  We study the effects of charged impurity scattering on the electronic
transport properties of <110>-oriented Si nanowires in a gate-all-around
geometry, where the impurity potential is screened by the gate, gate oxide and
conduction band electrons. The electronic structure of the doped nanowires is
calculated with a tight-binding method and the transport properties with a
Landauer-Buttiker Green functions approach and the linearized Boltzmann
transport equation (LBTE) in the first Born approximation. Based on our
numerical results we argue that: (1) There are large differences between
Phosphorous (P) and Boron (B) doped systems, acceptors behaving as tunnel
barriers for the electrons, while donors give rise to Fano resonances in the
transmission. (2) As a consequence, the mobility is much larger in P- than in
B-doped nanowires at low carrier density, but can be larger in B-doped
nanowires at high carrier density. (3) The resistance of a single impurity is
strongly dependent on its radial position in the nanowire, especially for
acceptors. (4) As a result of subband structure and screening effects, the
impurity-limited mobility can be larger in thin nanowires embedded in HfO2 than
in bulk Si. Acceptors might, however, strongly hinder the flow of electrons in
thin nanowires embedded in SiO2. (5) The perturbative LBTE largely fails to
predict the correct mobilities in quantum-confined nanowires.
