InGaN quantum dots studied by correlative microscopy techniques for enhanced light-emitting diodes

InGaN/GaN nanostructures form the active region of III-nitride emitters (light emitting diodes, laser diodes, single photon emitters) in the visible spectral range. In order to understand the optical performance of these nanostructures, it is necessary to obtain a direct correlation of alloy distribution and optical features. With this purpose in mind, laser-assisted atom probe tomography (La-APT) is a unique tool to visualize the three-dimensional distribution of chemical species at the nanometer scale. Recent advances in this technique also offer the possibility of recording simultaneously the photoluminescence spectrum of the specimen under investigation. Here, we report the results of a correlative study of a stack of InGaN/GaN quantum dots, combining high-resolution chemical and optical characterization by La-APT and in situ microphotoluminescence, compared to ex situ cathodoluminescence. We demonstrate that the structural information extracted from these techniques allows the precise modeling of these nanostructures, obtaining excellent agreement with the optical measurements. During the La-APT experiment, it was possible to resolve the emission of single quantum dots located in the nine quantum dot layers closer to the substrate. Single quantum dot lines display a spectral shift during the experiment which is assigned to the relaxation of elastic strain due to material evaporation. Our study conveys an image of the Stranski-Krastanov InGaN/GaN quantum dots that is very different from the GaN/AlN systems. The thickness of the InGaN wetting layer is thicker than the part of the dots that protrudes above the layer, and the indium mole fraction in the wetting layer is lower than that in the dots. This perturbed Stranski-Krastanov growth is explained by the surfactant effect of indium, which favors planar growth.