Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of A and B excitons (XX AB) as well as A biexcitons and their unbound biexciton are clearly observed. Through the FWM spectra and delay-time dependence, it is found that the interaction between A and B excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of XXAB (i.e. XX AB*). This interaction is found to play an important role in the FWM signals for all polarizations.
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