High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in (001)-oriented GaN epitaxial layers. For this, (00l) and (hkl) Bragg reflections with h or k nonzero were studied, the latter one measured in skew symmetric diffraction geometry. The defect analysis was applied to a variety of GaN layers grown by molecular-beam epitaxy under very different conditions. The outcome is a fundamental correlation between the densities of edge- and screw-type dislocations.

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cracks but also by the extension of cracks into the GaN layer and a thermal activated change in the defect structure.

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

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GaN layers were grown by molecular beam epitaxy and doped with carbon of nominal concentrations ranging from 1016 cm−3 to 1020 cm−3. The incorporation of carbon leads to a reduction of the background electron concentration by one order of magnitude but the material remains n-type. For high carbon concentrations a re-increase of the carrier concentration is observed which is related to selfcompensation. Investigations of the donor-acceptor-pair luminescence show that doping with carbon is accompanied by the generation of a new donor exhibiting a thermal activation energy of about 55 meV. Layers grown by atomic layer epitaxy are marked by an increased intensity of the donor-acceptor-pair band luminescence which is attributed to the enforced incorporation of carbon onto the nitrogen sublattice. The yellow luminescence is found to be a typical feature of all carbon doped layers in contrast to nominally undoped samples.