2-DIMENSIONAL HOLE GAS AND FERMI-EDGE SINGULARITY IN BE DELTA-DOPED GAAS

The subband structure of the quasi-two-dimensional hole gas (2DHG) for med at a single Be delta-doped layer in GaAs has been studied by photo luminescence spectroscopy. To confine the photogenerated minority carr iers, and thus to enhance the efficiency of radiative recombination fr om the 2DHG, the delta-doping spike was placed in the center of an Alx Ga1-xAs/GaAs/AlxGa1-xAs double heterostructure. Recombination involvin g different hole subbands has been resolved which enabled us to analyz e the subband occupation as a function of dopant concentration and sam ple temperature. In sample structures where the Fermi level is located close to unoccupied subbands, a pronounced Fermi-edge singularity (FE S) is observed in the low-temperature (<20 K) luminescence spectrum. T he temporal evolution of the FES has been studied by time-resolved lum inescence spectroscopy. The enhancement in emission intensity at the F ermi edge can be understood in terms of a transfer of excitonic oscill ator strength from the unoccupied subbands to nearby occupied states a t the Fermi energy. Self-consistent subband calculations have been per formed to compute the hole confining potential and the subband energie s for the present delta-doped structures. The results of these calcula tions, which take into account the finite spread of the dopant atoms i n accordance with secondary-ion-mass spectroscopic data, are in good a greement with the measured subband spacings. The assignment of light- and heavy-hole transitions is supported by luminescence measurements u sing circularly polarized light.