Huge binding energy of localized biexcitons in CdS/ZnS quantum structures

Biexcitons localized in ultrathin CdS/ZnS single quantum wells with fluctua ting well thicknesses are investigated by two-photon absorption and single- exciton spectroscopy. For the binding energy of the confined biexciton, a v alue of Delta E-B=38 meV is determined. Comparing Delta E-B with the CdS bu lk exciton binding energy E-Ryd=28 meV, the largest known ratio Delta E-B/E RRyd=1.4 is found. Additionally, a strong enhancement of exchange splitting effects is observed resulting in a linearly polarized exciton doubler and a single, linearly polarized biexciton line.