Doping dependence of the E1 and E1 plus Delta1 critical points in highly doped n- and p-type GaAs: Importance of surface band bending and depletion

Using rotating analyzer ellipsometry, we determined at room temperature and at 30 K the doping dependence of the E1 and E1 plus Delta1 critical points of n-type highly Si-doped GaAs(100) (up to carrier concentration of 1.9 x 10 high19 cm high-3) exposed to air. For both temperatures the E1 and E1 plus Delta1 critical points shift to lower energies with increasing dopant concentration. This redshift, however, unexpectedly saturates at a dopant concentration of about 3 x 10 high18 cm high-3. We found these measurements to be strongly influenced by surface effects. UHV-cleaved GaAs(110) samples (n- and p-type), exhibiting flat bands at the surface, show a significantly smaller energy shift over the whole doping range following a power-law dependence corresponding rather well to the predicted effect of screened impurities, i.e., dopants. We believe that this represents the behavior of bulk doping. fermi-level pinning introduced by hydrogen absorption after cleaving produces a behavior simi lar to that observed for air-exposed surfaces. These measurements are influenced by band-bending and carrier depletion in the surface region. Measurements of undoped material in an electric field indicate that the presence of this field does not account for all the band-bending effects. We propose that unscreening of dopants in the depletion region makes an important contribution