By means of ab initio simulations, we investigate the structural, electronic, and transport properties of boron and phosphorus doped silicon nanowires. We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Important differences between p- and n-type doping are observed. Our results rationalize several experimental observations.
