Abstract
Biexcitons localized in ultrathin CdS/ZnS single quantum wells with fluctuating well thicknesses are investigated by two-photon absorption and single-exciton spectroscopy. For the binding energy of the confined biexciton, a value of $\ensuremath{\Delta}{E}_{\mathrm{B}}=38$ meV is determined. Comparing $\ensuremath{\Delta}{E}_{\mathrm{B}}$ with the CdS bulk exciton binding energy ${E}_{\mathrm{Ryd}}=28$ meV, the largest known ratio $\ensuremath{\Delta}{E}_{\mathrm{B}}{/E}_{\mathrm{Ryd}}=1.4$ is found. Additionally, a strong enhancement of exchange splitting effects is observed resulting in a linearly polarized exciton doublet and a single, linearly polarized biexciton line.
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