Electroreflectance Spectra due to Free Carriers in Semiconductors

The modulation of the spectral reflectivity of a semiconductor-electrolyte interface has been observed in the deep infrared (200-600 ${\mathrm{cm}}^{-1\mathrm{}}$). In $n$-type Ge and GaAs the resulting electroreflectance spectra show pronounced characteristic structure, with peak modulation of several percent. These observations can be understood as resulting from a depletion region of variable thickness at the semiconductor interface which acts as a dielectric optical coating. More detailed considerations predict "resonances" in the modulation near lattice reststrahl and free-carrier plasma frequencies, as observed.