Charged and neutral exciton phase formation in the magnetically quantized two-dimensional electron gas

We report on a spectroscopic study of the lowest-energy electron-hole transitions of the two-dimensional electron gas (2DEG), with a density varied by photoexcitation in GaAs/${\mathrm{Al}}_{0.1}$ ${\mathrm{Ga}}_{0.9}$As quantum wells and under a perpendicularly applied magnetic field B. The transition into the phase consisting of a charged exciton singlet (${\mathit{X}}_{\mathit{s}}^{\mathrm{-}}$) ground state and neutral exciton (X) excited state occurs at a filling factor ν=1. The relative intensities of the ${\mathit{X}}_{\mathit{s}}^{\mathrm{-}}$ and X transitions measured by photoluminescence excitation at T=2 K, as a function of ν≤1 and of B, are shown to depend on the relative area occupied by the magnetic-field–localized 2DEG and on the reduced orbit of the additional electron bound to ${\mathit{X}}_{\mathit{s}}^{\mathrm{-}}$. © 1996 The American Physical Society.