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The optical properties of Al x Ga 1--x N samples (x < 0.7) have been studied by photoluminescence (PL) and reflectivity in the 10-300 K temperature range. Various physical properties have been studied as a function of composition, such as Stokes shift, alloy broadening, exciton localization, and Huang-Rhys factor. Up to x % 0.3, a band gap bowing factor of $0.9 eV accounts for the variation of PL and reflectivity energies. At higher compositions, luminescence energies deepen with regard to this behaviour. This is interpreted as a consequence of the G 9 -G 7 crossover of the valence band maxima. This is confirmed by the linear polarization of the luminescence studied under oblique observation.

We report a study by photoluminescence (PL), Raman scattering, and highly resolved X‐ray diffraction (HRXRD) of a series of Si‐doped n‐type GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire (0001) with the carrier concentration of 2.3 × 1017 – 9 × 1018 cm–3. We found that the band gap reduction deduced from the PL spectra analysis is due to both band gap narrowing (BGN) effect and change of the nature of stress in the GaN:Si layers. The HRXRD spectra show that at high Si‐doping levels (>1.6 × 1018 cm–3) the GaN films become under tensile stress. Progressive decreases of the E2 Raman mode frequency with Si concentration confirm this observation. The stress induced E2 mode frequency shift is estimated to 1.6 cm–1/GPa. Additionally, dislocation densities are determined by HRXRD by employing a model that uses the line width of X‐ray rocking curves. Atomic force microscopy (AFM) is also used to investigate the defect surface structures of GaN:Si layers which are dominated by pinned steps and surface depressions related to threading dislocations. We find that dislocation densities given by AFM measurements are in agreement with those obtained from rocking curve line widths. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)