Effect of carrier concentration on the reflectivity spectrum of Ge and InSb

We have measured at 5 K the derivative spectra (1/R) dR/dE around the E₁ and E₁ + Δ₁ structures, of Ge and InSb with different impurity concentrations. In the more highly doped samples, the peaks are broadened and shifted slightly to lower frequencies. The change in the value of (1/R) dR/dE is much larger than recently predicted by Seraphin and Aspnes, based on the surface field effect. Different possibilities for the effect are discussed. Wavelength modulation measurements on samples with an applied surface electric field have shown no difference.