DOPING DEPENDENCE OF THE E1 AND E1-1 CRITICAL-POINTS IN HIGHLY DOPED N-TYPE AND P-TYPE GAAS - IMPORTANCE OF SURFACE BAND BENDING AND DEPLETION(LAMBDA)

Using rotating analyzer ellipsometry, we determined at room temperatur e and at 30 K the doping dependence of the E1 and E1 + DELTA1 critical , points of n-type highly Si-doped GaAs(100) (up to carrier concentrat ions of 1.9 X 10(19) CM-3) exposed to air. For both temperatures the E l and El + DELTA1 critical points shift to lower energies with increas ing dopant concentration. This redshift, however, unexpectedly saturat es at a dopant concentration of about 3 x 10(18) CM-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 surfa ce, show a significantly smaller energy shift over the whole doping ra nge following a power-law dependence corresponding rather well to the predicted effect of screened impurities, i.e., dopants. We believe tha t this represents the behavior of bulk doping. Fermi-level pinning int roduced by hydrogen adsorption 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 e ffects. We propose that unscreening of dopants in the depletion region makes an important contribution.