Abstract
Competitive mechanisms contribute to image contrast from dislocations in annular dark-field scanning transmission electron microscopy (ADF-STEM). A clear theoretical understanding of the mechanisms underlying the ADF-STEM contrast is therefore essential for correct interpretation of dislocation images. This paper reports on a systematic study of the ADF-STEM contrast from dislocations in a GaN specimen, both experimentally and computationally. Systematic experimental ADF-STEM images of the edge-character dislocations reveal a number of characteristic contrast features that are shown to depend on both the angular detection range and specific position of the dislocation in the sample. A theoretical model based on electron channelling and Bloch-wave scattering theories, supported by numerical simulations based on Grillo's strain-channelling equation, is proposed to elucidate the physical origin of such complex contrast phenomena.
Highlights
Defect analysis in crystalline materials provides important insights into many properties of materials across a broad range of applications [1]
Due to the complexity of the mechanisms contributing to contrast from defects in annular darkfield (ADF)-scanning TEM (STEM) images, it is often essential to couple image simulations with experimental data for a correct interpretation of defect contrast [16, 19, 20]
Contribution of the Bragg scattering decreases in the medium-angle ADF (MAADF) regime, and in the high-angle ADF (HAADF) regime it is overtaken by thermal diffuse scattering (TDS) and Rutherford scattering
Summary
Defect analysis in crystalline materials provides important insights into many properties of materials across a broad range of applications [1]. Maher and Joy were the first to apply the principle of reciprocity for defect image interpretation by using fixed-beam dynamical theory of electron diffraction [15] They showed that the geometry of crystalline defects could be assessed by STEM diffraction analysis methods as in CTEM. Perovic et al used Bloch-wave scattering theory as an alternative approach to elucidate the effect of elastic strain on the contrast of the ADF images [10, 18]. We use Bloch-wave scattering and electron-channelling theories to further report on the ADF-STEM dislocation contrast mechanism. We apply this approach to interpret the contrast effects that ascertained to depend on both ADF detection range and specific position of the defect in the sample. We couple the experimental images with simulations that have been done using an improved version of Grillo’s strainchannelling equation system to further explore the depth-dependent ADF dislocation contrast [32, 33]
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