Analysis of the extended defects in 3C-SiC.pdf - Nelson Mandela ...

2.8 Twinning
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Twinning is a process in which a region of the crystal is deformed through a
homogenous shear process. This produces the original crystal structure but in a
different orientation. The simplest case is the result in which the twinned area is a
mirror image of the original crystal. This is depicted in Fig. 2.18.
Fig. 2.18. A geometrical description of twinning in a crystal lattice (from Hull et al.
(1984))
The open circles represent the position of the atoms before twinning and the filled
circles after. The line x-y is termed the twin boundary with atoms above and below it
mirror images of one another. Twinning may occur due to plastic deformation or
introduced into the material during the CVD growth process. It differs from the
process of slip since no rotation of the lattice is taking place. In the fcc and zinc-
blende structures twinning takes place on the {111} system. It is as a result of
a shear causing a displacement of 1/6 on each consecutive plane and can be
seen as the motion of partial dislocations with b = 1/6 on consecutive {111}
planes lying parallel to x-y.
Furthermore a modification to the electron diffraction pattern obtained from a twinned
area in a crystal also occurs. The twinned sections generate a different set of
diffractions spots according to its crystallographic orientation which are superimposed
onto the diffraction pattern from the perfect crystal. Such a situation is depicted in
Fig. 2.19.