Ta. Fisher et al., FERMI-ENERGY EDGE SINGULARITY AND EXCITONIC ENHANCEMENT ASSOCIATED WITH THE 2ND SUBBAND IN ASYMMETRIC MODULATION-DOPED QUANTUM-WELLS, Physical review. B, Condensed matter, 48(19), 1993, pp. 14253-14263
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)].