Optical investigations of the high-density electron gas in pseudomorphic ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum-well structures

Low-temperature photoluminescence was studied in a large number of pseudomorphic modulation-doped transistor structures having an ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As (x<0.3) quantum well with sheet densities up to 2.5×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The Fermi edge appears clearly in the spectra, and its separation from the n=1 transition peak varies linearly with measured sheet densities of the two-dimensional electron gas. Appearance of a strong high-energy peak marks the occupation of the second electron subband if the electron density or well width is large, and a feature due to phonon interactions is present if the Fermi energy exceeds the optical-phonon energy. No enhancement is seen at the Fermi edge except for a slight increase when it lies less than about 10 meV below the second subband.