High quality GaAs nanowires (NWs) are nowadays experimentally grown on different substrates, regularly shaped and nicely oriented. At variance with the bulk phase, they show a prevalence of wurtzite structure with respect to zincblende and a p-type behavior with respect to the possible amphoteric behavior upon Si incorporation. Motivated by these recent experimental findings we investigate by first principles pseudopotential calculations the structural stability and electronic properties of GaAs NWs. The nature of Si dopants in GaAs NWs and the relative stability of donors and acceptors have been also studied. Our results show that wurtzite NWs are more stable than zincblende NWs for diameters of 50 Angstrom or less. On the basis of the formation energy only, Si dopants have amphoteric behaviour in GaAs NWs and segregates at the surface of the wire.
