FERMI-ENERGY EDGE SINGULARITY AND EXCITONIC ENHANCEMENT ASSOCIATED WITH THE 2ND SUBBAND IN ASYMMETRIC MODULATION-DOPED QUANTUM-WELLS

A study of the Fermi-energy edge singularity (FEES) in the second (n = 2) subband of asymmetric modulation-doped (AlGa)As-(InGa)As-GaAs quan tum wells (AMDQW's) is reported. In one of the AMDQW's a Schottky gate is employed to vary the electron density in the n = 2 subband (n(s,2) ) from 0 to 1 x 10(11) cm-2 . Temperature-dependent photoluminescence excitation (PLE) measurements clearly show that the n = 2 PLE feature has FEES character for n(s,2) greater than or similar to 0.4 x 10(11) cm-2. In contrast to PLE, photoluminescence (PL) intensity is not a tr ue measure of oscillator strength, since PL intensity can be affected by competing recombination pathways. Temperature-dependent PL measurem ents have been performed on two types of AMDQW. One type has n(s,2) ap proximately 0, with the Fermi energy close to the n = 2 subband energy . The other type has n(s,2) = 1 x 10(11) cm-2 and a FEES associated wi th n = 2 observed in PLE. We demonstrate that the very similar broaden ing and reduction in peak height of the n = 2 PL peak with temperature for the two types of samples can be accounted for in terms of spreadi ng of the electron or exciton populations near the n = 2 subband edge. Therefore, we conclude that temperature-dependent PL does not provide unequivocal evidence for a many-body enhancement of the n = 2 PL tran sition, in contrast to that reported by Chen et al. [Phys. Rev. Lett. 64, 2434 (1990)].