Bi-excitons in wide-gap II-VI quantum wells. Localization by alloy disorder

This paper studies, both experimentally and theoretically, bi-excitons in II-VI quantum wells with alloy disorder. Magneto-optics is used to prove the bi-exciton nature of a photoluminescence feature emerging on the low-energy side of the groundstate exciton at moderate excitation densities and To distinguish it from bound exciton transitions. On a set of differently designed structures, we elaborate that the bi-exciton binding energy increases with growing two-dimensional confinement. The fact that this energy is generally larger than the donor bound exciton binding energy represents unambiguous evidence that, similar like for a single exciton, localization of the bi-exciton on alloy fluctuations takes place. This is also revealed by the formation and decay dynamics of the complex seen in time-resolved photoluminescence. We present calculations of the bi-exciton binding energy and other relevant quantities beyond the virtual crystal approximation based on the density functional formalism. The results confirm the stability of a localized bi-exciton state. Most of the localization energy stems from the localization of the two independent excitons while their interaction energy is only slightly altered with respect to a situation with no alloy disorder.