Structural stability of Si(001) and Ge(001) in external electric fields

The effects of external electrostatic fields normal to surfaces, Ex,, on the structural stability of Si(001) and Ge(001) have been investigated using first-principles total energy calculations. It has been shown that the c(4 x 2) surface is more stable than the p(2 x 2) surface for both Si(001) and Ge(001) even with a finite electric field whose strength is typical of STM, experiments. On the other hand, it has been revealed that the energy barrier for dimer flipping, E-B, shows different dependences between the Si(001)- and Ge(001)-(2 x 1) surfaces: EB at Si(001) changes in proportion to E-ext, but hardly does at Ge(001). Such a difference is explained by the difference in the rehybridization of buckled dimers between the reconstructed surfaces of Si(001) and Ge(001), which originates from the difference in the s-p separation of valence orbitals between Si and Ge.