Condensation of bulk excitons on a magnetized two-dimensional electron gas in modulation-doped heterojunctions

Abstract

The evolution of photoluminescence (PL) spectra with increasing magnetic field $(B<~7 \mathrm{T})$ was studied in high-mobility, wide $\mathrm{GaAs}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ heterojunctions (HJ's) at lattice temperatures ${T}_{L}=1.9--25 \mathrm{K}.$ The two-dimensional electron-gas (2DEG) density in the studied samples is ${n}_{2D}^{0}=(0.9--3)\ifmmode\times\else\texttimes\fi{}{10}^{11} {\mathrm{cm}}^{\ensuremath{-}2},$ and it was varied with He-Ne laser illumination by optical depletion. For $B=0$ the PL is completely dominated by exciton recombination in the undoped GaAs layer. As B increases this band shows a typical exciton diamagnetic shift. For a filling factor $\ensuremath{\nu}<~2$ a strong PL transformation is observed: the exciton PL intensity decreases and a new, low-energy PL line abruptly appears and gains intensity at the expense of the exciton PL. We attribute this line to the 2DEG-free hole recombination, and propose that its appearance in the HJ's results from an increased free-exciton dissociation near the 2DEG at $\ensuremath{\nu}<~2.$ Thus, the evolution of the bulk free exciton to 2DEG-free hole PL with increasing B is due to ``condensation'' of the bulk excitons on the magnetized 2DEG layer at $\ensuremath{\nu}<~2.$