International audienceIn this paper we report on a spontaneous 2D/3D transition observed in InGaN alloys after 60 nm of growth. This transition is responsible for the formation of a stack of distinct InGaN layers. The driving mechanism is shown to be lateral fluctuations of the indium composition, that arise to accommodate the increasing strain energy of the InGaN layer. Three distinct stages of growth have been identified. First, a homogeneous, 2D InGaN layer forms, pseudomorphically strained on the underlying GaN. Then, at around 30 nm large lateral fluctuations of the indium composition are observed and a second pseudomorphic layer, composed of indium-rich and indium-poor clusters, is formed. Finally induces a 2D/3D transition at 60 nm and a 3D InGaN layer is formed
Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.
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