Optical investigations of AlGaN on GaN epitaxial films

We investigated coherently strained AlxGa1-xN/GaN heterostructures (0<x<0.2 2) grown by metalorganic vapor phase epitaxy on sapphire with photoluminesc ence (PL), reflexion and cathodoluminescence experiments. The energetic pos itions of the free A exciton as a function of the alloy compositions are de duced from temperature dependent PL and from reflexion measurements. We obt ain a small bowing parameter and no evidence for a Stokes shift between abs orption and emission. Compositional inhomogeneities are present, but the fl uctuations are too small to be important for carrier localization. The broa dening of the luminescence linewidth in the alloys can be described by stat istical disorder of a random alloy. (C) 1999 American Institute of Physics. [S0003-6951(99)03417-8].