Electro-Absorption Effects at the Band Edges of Silicon and Germanium

The change in the absorption coefficient $\Delta \alpha =\alpha (\omega , \mathcal{E})-\alpha (\omega , 0)$ due to the presence of an electric field $\mathcal{E}$ has been measured for the indirect-absorption edges of silicon and germanium and for the direct-absorption edge in germanium. The results show good qualitative agreement with the theoretical predictions. The quantitative deviations are caused by the presence of excitons and thermal and electric-field broadening, which have not been taken into account in the theory. The energies of the phonons which take part in indirect optical absorption have been determined from the room-temperature data and are in very good agreement with the values found at liquid-helium temperatures by other means.