Optical transitions and sum rules at clean semiconductor surfaces

The experimental results of surface differential reflectivity and reflectance anisotropy spectroscopy show that in Si(111)-2 x 1, Ge(Ill)-2 x 1 and GaAs(001)-2 x 4 a sum rule for the imaginary part of the (surface) dielectric function is verified both for the isotropic and anisotropic parts of epsilon(s)". It is shown that the sum rule together with the dependence of the spectra upon oxygen contamination are useful in the interpretation of the optical transitions of the above surfaces. In particular, for the case of GaAs(001)-2 x 4 the above analysis has allowed the distinction between optical transitions associated to true surface states and bulk states modified by the surface near the 3 eV critical point.