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

Abstract

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 high resolution images of edge dislocations. It is shown that at the 5 Å 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.