Onset of exciton absorption in modulation-doped GaAs quantum wells

We study the evolution of the absorption spectrum of a modulation-doped GaAs/AlxGa1-xAs semiconductor quantum well with decreasing the carrier density. We find that at some critical electron density there is a sharp change in the line shape and the transitions energies of the exciton peaks. We show that this critical density marks an abrupt transition from a simple excitonic behavior to a Fermi edge singularity.