Atomic geometry and electronic structure of Al0.25Ga0.75N(0Â 0Â 0Â 1) surfaces covered with different coverages of cesium: A first-principle research

We investigate cesium adsorption on Al0.25Ga0.75N(0 0 0 1) surface at different coverages using first principle method based on density functional theory. Adsorption energies, atomic structure, Mulliken charge distribution, electron transfer, band structures, and density of states of the adsorption systems corresponding to different Cs coverages were obtained. Total-energy calculations show that cesium adsorption on Al0.25Ga0.75N(0 0 0 1) surface is more and more difficult as the increase of cesium coverage. A single cesium adatom is preferred to locate at the top of Ga atom (TGa). Meanwhile, it is not the most stable configuration when two cesium atoms were located on the top of two Ga neighbors at the same time. This is mainly because the distance of Cs adatoms is so small that repulsive force between adatoms rises. At low coverage, electrons transfer from Cs adatom to Ga atoms on the topmost and second topmost bilayers. Meanwhile, the efficiency of electron transfer decreases as the increasing of Cs coverage. There appear new bands at −25 to −23 eV and −14 to −10 eV, which were caused by Cs 5s and Cs 5p state electrons. Under the joint effect of Cs 5s and 5p state electrons, density of states at Fermi level increases, and the adsorption surfaces show more metal properties. Electrons transferring from Cs adatoms to Al0.25Ga0.75N substrate induces dipole moment, which is useful to lower work function. What is more, there exists an optimum of cesium coverage to obtain the lowest work function.