Excitonic absorption in modulation-doped GaAs/AlxGa1−xAs quantum wells

We have performed transmission measurements in a range of undoped through heavily-modulation-doped GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multiple-quantum-well structures. The observed absorption spectra demonstrate quenching of the excitonic oscillations with increasing quasi-two-dimensional electron gas. The electron density corresponding to the total bleaching of the lowest excitonic oscillation is greater than or equal to 3×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ for a quantum well size of 200 Å. Theoretical calculations of the absorption spectra which include the effect of carrier screening have been made. The results show that both long-range and short-range many-body effects should be included to explain the experimentally observed spectra. In the modulation-doped case, we conclude that the phase-space filling and exchange of the electron gas are the dominant effects on excitonic absorption. From the observation that the linewidth increases with the electron density, we demonstrate that the exciton lifetime reduces due to the interaction between the electrons and the excitons.