Photoreflectance study of the surface Fermi level at (001) <i>n</i>- and <i>p</i>-type GaAs surfaces

We report a photoreflectance study of Fermi level pinning (VF) on (001) n- and p-type GaAs with large, uniform electric fields. Surface photovoltage (Vs) effects were evaluated as a function of temperature (77 K&amp;lt;T&amp;lt;450 K), pump beam wavelength (633 and 407 nm) and W–metal coverage (in situ). The dependence of the measured barrier height, Vb (=VF−Vs), on T can be explained by a modification of the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding values of VF =0.77±0.02 V and VF =0.75±0.02 V for n- and p-type GaAs, respectively, at 300 K. In addition, by introducing the ratio (r) of the area of the surface states to the illuminated area into the theory of Hecht we have been able to estimate the density of surface states on the GaAs surface. The effect of metal coverage is to increase r and reduce the influence of Vs.