Ab-initio modeling of electromechanical coupling at Si surfaces

The electromechanical coupling at the silicon (100) and (111) surfaces was studied via density functional theory by calculating the response of the ionization potential and the electron affinity to different types of strain. We find a branched strain response of those two quantities with different coupling coefficients for negative and positive strain values. This can be attributed to the reduced crystal symmetry due to anisotropic strain, which partially lifts the degeneracy of the valence and conduction bands. Only the Si(111) electron affinity exhibits a monotonously linear strain response, as the conduction band valleys remain degenerate under strain. The strain response of the surface dipole is linear and seems to be dominated by volume changes. Our results may help to understand the mechanisms behind electromechanical coupling at an atomic level in greater detail and for different electronic and atomic structures.