A. Howie

Elastic strains and the energy balance for multiply twinned particles

Abstract The energetics of multiply twinned particles (MTPs) are investigated using elasticity theory. This allows the homogeneous strain models to be critically compared with the disclination model for the strains in decahedral particles and with a new model for the strains in icosahedral particles based on inhomogeneous elasticity. The overall energy balance between MTPs and single crystals is then evaluated, including the significant cost of elastially distorting the surface and using two extreme models of the faceting. The results of this analysis indicate that icosahedral MTPs will be more stable than single crystals for small sizes only for strong faceting conditions, decahedral MTPs being true intermediaries between the two. Experimentally observed stress-relief mechanisms provide indirect evidence for the inhomogeneous strain models.

Field-emission SEM imaging of compositional and doping layer semiconductor superlattices

Field-emission scanning electron microscopy (FE-SEM) has been used to study several semiconductor multilayer heterostructures. Compositional superlattices based on Ge x Si 1−x /Si and Al x Ga 1−x As/GaAs have been studied in both cross-sectional and oblique plan-views after indentation. Secondary and backscattered electron images reveal strong atomic number contrast which is primarily structural in origin. Secondly, for the first time, heterostructures containing n and p doping have been directly imaged at low voltages (0.5-1 kV) including: (i) Si- and Be-doped GaAs layers and (ii) B- and As-doped Si layers. Secondary electron images reveal strong contrast at doping concentrations as low as 10 17 cm −3 . The results have been interpreted in terms of energy band-bending effects between n- and p-doped layers

Imaging elastic strains in high-angle annular dark field scanning transmission electron microscopy

Abstract High-angle annular dark field (HAADF) imaging in a dedicated scanning transmission electron microscope (STEM) has been applied to the study of imperfect crystals. Firstly, a study of B-doped layers in Si has revealed significantly stronger contrast and of opposite sign relative to simple atomic number contrast ( Z -contrast) predictions. It is shown that misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect. Multi-beam Bloch-wave theory has been used to quantitatively predict experimental contrast levels. Secondly, HAADF-STEM imaging of inclined dislocation segments revealed a number of novel contrast effects which depend on the specific position of the dislocation in the foil. Unlike conventional diffraction contrast from dislocations, HAADF dislocation contrast is neither similar nor complementary at the entrant and exit surfaces of the specimen. A qualitative Bloch-wave scattering description has been developed consistently to describe the dislocation contrast features.

Defects in electron-irradiated germanium

Abstract The growth and structure of defects in electron-irradiated Ge has been followed at room temperature and at higher temperatures between 200° and 300°C. At room temperature, small interstitial loops of approximately edge character on {311} planes are observed and eventually grow into dendritic structures. At the higher temperatures, rods elongated along the 〈110〉 directions form first but then tend to widen into plates of interstitial type on {311} planes with a displacement vector R = 0.04〈611〉. It is possible that all of these defects are of the same type and correspond to aggregates of Ge interstitials, but the possibility of impurity effects cannot be ruled out. The defects tend to transform into perfect dislocation loops on annealing.

Interpretation of electron micrographs and diffraction patterns of amorphous materials

Abstract The image contrast from amorphous materials studied by dark field or by interference electron microscopy using tilted illumination is quantitatively discussed in terms of the microcrystallite model and of the random network model using a network model for SiO2. The experimental observations of bright spots of diameter > 10 A. in dark field and of lattice fringes in interference micrographs are well explained by the microcrystallite model but cannot be accounted for by the random network model. The random network gives rise to a number of interesting diffraction effects, however, in particular to very small rather intense bright spots of diameter <3 A. in dark field images. On the basis of the microcrystallite model some further improvements are described for the analysis of the diffraction pattern interference function to allow for intercrystalline effects and grain boundary material.

Some electron diffraction contrast effects at planar defects in crystals

Abstract Intensity profiles corresponding to transmission electron microscope images of planar defects in crystals have been calculated for a range of values of g. R, where g and R have their usual meaning. The results show that when two-beam theory is used, the defects will be effectively invisible if R.g differs from an integer by less than 0·02. When systematic many-beam theory is used, this criterion may be modified. In addition, many-beam profiles have been calculated for a number of specific cases relating to a stacking fault in silicon, and the results compared with experiment.

Study of single-electron excitations by electron microscopy I. Image contrast from delocalized excitations

Abstract The inelastic scattering of fast electrons by the excitation of L-shell electrons at a stacking fault in silicon has been studied with a scanning transmission electron microscope. It was f...

Column approximation effects in high resolution electron microscopy using weak diffracted beams

Abstract It is pointed out, and confirmed by calculation in a specific case, that the effects of the column approximation in calculating weak beam images of defects can be greatly reduced if the weak beams used make small angles with the Bragg planes.

New imaging methods for catalyst particles

The application is described of various bright field and dark field imaging methods to the study of catalyst and other small particles in the presence of scattering effects from the support. A major advance has been possible with the use of annular dark field, Z contrast and microdiffraction techniques in the STEM. Microanalysis of individual particles can now be carried out almost routinely. Methods of imaging surface structure are under development and it may also be possible to extract useful information about valence electron properties.

Calculations of lattice defect images for scanning electron microscopy

Abstract The profiles of screw dislocations and stacking faults in thin crystals, when imaged in the back-scattered mode of the scanning electron microscope, have been calculated using dynamical diffraction theory. These results may be extended to thick crystals by the inclusion of a suitable background term to allow for multiple scattering effects. The theoretical contrast of dislocation images in thick crystals is similar to that in calculated channelling patterns. The influence of probe size and beam divergence on the image is discussed, and it is apparent that the use of higher accelerating voltages offers advantages not only because of increased gun brightness but also because of more favourable diffraction and aberration conditions. It is concluded that to detect dislocations in a thick crystal using scanning microscopes with conventional electron guns and detectors the scanning times required are unreasonably long. However, in view of the improved contrast available from more efficient detectors, i...