Spatial variation of luminescence of InGaN alloys measured by highly-spatially-resolved scanning cathodoluminescence

Cathodoluminescence (CL) measurements were performed on thick InGaN samples over a wide range of indium concentrations ([ln] = 0.03 - 0.20). The layers were grown on c-plane sapphire using an AlGaN buffer layer, followed by 4 μm GaN, and 100 nm InGaN films. These thick InGaN layers were exceptionally specular, especially at low indium concentrations. Some degree of microscopic roughness was observed for x > 0.15. With the objective of correlating the microstructure to the spatial dependence of the luminescence, average CL spectra over large areas were compared with high-spatial-resolution CL measurements. The variation of the indium-related peaks over large areas was observed under low magnification scanned CL. The full width at half-maximum in the spot mode is typically smaller than wide-area scans by a factor of 2 to 3. The size of the areas with constant emission wavelength changes significantly with indium content. It has also been observed that the distribution of the local CL peak wavelength changes from single-mode Gaussian to a multimodal distribution for x > 0.1. A shoulder of the main peak at the low energy side appears at higher indium content. Spot-mode spectra show that this low energy shoulder consists of single lines at specific wavelengths. In highly resolved monochromatic images we correlate the low energy side shoulder to two structural defects: pinholes and grooves. Particularly, bright luminescence occurs at the center of the grooves.