Abstract
We report the localization dynamics of biexcitons in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals under exciton resonant excitation at low temperatures. During a few tens of picoseconds just after intense laser excitation, the photoluminescence (PL) spectral shape obeys an inverse Maxwell--Boltzmann distribution and free biexcitons dominate the PL spectrum. With a further increase in the delay time, the biexciton PL peak energy and edge energy shift to lower energies. These redshift behaviors in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals are completely different from the behaviors of free biexcitons in GaN crystals. Our observations reveal the rapid transformation dynamics from free to localized biexcitons in band-tail states in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals.
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