Surface influence on the doping dependence of the E1 plus Delta1 critical points of GaAs - electric field and inpurity unscreening effect

We report on ellipeometrie measurements of the doping dependence of the E1 and 1+ Delta1 critical points in highly doped n-type GaAs(100) exposed to air (up to doping concentrations of n=1.9x10high19cmhighminus3). The critical points shift to lower energy and broaden following a power-law dependence on doping n up to about n=3x10high18cmhighminus3. For higher doping levels, however, we observe an unexpected saturaton of the red-shift an broadening. We found surface effects, i.e., surface band bending, to account for most of the observed doping effects. Cleaved GaAs(110), having flat bands at the surface, shows significantly smaller red-shift following a power-law dependence over the entire doping range. The removal of the free carriers in the surface depletion layer of oxidized GaAs(100) - the unscreening of impurities - enhances the impurity potential influence on the band structures. Only this region is probed at low doping levels, whereas at higher doping levels bulk behavior starts to contribute, because the depletion layer with reduces below the optical penetration depth.