Crystal structures of GaAs nanowires prepared by employing molecular beam epitaxy technique are often dominated by the wurtzite (WZ) phase.Recently,Galicka et al.found that the WZ GaAs nanowires grown along the[0001]direction in smaller size are energetically more favorable than other nanowires with the zinc blende phase grown along a specific direction (2008 J.Phys.:Condens.Matter 20 454226).The native nanowire usually has abundant unsaturated surface dangling bonds (SDBs) inducing significant surface states,leading to electrons accumulating at the nanowire surface. Thus the electrical property of the nanowire is very sensitive to the surface condition.However,surface passivation can effectively remove the surface states from the SDBs,and optimize the device performance.In this paper,using the first-principle calculations in combination with density function theory,we investigate the effect of surface passivation on the electronic structure of the GaAs nanowires grown along the[0001]direction.Various passivation species (hydrogen (H),fluorine (F),chlorine (Cl) and bromine (Br)) with different coverage ratios are considered.The GaAs nanowires hydrogenated with different locations and coverage ratios display different electronic properties.It is found that the GaAs native nanowire with a smaller diameter shows a semiconductor characteristic with indirect band gap,which originates from the fact that at smaller diameter,the surface stress becomes more remarkable,and then leads to surface atomic reconstruction.After passivation,the indirect band gap is translated into the direct band gap.For the GaAs nanowire with an As SDB hydrogenated,one deep donor level is located in the gap,and its band structure shows an n-type characteristic.For the GaAs nanowire with a Ga SDB hydrogenated,one shallow acceptor level is located in the gap,and its band structure shows a p-type characteristic.For the GaAs nanowire with a Ga-As dimer hydrogenated, its band structure shows an intrinsic semiconductor characteristic.For the GaAs nanowire with all of the Ga SDBs hydrogenated,the band structure shows a metallic characteristic.The band gap of the GaAs nanowire gradually increases as the hydrogen passivation ratio increases.For 50% hydrogen passivation,the band gap for the symmetrical passivation is slightly bigger than that for the half-side passivation.For the F-,Cl-and Br-passivation,the band gap decreases compared with for H-passivation.This is due to the fact that the ability of passivating atoms to compensate for surface atoms is weak,thereby reducing the band gap.The mechanism for the surface passivation is the suppression of surface states by the ability of the passivating atoms to compensate for surface atoms.These results show that the electronic properties of GaAs nanowires can be modulated by surface passivation,which is helpful for using GaAs nanowires as components and interconnections of nanoscale devices.