Causal self-energies for NEGF modelling of quantum nanowires

Electron-phonon scattering is shown to increase dramatically at small nanowire cross-sections so that the transport is not ballistic. Non-ballistic dissipative device modelling requires the full complexity of the non-equilibrium Green Function (NEGF) method. The role of causality in obtaining spectral sum rule-conserving approximations to the electron-phonon self-energies is demonstrated and applications given for wrap-round gate silicon nanowire field effect transistors. Causality violations give erroneous density of states and current densities.