise at 0.5 ML, which is in well consistent with our calculation results.