3.5.6 $C_{2h}$ symmetry

Higher symmetry reduction results from deforming the base of the Bravais parallelepiped of the orthorhombic system so that the angle between its edges is changed. In this way the invariant parallelelided of the system $C_{2h}$ is obtained from the cubic lattice $O_h$ [Bir74]. It contains four symmetry operations given in Table 3.2 with only one twofold symmetry axis.

Uniaxial stress in [120] direction can achieve this kind of symmetry reduction. The strain tensor has three different nonzero diagonal components and one off-diagonal component

$$\ensuremath{{\underaccent{\bar}{\varepsilon}}} = \begin{pmatrix}{... ...{\varepsilon_{22}}} & 0\\ 0 & 0 & {\ensuremath{\varepsilon_{33}}}\end{pmatrix}.$$ (3.45)

A irreducible volume can be chosen according to Figure 3.11. The volume can be mapped onto twelve wedges of the unstrained lattice in the limit of vanishing strain. The twelve wedges are labeled in Figure 3.11 and can be transformed into the first wedge by proper transformations.

Figure 3.11: Irreducible wedge of the first BZ of a diamond structure stressed along $[120]$.

E. Ungersboeck: Advanced Modelling Aspects of Modern Strained CMOS Technology