Exciton and electron-hole plasma formation dynamics in ZnO
We employ optical pump-THz probe measurements to study the formation of excitons and electron-hole plasmas following photogeneration of a hot electron-hole gas in the direct gap semiconductor zinc oxide. Below the Mott density, we directly observe the evolution of the hot electron-hole plasma into an insulating exciton gas in the 10 to 100 ps following photoexcitation. The temperature dependence of this process reveals that the rate determining step for exciton formation involves acoustic phonon emission. Above the Mott density, the density of the hot electron-hole plasma initially decreases very rapidly (~1.5 ps) through Auger annihilation until a stable plasma is formed close to the Mott density. In contrast to exciton formation, Auger annihilation is found to be independent of lattice temperature, occurring while the plasma is still hot.