Transmission electron microscopy is the most valuable technique to access composition and morphology of semiconductor heterostructures. Nanostructures containing ternary layers (e.g. InxGa1-xAs and AlxGa1-xAs) are subject of intensive research because of their application in novel optoelectronic devices. The CELFA technique aims to provide a well applicable tool for near atomic scale spatial resolution composition determination in ternary semiconductor nanostructures, giving insight into kinetic growth effects such as segregation and migration.
The image shows the local In concentration in a Stranski-Krastanow InGaAs quantum dot structure, that was grown by molecular beam epitaxy. It was taken on a Philips CM200 FEG microscope and evaluated with the CELFA method. The red region corresponds to a quantum dot embedded in a wetting layer with smaller In-concentration. The CELFA method uses a 2 beam interference of the undiffracted (000) beam with the chemically sensitive (002) beam. The simplicity of this condition provides for a simple thickness dependence of the (002) beam amplitude.
The local (002) Fourier component of the image intensity is evaluated from the resulting lattice fringe image. The composition x is obtained by a comparison with Bloch-wave calculations, carried out with the EMS program package.
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