Observation of Mixed-Mode Excitons in the Photoluminescence of Zinc Oxide

Mixed-mode excitons have been previously reported in the absorption spectrum of uniaxial II-VI crystals when plane-polarized light is used with its E field in a plane containing both the photon's wave vector k and the $c$ axis of the crystal. Lines due to annihilation of mixed-mode excitons should also be observed in the emission spectrum of these uniaxial crystals. We describe here the observation of such emission lines in the photoluminescence of ZnO at 77°K. The strengths of these lines are essentially zero when $\mathrm{k}\perp c$, but increase rapidly as the angle between k and $c$ is reduced below 90°. The energies of the $A$ and $B$ longitudinal excitons in Thomas's line assignment are found to be 3.373 and 3.386 eV, respectively, in good agreement with results obtained from reflectivity data.