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

The subband structure of the quasi-two-dimensional hole gas (2DHG) formed at a single Be delta-doped layer in GaAs has been studied by photoluminescence spectroscopy. To confine the photogenerated minority carriers, and thus to enhance the efficiency of radiative recombination from the 2DHG, the delta-doping spike was placed in the center of an AlxGa1-xAs/GaAs/AlxGa1-xAs double heterostructure. Recombination involving different hole subbands has been resolved which enabled us to analyze the subband occupation as a function of dopant concentration and sample temperature. In sample structures where the Fermi level is located close to unoccupied subbands, a pronounced Fermi-edge singularity (FES) is observed in the low-temperature (<20 K) luminescence spectrum. The temporal evolution of the FES has been studied by time-resolved luminescence spectroscopy. The enhancement in emission intensity at the Fermi edge can be understood in terms of a transfer of excitonic oscillator strength from the unoccupied subbands to nearby occupied states at the Fermi energy. Self-consistent subband calculations have been performed to compute the hole confining potential and the subband energies for the present delta-doped structures. The results of these calculations, which take into account the finite spread of the dopant atoms in accordance with secondary-ion-mass spectroscopic data, are in good agreement with the measured subband spacings. The assignment of light- and heavy-hole transitions is supported by luminescence measurements using circularly polarized light.