Exciton freeze-out and thermally activated relaxation at local potential fluctuations in thick AlxGa1−xN layers

${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers with $\text{0.05⩽x⩽0.25}$ were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by ${\mathrm{-\sigma }}_E^2{\mathrm{/k}}_B\mathrm{T,}$ where ${\sigma }_E$ is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, $\mathrm{x,}$ was found to systematically increase from 7 meV at $\text{x=0.05}$ to 21 meV at $\text{x=0.25,}$ following the theory for alloy disorder. The recombination and relaxation kinetics investigated using time-resolved CL are fully consistent with our potential fluctuation model. At 5 K, when the excitons are strongly localized, the exciton lifetime increases monotonically with aluminum content. At elevated temperatures, when the excitons are delocalized, the decay is significantly faster and preferentially nonradiative, regardless of the aluminum content.