Nature of Friedel oscillations around Si dopants in the GaAs(110) accumulation layer

The screening of sub-surface Si impurities in an accumulation layer at the GaAs(110) surface is calculated. Such an accumulation layer can be induced by a scanning tunnelling microscope tip, and surface Friedel oscillations h ave been imaged around the Si dopants. This study uses the effective mass a pproximation to describe the electrons in the GaAs conduction band, and a f itted model potential for the impurity. Two-dimensional effects dominate, w ith a doubly occupied bound state pulled off the lowest sub-band by the imp urity, and a depletion of one electron in the conduction states. The bound state gives a large central peak in the surface induced charge, with the Fr iedel oscillations coming from the change in the conduction states. To expl ain the amplitude of the observed oscillations, it is necessary to reduce t he tunnelling contribution from the bound state.