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 (Gamma(5)) and forbidden (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. (C) 2000 American Institute of Physics. [S0021-8979(00)00316-9].