Picosecond spectroscopy of excitons and biexcitons in ZnO at high density

The coexistence of excitons and biexcitons at high density is investigated in ZnO using the simultaneous time-resolved picosecond spectroscopy of luminescence and gain. The sample is excited by means of two-photon absorption to ensure a homogeneous generation of electron-hole pairs. Radiative recombination of biexcitons is observed at high excitation simultaneously with radiative exciton-exciton collisions. We show that the exciton-biexciton mixture is stable up to concentrations around 2×{}10¹⁸ cm^-3, which is typically 1 order of magnitude higher than the theoretical Mott density in ZnO. We suggest that this stability of bound states at high density results from the weakness of the dipole-dipole screening of excitons. We also deduce the magnitude of the matrix element M of the biexciton recombination into one Γ₅T exciton and one photon which yields 10^-23 eV² cm³{≤}M²{≤}7×{}10^-23 eV² cm³. The kinetics of the biexciton temperature cooling and the kinetics of the biexciton density decay are also reported.