Optical investigation of exciton localization in AlxGa1−xN

The optical properties of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ epilayers with $x$ ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ layers. The localization parameter $\sigma$ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of $52\mathrm{meV}$ at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with $x$ indicating that the degree of localization again increases with $x$ . In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas.