Time-resolved photoluminescence lifetime measurements of the Γ5 and Γ6 free excitons in ZnO

Time-resolved photoluminescence spectroscopy at 2 K was used to measure the radiative recombination lifetime of the allowed ${\mathrm{(\Gamma }}_5)$ and forbidden ${\mathrm{(\Gamma }}_6)$ free excitons in ZnO. The measurements were made on a sample containing internal strain, which altered the sample symmetry, and resulted in relaxed selection rules, allowing the ${\Gamma }_6$ exciton to be observed. A radiative recombination lifetime of 259 ps was measured for the ${\Gamma }_5$ exciton and 245 ps for the ${\Gamma }_6$ exciton. The decay of the free excitons was of single-exponential form, and the decay times were obtained using a least-squares fit of the data.