Evidence of strong sequential band filling at interface islands in asymmetric coupled quantum wells

We report the observation of relative saturation among excitonic emission peaks at different sets of interface islands in growth-interrupted asymmetric-coupled quantum well GaAs/Al_0.2Ga_0.8As structures. The saturation is due to sequential filling of excitonic states at different sets of interface islands. In contrast to free excitonic states, the small total area density of excitonic states at the interface islands makes their filling observable at much lower excitation levels. As a result of the relative saturation, an effective blue shift of the apparent excitonic emission peak at the islands, with a magnitude as large as ∼6.1 meV is observed when the excitation intensity increases from ∼1.6 to ∼215 W cm^-2. The highest intensity required to observe the effective blue shift is about two orders of magnitude lower than that needed to observe a similar effect in a free excitonic emission peak.