First-principles calculations of the non-equilibrium polarization in ultra-small Si nanowire devices

The paper presents a method for the first principles calculations of polarization charge in non-equilibrium states of silicon nanowire transistor. The method combines the density-functional perturbation theory and piecewise equivalent model representation which enables us to reduce the numerical burden and eliminate erroneous contributions from the occupied electronic states. In the mean field approximation, the polarization response of the device material is taken into consideration by using appropriate dielectric constants in the Poisson solver. Our results suggest that this approximation generally fails and the electric field in the device area may give rise to a nonzero macroscopic polarization charge which is completely ignored in common device simulations.