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The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAsN (x = 0.01) layers - with different H doses and similar low-energy irradiation conditions - was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAsN reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

We addressed the issue of atomic ordering in a series of In-rich In xGa 1 − xN epilayers grown on a GaN/AlN/sapphire(0001) epitaxial template by quantitatively measuring the Ga-cation coordination in the second atomic shell of Ga. We used Diffraction Anomalous Fine Structure Spectroscopy (DAFS) at the Ga K-edge to select the signal coming from the Ga atoms in the epilayer and eliminate any contribution from the buffer layer. DAFS measurements were also performed on hydrogenated epilayers in order to investigate the possible formation of Ga-H complexes. We find that deviation from cation random distribution in the form of Ga clustering is present in both untreated and hydrogenated samples and that in the latter ones the second atomic shell around Ga is modified, suggesting an additional perturbation of the cation sublattice related to hydrogen. However, comparing the present results with In K-edge x-ray absorption spectra previously reported, we conclude that the formation of Ga-H complexes vs. In-H ones is unlikely. Cation clustering has to be anyway taken into account as a possible channel playing in the determination of the optical bandgap of these technology-oriented semiconductors.We addressed the issue of atomic ordering in a series of In-rich In xGa 1 − xN epilayers grown on a GaN/AlN/sapphire(0001) epitaxial template by quantitatively measuring the Ga-cation coordination in the second atomic shell of Ga. We used Diffraction Anomalous Fine Structure Spectroscopy (DAFS) at the Ga K-edge to select the signal coming from the Ga atoms in the epilayer and eliminate any contribution from the buffer layer. DAFS measurements were also performed on hydrogenated epilayers in order to investigate the possible formation of Ga-H complexes. We find that deviation from cation random distribution in the form of Ga clustering is present in both untreated and hydrogenated samples and that in the latter ones the second atomic shell around Ga is modified, suggesting an additional perturbation of the cation sublattice related to hydrogen. However, comparing the present results with In K-edge x-ray absorption spectra previously reported, we conclude that the formation of Ga-H complexes vs. In-H...