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×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, 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 ${\Gamma }_5^T$ exciton and one photon which yields ${10}^{\mathrm{-}23}$ ${\mathrm{eV}}^2$ ${\mathrm{cm}}^3$≤${\mathrm{M}}^2$≤7×${10}^{\mathrm{-}23}$ ${\mathrm{eV}}^2$ ${\mathrm{cm}}^3$. The kinetics of the biexciton temperature cooling and the kinetics of the biexciton density decay are also reported.