We present reflectance and total photoelectric yield measurements of commercially available silicon wafers. The measurements have been performed at the xuv reflectometer station at HASYLAB(DESY) in the photon energy range of between 50 and 900 eV. Fitting of the reflectance as a function of angle of incidence using Fresnel's equations yielded values for the thickness of the native oxide layer and optical constants of the Si wafer. The surface roughness of Si wafers could also be determined. The optical constants derived from angular dependent total photoelectric yield measurements are in good agreement with those obtained from reflectance measurements on the same Si wafer. Some of the Si wafers investigated were cleaned by a combination of chemical etching and in situ thermal treatment in order to reduce the oxide layer. Si wafers which were solely etched revealed an oxide layer clearly thinner than that of unetched samples as well as a drastic decrease in their surface roughness. Subsequent in situ thermal treatment resulted in oxide-free Si wafers. This treatment, however, always brought about a strong increase in surface roughness.