Recombination dynamics of free and localized excitons in ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{G}\mathrm{a}}_{0.93}{\mathrm{Al}}_{0.07}\mathrm{N}$ quantum wells

Time-resolved photoluminescence studies have been performed on ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{G}\mathrm{a}}_{0.93}{\mathrm{Al}}_{0.07}\mathrm{N}$ multiple quantum wells grown by molecular-beam epitaxy on a sapphire substrate. Radiative recombinations of excitons in both the wells and barriers exhibit average decay times of $\sim 330\mathrm{ps},$ at $T=8\mathrm{}\mathrm{K},$ with a significant spectral distribution of times. This distribution is interpreted in terms of localization of carriers by potential fluctuations due to alloy disorder and to well width and depth variations. The temperature dependence of the radiative lifetime ${\tau }_r$ of excitons in the wells is deduced from the measurement of both the photoluminescence decay time and intensity. We find that ${\tau }_r$ increases linearly with temperature dependence $\partial {\tau }_r/\partial T=20.5\mathrm{}\pm 0.7{\mathrm{ps}\mathrm{}\mathrm{K}}^{\mathrm{-}1},$ for $T<\mathrm{}60\mathrm{}\mathrm{K}.$ From this result, we deduce the radiative lifetime of free excitons in the low-temperature limit (related to the oscillator strength) to be of 2.4 ps, i.e., half one order of magnitude smaller than for GaAs/Ga-Al-As quantum wells.