DOPING-DENSITY DEPENDENCE OF PHOTOLUMINESCENCE IN HIGHLY SI-DOPED GAAS/ALXGA1-XAS QUANTUM-WELLS FROM BELOW TO ABOVE THE METALLIC LIMIT

The development of the photoluminescence spectra with doping density has been studied for a series of single GaAs/AlxGa1-xAs quantum wells center doped with Si. The trends observed are found to be very different from those observed for equivalent doping levels in bulk GaAs. In particular, excitons continue to dominate the radiative recombination at doping levels right up to the metallic limit. The electron density at which the exciton is quenched is found to be considerably higher (about 10(12) cm-2) in the center-doped case than has previously been demonstrated for modulation-doped quantum wells (about 4 x 10(11) cm-2). This result is understood in terms of the difference in phase-space filling for the center-doped quantum well when compared with the modulation-doped case. The impurity concentration at which the material becomes degenerate is also found to be significantly higher than in bulk GaAs. The recombination dynamics at the metallic limit have been studied using a time-resolved photoluminescence technique and demonstrate that localization, resulting from either interface roughness or the high doping level, is reduced as the density is increased above the degenerate limit. The apparent relaxation of momentum conservation observed in the photoluminescence spectra for highly doped samples is interpreted as being due to recombination from strongly localized holes.