Exciton and electron-hole plasma formation dynamics in ZnO

Abstract

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 ͑ϳ1.5 ps͒ 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.

Two types of excited electron dynamics in zinc oxide

“…The experiments of Yamamoto et al 12 have been performed for 230 nm thick epitaxial films of ZnO, the experiments of Sun et al 13 have been done for 80 nm thick nanorods, and the results of Hendry et al 9 were obtained from measurements of bulk ZnO samples. So the data obtained in these experiments can be associated with the bulk ZnO.…”

“…A great part of them concerns with the dynamics of exciton states important for optical applications. [6][7][8][9][10][11] These researches have permitted to evaluate the rates of the main excitonic processes, i.e., relaxation, recombination, and capture by impurities. Since the concentration of excitons in oxide semiconductors is typically low, in the photocatalytic processes more important is the dynamics of free excited carriers, electrons and holes.…”

“…In the pump-probe experiments of Wen et al 6 the electron relaxation time at room temperature has been estimated as about 800 fs at the energy above 3.45 eV but essentially increasing with the reduction in energy, up to 34 ps at 3.26 eV. Hendry et al 9 performed for ZnO the pump-probe experiments with probing in tetrahertz energy region. At temperature about 20 K and excitation energy of 3.21 eV, i.e., below the fundamental band gap, they found the time of binding of free electrons and holes into excitons as 20 ps.…”

Excitons versus free charges in organo-lead tri-halide perovskites

“…15) we can neglect exciton-exciton interaction, which only occurs for n410 20 cm À 3 , because there is essentially no wavefunction overlap. Consequently, we can safely assume that the binding energy is independent of excitation density 23,25 . Within such an approximation, we can apply the law of mass action for Wannier-Mott excitons, as described by the Saha-Langmuir equation 13,14 .…”

Ultrafast plasmonic nanowire lasers near the surface plasmon frequency

“…From simulations, we know that , and so , which is the time taken to establish the first output pulse. Here, we observe a 1.1 ps which is determined by the thermalization of the EHP 41 . Since EHP thermalization is solely dependent on the gain material, we expect to be independent of the electromagnetic environment (see Fig.…”