Excitonic fine structure and recombination dynamics in single-crystallineZnO

The optical properties of a high quality bulk $\mathrm{ZnO}$, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at $3.3605\mathrm{eV}$ having a linewidth of $0.7\mathrm{meV}$ and dominating the PL spectrum at $10\mathrm{K}$. This bound exciton transition was visible only below $150\mathrm{K}$, whereas the A-free exciton transition at $3.3771\mathrm{eV}$ persisted up to room temperature. A-free exciton binding energy of $60\mathrm{meV}$ is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of $\sim 170$ and $\sim 864\mathrm{ps}$ for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.