Computed electron microscope images of atomic defects in f.c.c. metals

Limitations on the calculation of high-resolution detail in electron microscope images of defects in crystals are discussed and it is shown how images of crystals with arbitrary strain can be calculated by means of a reinterpretation of the normal dynamical equations. An alternative approach, which in some cases is preferable for numerical calculations, is the method of periodic continuation. These methods, and various approximations to them, are applied to the study of highresolution images of edge dislocations. It is shown that at the 5 ]k level of resolution, the column approximation is adequate for calculating the important features of weak-beam images of an edge dislocation with an extended core. However, when applied to the calculation of lattice fringes near a core, it may lead to serious error. Calculations are presented which show that, because of spherical aberration, care is required in the interpretation of lattice-fringe images near a dislocation core even for thin crystals.

Section: Approximationsmentioning

confidence: 99%

Section: Computations In Reciprocal Spacementioning

confidence: 99%

The calculation and interpretation of high-resolution electron microscope images of lattice defects

Anstis¹,

Cockayne²

1979

“…It can readily include surface relaxation or reconstruction without any serious complications. Furthermore, it may be applicable to simulations of defects using the periodic continuation approximation (Fields & Cowley, 1978). The effect of anomalous absorption resulting from the depletion of the elastic wave by inelastic scat- tering can be easily taken into account by adding additional complex contributions Vg (z) and Vg to (2) and (8) (Yoshioka, 1957; Nagano, 1~90) in two-and three-dimensional Bloch-wave theories, respectively.…”

Section: Discussionmentioning

confidence: 99%

Newn-beam dynamical calculations combining two- and three-dimensional Bloch waves

Mitsuishi¹,

Watanabe²,

Hashimoto³

1994

A new scheme for the matrix representation of highenergy electron diffraction by a crystal is developed. The theory consists of the matrix formula of twodimensional Bloch waves and that of threedimensional ones combined with the layer-doubling method. The new method reduces computing time to about one-tenth of that required for two-dimensional Bloch waves alone and makes it possible to include the surface effect accurately.

“…Such images should thus not be termed structure images -the expression 'interference lattice image' seems more appropriate since this denotes that the images arise from the mutual interference of diffracted beams provided only that the incident illumination has sufficient coherence (Smith, Camps & Freeman, 1981). Finally, in the vicinity of lattice defects, image interpretation in terms of atomic positions is not possible without extensive image simulation and considerable prior knowledge of the defect (see, for example, Fields & Cowley, 1978). …”

Section: Introductionmentioning

confidence: 99%

Conditions for direct structure imaging in silicon carbide polytypes

Smith¹,

O’Keefe²

1983

The conditions appropriate for direct structure imaging of silicon carbide polytypes in the high-resolution electron microscope have been investigated. Weakphase-object calculations confirm that a resolution of better than 2.5 A is necessary before polytypic stacking sequences can be identified directly. Furthermore, resolutions closely approaching 1 A are required to resolve projected pairs of Si-C atoms, and considerably better than 1 A is necessary to differentiate between the two species. Extensive multi-slice calculations, based on both current and projected electron-optical characteristics, show that polytype stacking should be recognizable at 500 kV up to thicknesses of 45-75 A, but not at 100 kV, except possibly at the 'reversed'