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

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 $\Delta E_{\mathrm{B}}=38\mathrm{}$ meV is determined. Comparing $\Delta E_{\mathrm{B}}$ with the CdS bulk exciton binding energy $E_{\mathrm{Ryd}}=28\mathrm{}$ meV, the largest known ratio $\Delta E_{\mathrm{B}}{/E}_{\mathrm{Ryd}}=1.4\mathrm{}$ 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.