Optical investigations of AlGaN on GaN epitaxial films

We investigated coherently strained ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}$ N/GaN heterostructures $\text{(0<x<0.22)}$ grown by metalorganic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodoluminescence experiments. The energetic positions of the free A exciton as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localization. The broadening of the luminescence linewidth in the alloys can be described by statistical disorder of a random alloy.