Diffraction contrast STEM of dislocations: Imaging and simulations
Highlights
► STEM defect analysis has been extended to include dislocations. ► Systematic row, zone axis & 3g diffraction conditions are all found to be useful for general defect observations in STEM mode. ► Conventional contrast visibility rules for diffraction contrast are found to remain valid for STEM observations. ► Multi-beam dynamical scattering matrix simulations provide excellent agreement with experimental images.
Introduction
Defect analysis in crystalline materials, a major and mature branch of electron microscopy, has been and continues to be dominated by conventional transmission electron microscopy (CTEM) imaging methods. However, as early as the 1970s, it was demonstrated that scanning transmission electron microscopy (STEM) could also be employed successfully in defect analysis [1], [2], [3], [4], [5]. Beyond the direct application of STEM to defect analysis, these early publications highlighted many additional benefits of STEM not realized in CTEM. For example, it has been shown that STEM imaging can be performed on thicker samples than CTEM, and that bend contour and auxiliary contrast effects can be suppressed while retaining defect contrast. It was also shown that, under certain conditions, traditional CTEM and invisibility criteria remain applicable, where g, b, and R indicate the operative diffraction vector, Burgers vector, and displacement vector (of a stacking fault), respectively [2], [4], [5].
The imaging mechanisms of STEM and CTEM have been shown to be equivalent through the principle of reciprocity, which necessitates that the angles of incidence and collection in CTEM equal the angles of collection and incidence in STEM, respectively [4], [5], [6]. In practice, however, to achieve the aforementioned benefits of STEM, the STEM collection angle is often taken to be much larger than the CTEM incidence angle. As such, computational validation is required for a proper interpretation of STEM defect images. The principle of reciprocity and further advantages of STEM are discussed in great detail in a previous publication by the authors [7], which systematically explores stacking fault contrast under various experimental conditions and provides computational validation for STEM defect observations. The main purpose of the present paper is to extend the previously developed methodology of STEM diffraction contrast to dislocations. Imaging modes similar to CTEM bright field (BF), dark field (DF), and weak-beam dark field (WBDF) will be explored; experimental as well as computational results will be provided.
Section snippets
STEM image collection
A schematic of the STEM imaging mode, in a systematic row configuration, is shown in Fig. 1. The converged probe provides a conical illumination on the sample, of opening angle , as opposed to the parallel beam case in CTEM. The camera length (CL), the distance between the sample and the detector plane, is an adjustable parameter that directly influences image contrast, as it controls the bright field (BF) and annular dark field (ADF) detector acceptance angles. Table 1 contains a list of
Experimental and computational results
A sample area containing several dislocations was selected for all observations and image simulations. Fig. 2 shows a schematic illustration of four dislocation line segments, labeled 1 through 4; these dislocations have line directions near the [1 1.0] direction, and Burgers vectors of the type ±1/3[1 1.0], so they are near perfect a-type screw dislocations. The distance between the centers of dislocations 2 and 3 is approximately 93 nm, and sets the scale for all experimental images in this
Conclusion
STEM imaging has been shown to be a useful method for crystalline defect analysis for a variety of diffraction configurations. Computational micrographs validate the experimental results, which remain in good agreement throughout. The traditional CTEM diffraction contrast and criteria hold in STEM; invisibility conditions can also be achieved. It is important to stress that experimental parameters, such as , , CL, and detector geometry, must all be accounted for, as these directly
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