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 nu