Theoretical and observed electron microscope images of impurity atoms in thin crystals formed by L-shell ionization electrons

Elemental mapping in achromatic atomic-resolution energy-filtered transmission electron microscopy

We present atomic-resolution energy-filtered transmission electron microscopy (EFTEM) images obtained with the chromatic-aberration-corrected FEI Titan PICO at the Ernst-Ruska Centre, Jülich, Germany. We find qualitative agreement between experiment and simulation for the background-subtracted EFTEM images of the Ti-L2,3 and O-K edges for a specimen of SrTiO3 oriented down the [110] zone axis. The simulations utilize the transition potential formulation for inelastic scattering, which permits a detailed investigation of contributions to the EFTEM image. We find that energy-filtered images of the Ti-L2,3 and O-K edges are lattice images and that the background-subtracted core-loss maps may not be directly interpretable as elemental maps. Simulations show that this is a result of preservation of elastic contrast, whereby the qualitative details of the image are determined primarily by elastic, coherent scattering. We show that this effect places a constraint on the range of specimen thicknesses which could theoretically yield directly useful elemental maps. In general, interpretation of EFTEM images is ideally accompanied by detailed simulations.

Imaging individual atoms inside crystals with ADF-STEM

The quantitative imaging of individual impurity atoms in annular dark-field scanning transmission electron microscopy (ADF-STEM) requires a clear theoretical understanding of ADF-STEM lattice imaging, nearly ideal thin samples, and careful attention to image processing. We explore the theory using plane-wave multislice simulations that show the image intensity of substitutional impurities is depth-dependent due to probe channeling, but the intensity of interstitial impurities need not be. The images are only directly interpretable in thin samples. For this reason, we describe a wedge mechanical polishing technique to produce samples less than <50 A thick, with low surface roughness and no amorphous surface oxide. This allows us to image individual dopants as they exist within a bulk-like silicon environment. We also discuss the image analysis techniques used to extract maximum quantitative information from the images. Based on this information, we conclude that the primary nanocluster defect responsible for the electrical inactivity of Sb in Si at high concentration consists of only two atoms.

Energy-filtered high-resolution electron microscopy and composition-sensitive imaging

Energy-filtered electron imaging is one of the future directions of high-resolution electron microscopy (HREM). In this paper, characteristics and applications of energy-selected HREM are illustrated. Image contrast can be dramatically improved with the use of an energy filter. High-resolution chemical-sensitive imaging using ionization edge loss electrons is demonstrated in studies on Ni/Ti and A1/Ti multilayer thin films. It has been shown that the spatial resolution of energy-selected ionization edge electron images is dominated by the signal-to-noise ratio. Experimental parameters which may be selected to improve the signal-to-noise ratio are discussed. It is pointed out that since the width of the energy-selecting window is small, the energy-filtered ionization edge electron image is sensitive to the specimen composition, but the calculation of the specimen composition based on the energy-filtered images relies on the integrated ionization cross-sections which are largely determined by the solid state effects. Therefore, the energy-filtered ionization image should be referred as composition-sensitive image rather than compositional image.