Optical properties of biexcitons in ZnS

Characteristics of biexcitons in ZnS epitaxial layers with biaxial tensile strain have comprehensively been studied by means of photoluminescence excitation spectroscopy, magnetophotoluminescence spectroscopy, and time-resolved photoluminescence spectroscopy. Photoluminescence excitation measurement of biexcitons allowed us to observe two-photon absorption of biexcitons and to determine the biexciton binding energy of 8.0±1.2meV. Under a magnetic field, the luminescence line due to the radiative recombination of light-hole free excitons slightly shifted to the lower-energy side (about 0.3 meV at 9 T). On the other hand, the luminescence line due to the radiative recombination of biexcitons split into two components. This splitting (about 1.2 meV at 9 T) originated from the Zeeman splitting of light-hole free excitons in the final state of the optical transition for biexciton luminescence. In addition, the luminescence decay times of both light-hole free excitons and biexcitons were estimated to be about 130 and 80 ps, respectively. The observed temporal behavior was consistent with the expected recombination dynamics between excitons and biexcitons.