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We investigated coherently strained AlxGa1−xN/GaN heterostructures (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.

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We investigated AlGaN/GaN heterostructures grown by metal-organic vapor-phase epitaxy on sapphire by calorimetric absorption, transmission and reflection spectroscopy (CAS/CTS/CRS) at 47 mK. The AlGaN film on a 2-µm-thick GaN layer introduces additional compressive strain into the GaN layer. A blue shift of the A-and B-exciton line positions is directly proportional to the AlN molar fraction in the films. The amount of strain in the GaN layers is quantified by micro-Raman experiments. We can explain the results by taking into account the elastic properties of GaN and AlGaN.