Composition evaluation by lattice fringe analysis

In this paper we present a compositional analysis, by high-resolution transmission electron microscopy of a 2 nm In(x)Ga(1-x)As layer (nominal In-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 axis. 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 that 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 bijective function of x. Furthermore, we show that the suggested method does not require the exact knowledge of the objective lens defocus and the sample 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-concentration profile. The results of the finite-element calculation are compared with an evaluation of the image using atomic scale strain measurements. The investigations of the In(x)Ga(1-x)As layer showed that the GaAs overgrowth caused a structural modification of the initial island structure which is transformed into an In(x)Ga(1-x)As layer with a rather homogeneous thickness and an In-content that Varies locally. Reasons for the morphological transition are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.