Biexciton binding energy in CuCl quantum dots

Induced absorption from the exciton state to the biexciton state was clearly observed under the site-selective excitation of the inhomogeneously broadened ${\mathit{Z}}_3$ exciton band in CuCl microcrystallites embedded in NaCl crystals. This allows us to obtain the size-dependent biexciton binding energy in CuCl quantum dots. The binding energy of biexcitons increases with the decrease in their size and its size dependence and bulk value are well represented by the expression 78/${\mathit{a}}^{\mathrm{*}2}$+52/${\mathit{a}}^{\mathrm{*}}$+33 (meV), where ${\mathit{a}}^{\mathrm{*}}$ is the effective radius of microcrystallites in a unit of nm. The experimental results are compared with an available theoretical result. The enhanced Coulomb interaction in microcrystallites still increases the biexciton binding energy in the large-size regime, where the quantum confinement energy of excitons is not considerable. The size-dependent biexciton binding energy cannot be explained by the weak-confinement model of biexcitons.