COMPOSITION EVALUATION BY LATTICE FRINGE ANALYSIS

In this paper we present a compositional analysis, by high-resolution transmission electron microscopy of a 2 nm InxGa1-xAs layer (nominal I n-content 60%) which was deposited on GaAs (0 0 1) and capped with 10 nm GaAs. A three-beam condition close to the (1 0 0) zone axis is used . The lattice fringe images result from the interference of the (0 0 0 ), (0 0 4) and (0 0 2) beams with the latter centered on the optical a xis. Due to the chemical sensitivity of the (0 0 2) beam, the obtained contrast patterns strongly depend on the In-content x averaged along the electron beam direction. We suggest a simple analysis procedure th at uses the ratio A(002)/A(004) of the (0 0 2) and (0 0 4) reflections of local image cell diffractograms, respectively, which is a bijectiv e function of x. Furthermore, we show that the suggested method does n ot require the exact knowledge of the objective lens defocus and the s ample thickness in the electron beam direction. In order to check the reliability of the gained results, a finite-element calculation of the strain state is performed which is based on the evaluated In-concentr ation profile. The results of the finite-element calculation are compa red with an evaluation of the image using atomic scale strain measurem ents. The investigations of the InxGa1-xAs layer showed that the GaAs overgrowth caused a structural modification of the initial island stru cture which is transformed into an InxGa1-xAs layer with a rather homo geneous thickness and an In-content that Varies locally. Reasons for t he morphological transition are discussed. (C) 1998 Elsevier Science B .V. All rights reserved.