Electronic properties of Na overlayers on the GaAs(110) surface

A seven-layer slab was used to model the ideal GaAs(110) surface. Two coverages, ${\Theta }_t$=1/2 and 1 monolayer (ML) of Na atoms on the surface were studied. The total-energy calculations with the self-consistent pseudopotential method was used to determine the positions of Na atoms on the ideal GaAs(110) surface. We find that the charge transfer is from the Na to the Ga atoms. The shift of the Fermi level with respect to the valence-band maximum from coverage of 1/2 to 1 ML is 0.33 eV downward. This result agrees reasonably with the measured value of 0.2 eV for the n-type GaAs(110) surface.