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 o n the low-energy side of the groundstate exciton at moderate excitatio n densities and To distinguish it from bound exciton transitions. On a set of differently designed structures, we elaborate that the bi-exci ton binding energy increases with growing two-dimensional confinement. The fact that this energy is generally larger than the donor bound ex citon binding energy represents unambiguous evidence that, similar lik e for a single exciton, localization of the bi-exciton on alloy fluctu ations takes place. This is also revealed by the formation and decay d ynamics of the complex seen in time-resolved photoluminescence. We pre sent calculations of the bi-exciton binding energy and other relevant quantities beyond the virtual crystal approximation based on the densi ty functional formalism. The results confirm the stability of a locali zed bi-exciton state. Most of the localization energy stems from the l ocalization of the two independent excitons while their interaction en ergy is only slightly altered with respect to a situation with no allo y disorder.