Effect of electron and hole accumulation on magneto-optical spectra of an undoped GaAs/GaxAl1−xAs quantum well

We have performed magnetophotoluminescence and magnetophotoluminescence-excitation spectroscopy at 1.8 K on a GaAs/${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$As quantum-well structure in which the carrier density in a single 50 Å quantum well can be varied from zero up to about 2×${10}^{11}$ carriers ${\mathrm{cm}}^{\mathrm{-}2}$ by use of a Schottky gate. This results from transfer of either electrons or holes photoexcited in thicker GaAs layers in the structure and thereby allows the investigation of carrier-density-dependent effects in a single sample. The data for the empty well are consistent with previous studies of magneto-optics of atomic excitons. With a Fermi sea of heavy holes or electrons present, the spectra show evidence of band-gap renormalization and phase-space filling. Also the lowest inter-Landau-level transition was observed to follow a linear field dependence for fields lower than filling factor ν=2 but to have a particularly weak magnetic field dependence at higher fields. This is consistent with crossover from free carrier to excitonic behavior at ν=2 and is compared to recent theoretical calculations.