Role of Hole Localization in the Magneto-Luminescence of a Two-Dimensional Electron Gas

We investigate the effect of hole localization on the magneto-luminescence emission of a two-dimensional electron system formed in a modulation-doped semiconductor heterojunction. The emission of heterojunctions containing a dilute delta-layer of Be acceptors at a large distance from the interface is compared with that of Be-free samples. A narrow Fermi-edge singularity emission line is observed at low temperature in Be-doped samples, involving electrons in the second confined state. The evolution of this line in a perpendicular magnetic field shows common characteristics with previously reported results in high mobility samples not containing Be. A new emission line appears abruptly at filling factor 2 originating by the recombination of electrons in the lowest Landau level with free valence holes, independent of the presence of Be. The abruptness of this transition, which is also observed in some samples at filling factor 1, reveals a simultaneous change in the electron system over a macroscopic sample area. The new optical emission shows marked deviations with respect to the single-particle behavior, which are tentatively interpreted as the formation of a complex state involving a free photocreated hole and the electron system. This complex unbinds when the Fermi level crosses the mobility edge.