Fermi enhancement and breakdown of the parity selection rule in the luminescence spectra of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As modulation-doped quantum wells

We report the results of an investigation of the optical properties of modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells with sheet-carrier concentration up to 1.8×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$. Photoluminescence and resonant Raman-scattering techniques have been used as complementary tools for the characterization of the intersubband transition. A two-band model of the two-dimensional plasma gives good agreement between the photoluminescence excitation data and Hall measurements. We point out that the large breakdown of the parity selection rule of the optical matrix element in the photoluminescence spectra is due to the localization of photogenerated holes at the heterointerface, which lifts the k-selection restriction. The enhancement in the photoluminescence intensity at the Fermi edge results from the strong correlation and multiple scattering of electrons near the Fermi edge by the localized holes.