Excitonic effects in the optical spectra of superlattices in an electric field

We consider theoretically the optical spectra of electron-hole Stark-ladder transitions in superlattices under an electric field. Electron and heavy-hole subband energies and envelope functions are calculated using a scattering phase-shift treatment of the quasibound, Stark-localized states. Exciton binding energies and oscillator strengths are obtained as functions of the electric field for several pairs of Stark-localized electron and hole states. Accurate and rapid computation of these quantities is achieved by using the simple approximation developed in the preceding paper. We show that observed deviations of the Stark-ladder fan diagram from the expected linear dependence on electric fields result from excitonic effects. We also calculate the absorption coefficient as a function of photon energy for various electric fields, and we compare our results with recent experimental data on short-period GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices.