Strong effects of carrier concentration on the Fermi-edge singularity in modulation-doped InP/InxGa1-xAs heterostructures

The presence of the Fermi-edge singularity (FES) in photoluminescence spectra of n-type modulation-doped InP/InxGa1-xAs quantum structures is found to be strongly dependent on the concentration of a two-dimensional electron gas (2DEG). The FES is observed in samples with a high concentration of the 2DEG, whereas it is not so in low-doped samples with a concentration of the 2DEG close to approximate to 4.0 x 10(11) cm(-2). The suppression of the FES in the latter case occurs even under optimal conditions for the FES observation, i.e., in quantum structures where (i) a hole localization is realized by the alloy fluctuation potential and (ii) an enhanced interaction between the 2DEG and photocreated holes is ensured by a close distance between the Fermi level and the empty upper electron subband. An analysis of the results obtained based on the self-consistent calculations of the band-bending, subband structure, and spatial distribution of carrier density suggests that the observed dependence of the FES on the 2DEG density presumably reflects intrinsic properties of the 2D system and is not related to a reduction of overlap between electron and hole wave functions with decreasing concentration of the 2DEG. This conclusion is also confirmed by a comparison with previously reported data. A detailed understanding of the observed dependence requires further theoretical studies.