Photoluminescence decay time studies of type II GaAs/AlAs quantum-well structures

We have investigated the temperature dependence of the photoluminescence (PL) decay kinetics of a series of GaAs/AlAs quantum-well structures where the GaAs thickness was kept constant at 25 Å and the AlAs was varied between 41 and 19 Å. In these structures the band alignment is type II and the dominant photoluminescence process at 4 K is due to recombination of excitons involving electrons confined at the AlAs X point and holes in the GaAs. At 4 K on the low-energy side of the zero-phonon type II transition the PL decay is a single exponential over at least two decades. The time constant of this decay is a strong function of the AlAs layer thickness. The variation of this decay time is described by a change in the oscillator strength of the type II process due to the change in the mixing between the Xz (AlAs) electron states and the Γ (GaAs) electron states. At higher temperatures (T>15 K) the photoluminescence intensity and the decay time decrease very rapidly with increasing temperature. This is due to the increased influence of nonradiative processes as the type II excitons become delocalized.