We have observed saturation of photoluminescence peak at extremely low pump intensities in growth-interrupted and undoped asymmetric-coupled quantum-well structures. We believe the saturation is due to filling of the exciton states localized at the interface islands. The exciton density required to completely fill the localized exciton states at the interface islands for one of the samples is two orders of magnitude lower than that for the previous sample. We have demonstrated that even under the same growth condition, two samples with the same structure exhibit distinctly different photoluminescence spectra and saturation characteristics. ©1997 American Institute of Physics.