Charged and neutral exciton phase formation n 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/Al sub (0.1) Ga sub (0.9) As quantum wells and under a perpendicularly applied magnetic field B. The transition into the phase consisting of a charged exciton singlet (X sup - sub s) ground state and neutral exciton (X) excited state occurs at a filling factor v=1. The relative intensities of the X sup - sub s and X transitions measured by photoluminescence excitation at T=2 K, as a function of v