4.2.3 Substrate Orientation (111)

On (111) substrate only one subband ladder forms which is sixfold degenerate. Biaxial in-plane strain does not split the $\Delta$ valleys, hence the subband ladders remain sixfold degenerate. The degeneracy can be lifted by applying stress along one out of the two in-plane directions, $[\bar{1}10]$ or $[11\bar{2}]$, since the resulting strain yields a different energy shift (3.47) for different subband ladders.

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