THICK SODIUM OVERLAYERS ON GAAS(110)

We report density-functional theory calculations of the electronic str ucture, total energy, and forces for the Na adsorption on GaAs (110) u sing the local-density approximation of the exchange-correlation funct ional and ab initio pseudopotentials. Results are presented for covera ges ranging from one adatom per substrate surface cell up to the thick overlayer limit. The atomic and electronic structure of the substrate is locally changed by the sodium adsorption on GaAs (110), depending on the coverage. In particular, we analyze the wave-function character of the states at the Fermi level, how it changes with sodium coverage , and we identify the formation of metal induced gap states (MIGS) at the interface. These MIGS are found to have mostly Ga dangling-bond ch aracter for all coverages. The calculated values of the p-type Schottk y barrier and of the variation of photothreshold as a function of cove rage are in good agreement with experimental data.