Optical energy gaps are measured for high-quality Al 1−x In x N-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of Ϸ6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13Ͻ x Ͻ 0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.

Study of the relationship between the composition and optical energies of In x Ga 1-x N has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from In x Ga 1-x N epilayers covering the full range of composition (0 < x < 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 < x < 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.