Control of biexciton localization and optical functionality in nitride-based ultraviolet alloy semiconductors

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We have studied the effect of localization due to alloy disorder on biexcitons in Ga-rich Al_x,Ga_<1-x>N ternary alloy epitaxial layers. We measured photoluminescence excitation spectra of biexcitons for five samples with different aluminum compositions from x=0.019 to 0.092. On the basis of the two-photon absorption of biexcitons, the binding energy of biexcitons was obtained as a function of aluminum composition. The strong enhancement of the biexciton binding energy was observed with an increase in aluminum composition. The biexciton binding energy in the Al_<0.002>Ga_<0.908>N ternary alloy epitaxial layer was estimated to be 16.6 meV, which was three times as large as the biexciton binding energy in GaN (5.6 meV). In addition, we defined the Stokes shift of biexcitons in order to evaluate the degree of biexciton localization quantitatively. We observed the strong localization of biexcitons. The ratio of biexciton Stokes shift relative to exciton Stokes shift was estimated to be 0.8 … More . The strong localization of biexcitons resulted in the strong enhancement of the biexciton binding energy. We have also studied the dynamical behavior of localized biexcitons in Al_xGa_<1-x>N ternary alloys by means of time-resolved luminescence spectroscopy. We analyzed the observed time-dependent luminescence using rate equations which expressed population dynamics of biexcitons and single excitons. The analysis was modified by considering the localization time of both biexcitons and single excitons individually in order to take account of the effect of localization due to alloy disorder. The analysis revealed the increase in the localization time of both excitons and biexcitons with increasing aluminum composition, which resulted from the increase in the degree of localization due to alloy disorder. The analysis also revealed the unchanged formation time of biexcitons in the present compositional range (x<0.1), which indicated the independence of the formation rate of biexcitons from the localization depth of single excitons. Less