GaAs electrolyte electroreflectance: low-field spectra and flat-band potential shift measurements

Abstract

The electrolyte electroreflectance (HER) spectra of GaAs crystals, both undoped, highly compensated ( N A ≈ N D = 3 × 10 15 cm −3 and n-type ( N d = 10 17cm −3), were studied as a function of the applied bias in the photon energy range corresponding to E E 0 + Δ 0 transitions. Unequivocal experimental evidence was obtained for the first time that the low-field limit of Aspnes third-derivative model applies to sufficiently low doped GaAs samples, as is the case for our undoped crystal. In this case, a reliable measurement of the flat-band potential ( V fb) could be obtained by means of HER voltammetries. The behaviour of the n-type sample was found to be different in two main aspects: first, low field conditions could not be fitted; second, V fb appeared shifted by about 2.0 V towards negative values because of a strong Fermi-level pinning effect associated with the surface electro-reduction of the electrode. These V fb shifts, which are associated with the n-type character, prevent any reliable V fb measurement, even if the concentration of free carriers is sufficiently low and low-field conditions are fulfilled.