Picosecond spectroscopy of excitons and biexcitons in ZnO at high density.

Abstract

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. %'e show that the exciton-biexciton mixture is stable up to concentrations around 2X 10' cm, 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. %e also deduce the magnitude of the matrix element I of the biexciton recombination into one I q exciton and one photon which yields 10 2' eV cm gM (7X10 eV cm . The kinetics of the biexciton temperature cooling and the kinetics of the biexciton density decay are also reported. I. INTRODUCTION The properties of the strongly excited electron-hole system in direct-band-gap materials have been investigated for about 10 years. In many experiments, the sample was excited by a nanosecond pulse laser, and the optical properties (i.e., luminescence, transmission, Raman scattering) were investigated. Many semiconductors and insulators have been studied using these nanosecond pulse techniques, mainly II-VI and DI-V compounds. A review of these works with numerous references can be found in Refs. 1 and 2. In a second family of experimental investigations, the sample is excited by a picosecond laser, and the time-resolved spectroscopy of luminescence or gain is studied. These picosecond or subpicosecond pulse techniques have been applied to many materials. %'e quote here several works in a nonexhaustive list [CuCl, i'