Bound Biexciton Photoluminescence in ZnO Epitaxial Thin Films

Photoluminescence (PL) due to bound biexcitons (BM bands) was observed in ZnO epitaxial thin films. The PL intensity of the BM bands increased superlinearly with increase in excitation density, whereas the peak energy and spectral shape of the BM bands did not change. These experimental results were consistent with previously reported results for other materials. Furthermore, the binding energies of the bound biexcitons were estimated and compared with those of other materials. Our experimental results suggest that the electron-hole mass ratio of ZnO that has been commonly used is overestimated.