Photoluminescence due to a bound-to-bound transition in a GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum-well structure

The low-temperature (2-K) photoluminescence (PL) spectra from a 200-Å GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum-well structure shows a peak Q, which occurs 1.0 meV lower in energy than the heavy-hole free-exciton (HHFE) transition. Previously transition Q was tentatively assigned to the collapse of an exciton bound to an ionized donor (${\mathit{D}}^{+}$,X) located at the well edge or the barrier center [Reynolds et al., Phys. Rev. B 40, 6210 (1989)]. The present work conclusively demonstrates that transition Q is neither exciton related nor free-to-bound related and most likely is due to a bound-to-bound transition. Time-resolved PL measurements using 70-ps pulsed excitation, which is resonant with the light-hole free-exciton formation energy, show that emission from transition Q peaks much earlier in time after excitation than emission from both free- and bound-exciton (HHFE, ${\mathit{A}}^0$,X, and ${\mathit{D}}^0$,X) transitions. Magnetic-field-dependent PL measurements show that the diamagnetic shift of transition Q is significantly smaller than that expected for a free-to-bound transition and is consistent with that expected for a bound-to-bound transition. The drastically reduced lifetime of Q compared with the HHFE can be qualitatively explained as being due to a bound-to-bound transition in which the electron and hole are more closely spaced than they are in the exciton state.