We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-μm-thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our measurements reveal that the peak position of the near-band-gap luminescence strongly depends on the distance to the substrate interface. The luminescence shifts continuously to lower energies with decreasing distance but a strong blue shift occurs directly at the interface. We correlate these effects with the inhomogeneous free-carrier distribution and the strain gradient found by our Raman experiments.

Thick AlxGa1−xN epilayer with microcracks grown by metalorganic vapor-phase epitaxy on a GaN buffer above a (0001) sapphire substrate was comprehensively characterized by spatially and spectrally resolved cathodoluminescence (CL) and micro-Raman (μ-Raman) spectroscopy. The variation of the CL line shift and the μ-Raman measurements between the microcracks are consistent with the interpretation that AlGaN is to a large extent stressed like a two dimensional film between the microcracks with nearly full relaxation close to the cracks. A satisfactory theoretical confirmation of this stress distribution was obtained by a three-dimensional finite-element application of the elasticity theory.

The optical properties of cubic InGaN samples with varying In content are investigated to provide insight into the processes responsible for optical amplification. The samples were grown by molecular beam epitaxy on GaAs substrates. The structural and optical properties were studied by means of time-resolved and time-integrated photoluminescence spectroscopy and cathodoluminescence microscopy, as well as gain measurements at various temperatures. From these measurements, localized states are proposed to be responsible as recombination mechanism. The cathodoluminescence measurements evidence a direct correlation of the degree of In fluctuation and the efficiency of optical amplification of the samples.

The strain relaxation due to cracks of different depths in AlGaN layers grown on GaN template layers has been investigated using spatially resolved cathodoluminescence spectroscopy, high-resolution x-ray diffraction and two-dimensional finite element simulations. The experimental data consistently show that the relief of tensile stress increases with decreasing crack spacing. The measured strain profiles between the cracks are well described by the theoretical calculations for small crack spacings; whereas, deviations for larger crack spacings have been found. The latter is discussed in terms of inelastic strain relaxation mechanisms, the reliability of the deformation potential for AlGaN employed in this article, and the spatial variations in the properties of the AlGaN, e.g., its composition.

The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer.