Photoluminescence of a two-dimensional electron gas in a modulation-doped GaAs/AlxGa1-xAs quantum well at filling factors nu < 1

The evolution of the 2DEG-hole magneto-photoluminescence (PL) with decreasing 2DEG density is studied in a high quality 25 nm-wide modulation-doped GaAs/AlGaAs quantum well in magnetic fields Bless than or equal to7 T and lattice temperature T-L=1.9 K. The 2DEG density was varied by optical depletion (with He-Ne laser illumination) in the range of n(2D)=(1-7)x10(10) cm(-2). As the filling factor decreases below nu=1, a high-energy PL band (H) emerges; its evolution with B and electron density n(2D) is studied. At nusimilar to0.4, two additional PL lines split off the H-band, and these are assigned to the charged triplet X-t(-) and neutral exciton X-0 PL. The evolution from free-hole-2DEG to charged exciton PL with decreasing n(2D) and with increasing magnetic field (at nu0.4) is attributed to the appearance of regions containing localized electrons in the photoexcited quantum well. Localization results in simultaneous presence of the free-hole-2DEG PL from the electron puddles (H-band) and the charged exciton PL lines (spin-singlet, X-s(-) and spin-triplet, X-t(-)) from areas containing localized electrons.