The correlation between photoluminescence (PL) and structural properties of ZnO crystals has been investigated using fine particles prepared by a precipitation method and subsequent heat treatment. The ZnO particles heat-treated at temperatures less than 600°C exhibited red emission centered at 1.9 eV for a weak light excitation similar to that observed in ZnO thin films sputter-grown under thermally non-equilibrium conditions. Heat treatment at high temperatures reduced the red PL while it enhanced the green PL around 2.45 eV. Raman scattering and X-ray diffraction measurements showed that the ZnO particles obtained at lower temperatures have a greater lattice disorder accompanied by a specific lattice expansion along the c -axis of the ZnO crystal which would introduce defect states in the band gap. In addition, electrical conductivity measurements suggested that the lattice disorder is associated with interstitial Zn. These results indicate that the lattice disorder along the c -axis of the ZnO crystal, i.e., the interstitial Zn is most likely responsible for the red PL observed for both ZnO particles and thin films. On the other hand, recombination centers such as singly charged oxygen vacancies contributing to the green emission may be too low in concentration to affect the structural quality.
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Gomi et al. (2003) studied this question.