Biexciton luminescence in high-quality ZnO epitaxial thin films

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Abstract
A photoluminescence (PL) band, the M band, was observed in photoluminescence spectra for various excitation densities in high-quality ZnO epitaxial thin films. The M band intensity increased superlinearly with an increase in the excitation intensity, suggesting that the observed PL band is due to a biexciton state. In order to prove this, the photoluminescence excitation (PLE) spectrum of the M band and time dependence of the PL intensity were measured. A shoulder that originates from two-photon absorption of the biexciton state appeared in the PLE spectrum. The biexciton binding energy was estimated to be 15 meV. The temporal behavior of the PL intensities of the M and free-exciton bands can be explained by modified rate equations assuming that the M band is caused by radiative annihilation of a biexciton leaving a free exciton and that the rate of creation of biexcitons is proportional to the 1.5th power of the exciton density. This power dependence is consistent with experimental results showing that the M band intensity is proportional to the 1.5th power of the excitation density.