Ultrafast screening and carrier dynamics in ZnO: Theory and experiment

TL;DR

This study combines ultrafast pump-probe experiments and theoretical modeling to investigate how high carrier densities affect exciton resonances and optical spectra in ZnO, revealing the Mott density and carrier dynamics.

Contribution

The paper introduces a comprehensive theoretical framework and experimental validation for carrier dynamics and optical properties in ZnO at high densities, including the determination of the Mott density.

Findings

Mott density in ZnO is approximately 1.5×10^{24} m^{-3} at 300 K.

Theoretical reflectivity spectra match experimental data well.

Carrier dynamics involve three-photon absorption, cooling, and surface trapping.

Abstract

At carrier densities above the Mott density Coulomb screening destroys the exciton resonance. This, together with band-gap renormalization and band filling, severely affects the optical spectra. We have experimentally studied these effects by ultrafast pump-probe reflectivity measurements on a ZnO single crystal at various wavelengths around the exciton resonance and in a broad carrier-density range. Theoretically we determined the Mott density in ZnO to be $1.5\times10^{24}$ m$^{-3}$ at 300 K. Taking a field-theoretical approach, we derived and solved the Bethe-Salpeter ladder equation and we computed the density-dependent reflectivity and absorption spectra. A carrier dynamics model has been developed, containing three-photon absorption, carrier cooling, and carrier trapping near the surface. The agreement between the theoretical reflectivity based on our model and the experimental data is excellent.