First Observation of InxGa1−xAs Quantum Dots in GaP by Spherical-Aberration-Corrected HRTEM in Comparison with ADF-STEM and Conventional HRTEM

InxGa1−xAs quantum dots in GaP(100) crystals prepared by the OMVPE technique are observed along the [011] direction with a newly developed 200-kV spherical aberration(Cs)-corrected HRTEM, a 200-kV annular dark-field (ADF)-STEM, and a 200-kV conventional HRTEM equipped with a thermal field-emission gun. The dots are 6–10 nm in size and strongly strained due to the misfit of about 9% with the GaP substrate and GaP cap layer. All of the cross-sectional high-resolution electron micrographs show dumbbell images of Ga and P atomic columns separated by 0.136 nm in well-oriented and perfect GaP areas, but the interpretable images are limited to those taken with the Cs-corrected HRTEM and ADF-STEM with Fourier filtering of the images. The Cs-corrected HRTEM and ADF-STEM are comparable from the viewpoint of interpretable resolution. A detailed comparison between the Cs-corrected HRTEM images and the simulated ones with electron incidence tilted by 1° to 5° from the [011] zone axis gives information on local lattice bending in the dots from the images around 0.1 nm resolution. This becomes one of the useful techniques newly available from electron microscopy with sub-Ångstrom resolution.

High-Resolution Electron Microscopy of Dislocation Cores in NiAl

ABSTRACTThe atomic structure of dislocation cores in NiAl is studied by high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) calculations. Results are presented on dislocations with Burgers vectors b=a<100> and a<111>. A comparison with HRTEM image simulations indicates that the core of a 45° a <100> dislocation consists of Al atoms. The Burgers vector distribution shows a width of 2.2b. This corresponds very closely to MD results and is consistent with the relatively low Peierls stress of this dislocation. By detailed image analysis the angular dependence of the shear stress components of the dislocation are made visible. MD results obtained from 45° dislocations with opposite screw components suggest, that the helicity of the screw component might be discernible from high-resolution electron micrographs. A a<111> dislocation with <110> line direction is shown which exhibits a rather wide dissociation, probably into two a/2<111> partials.

Analysis of Experimental Error in High Resolution Electron Micrographs

Analysis of Experimental Error in High Resolution Electron Micrographs

Electron crystallography

The idea of solving unknown crystal structures from experimental electron-diffraction intensities and high-resolution electron micrographs has remained a controversial topic in the 60 year history of electron crystallography. In this review it will be shown that the application of modern direct phasing techniques, familiar to X-ray crystallographers, has decisively proven that such ab initio determinations are, in fact, possible. This statement does not, by any means, refute the existence of the several significant scattering perturbations identified by diffraction physicists. Rather, it does affirm that experimental parameters can be controlled to ensure that a `quasi-kinematical' data set can be collected from many types of specimens. Numerous applications have been made to various types of specimens, ranging from small organics to proteins, and also some inorganic materials. While electron crystallography may not be the optimal means for determining accurate bonding parameters, it is often the method of choice when only microcrystalline specimens are available.

IMAGE ANALYSIS OF A NEGATIVELY CURVED GRAPHITIC SHEET MODEL FOR AMORPHOUS CARBON

High-resolution electron micrographs are presented which show essentially curved single sheets of graphitic carbon. Image calculations are then presented for the random surface schwarzite-related model of Townsend et al. (Phys. Rev. Lett.69, 921–924, 1992). Comparison with experimental images does not rule out the contention that such models, containing surfaces of negative curvature, may be useful for predicting some physical properties of specific forms of nanoporous carbon. Some difficulties of the model predictions, when compared with the experimental images, are pointed out. The range of application of this model, as well as competing models, is discussed briefly.

Modeling and simulation of octahedral Pb inclusions in Al

Pb inclusions in Al have been extensively studied for their unusual melting/solidification behavior. Pb inclusions have a cube on cube parallel orientation relationship with the Al matrix and assume cuboctahedral shapes faceted on {111} and {100}. Al and Pb are both fcc structures but with very different lattice parameters: aAl = 0.405 nm, apb = 0.495 nm. Thus 5 Al spacings match approximately 4 Pb spacings giving rise to a moire pattern visible in HREM images.High resolution electron micrographs in the <110> zone axis orientation were recorded on the Berkeley ARM at an accelerating voltage of 800 kV. In this orientation the cuboctahedra project as truncated parallelograms as shown in Fig. 1. Although the four (111) interfaces revealed in Fig. 1 are imaged edge-on, the Al lattice overlaps the Pb lattice above and below, because the other four (111) interfaces are inclined. Therefore, even though the (111)Al lattice is clearly resolved, the determination of inclusion size is not straightforward because the contrast depends on defocus (Δf), particle size (s), depth of the inclusion in the matrix (z) and total sample thickness (t).