Franz-Keldysh Effect in the Space-Charge Region of a Germanium p − n Junction

By use of the carrier-depleted region of a reverse-biased $p\ensuremath{-}n$ junction, it has been possible to study the effect of large electric fields on the optical absorption coefficient of germanium at room temperature. In the energy range below the edge of the direct interband transitions, the absorption coefficient increases very strongly with field, and the experimental results are in fairly good agreement with the theoretical predictions of Franz, Keldysh, and others. In the region of the indirect transitions, the changes in the absorption coefficient with field oscillate between positive and negative values as a function of energy. This behavior can be related to the phonon-assisted processes first observed by Macfarlane et al.