Onset of exciton absorption in modulation-doped GaAs quantum wells

We study the evolution of the absorption spectrum of a modulation-doped GaAs/AlxGa12xAs 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. The absorption spectrum in the presence of a Fermi sea of electrons has been a subject of theoretical and experimental research for more than three decades. The interest in this problem was triggered by the pioneering work of Mahan, 1 who showed that in metals and bulk semiconductors the onset of the absorption spectrum should exhibit a power-law singularity of the form ( v2v 0) 2a . This singularity, which became known as the Fermi edge singularity ~FES!, reflects the final-state response of the Fermi sea electrons to the attractive potential of the valence-band hole. Mahan’s work was followed by other theoretical works, which established the many-body nature of the FES and provided the tools to treat it. 2,3 The FES was indeed observed in the x-ray absorption of metals and in the interband absorption of semiconductors. In semiconductor heterostructures it is manifested as a pronounced enhancement of the absorption threshold, with an asymmetrical line shape: a fast rise at the low-energy side and a slow fall at high energies. 4‐6 The high-energy tail is due to photon absorption, which is mediated by shake-up