Photoluminescence of a two-dimensional electron gas in a modulation-doped $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum well at filling factors $\nu <1\mathrm{}$

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 $B<~7\mathrm{T}$ and lattice temperature $T_L=1.9\mathrm{}\mathrm{K}.$ The 2DEG density was varied by optical depletion (with He–Ne laser illumination) in the range of $n_{2\mathrm{D}}=(1\mathrm{}–7)\times {10}^{10}{\mathrm{cm}}^{-2}.$ As the filling factor decreases below $\nu =1,$ a high-energy PL band (H) emerges; its evolution with B and electron density $n_{2\mathrm{D}}$ is studied. At $\nu \sim 0.4,$ 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 $n_{2\mathrm{D}}$ and with increasing magnetic field (at $\nu <0.4)$ 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.