Physical Principles of Electron Microscopy: An Introduction to TEM ...

114 Chapter 4
Crystals can also contain point defects, the simplest example being an
atomic vacancy: a missing atom. Lattice planes become distorted in the
immediate vicinity of the vacancy (as atoms are pulled towards the center)
but only to a small degree, therefore single vacancies are not usually visible
in a TEM image. However, the local strain produced by a cluster of
vacancies (a small void) gives rise to a characteristic bright/dark diffraction
contrast; see Fig. 4-16.
One example of a planar defect is the stacking fault, a plane within the
crystal structure where the atomic planes are abruptly displaced by a fraction
of the interatomic spacing. In a TEM image, the stacking fault gives rise to a
series of narrowly-spaced fringes (see Fig. 4-15) whose explanation requires
a Bloch-wave treatment of the electrons, as outlined below.
Figure 4-16. Diffraction-contrast image (1.0 �m � 1.5 �m) of point-defect clusters produced
in graphite by irradiation with 200-keV electrons at an elevated temperature. A bend contour,
visible as a broad dark band, runs diagonally across the image.
Other features in diffraction-contrast images originate from the fact that
the crystal can become slightly bent as a result of internal strain. This strain
may arise from the crystal-growth process or from subsequent mechanical
deformation. As a result, the orientation of the lattice planes gradually
changes across a TEM specimen, and at some locations the Bragg equation
is satisfied, resulting in strong diffraction and occurrence of a bend contour
as a dark band in the image; see Fig. 4-16. Unlike our previous examples, the
bend contour does not represent a localized feature present in the specimen.
If the specimen or the TEM illumination is tilted slightly, relative to the
original condition, the Bragg condition is satisfied at some different location
within the crystal, and the bend contour shifts to a new position in the image.
Thickness fringes occur in a crystalline specimen whose thickness is
non-uniform. Frequently, they take the form of dark and bright fringes that
run parallel to the edge of the specimen, where its thickness falls to zero. To