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 20454226). 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.%纳米线表面存在大量的表面态,它们能够引起电子分布在纳米线表面,使得纳米线的电学性质对表面条件变得更加敏感,严重地制约器件的性能.表面钝化能够有效地移除纳米线的表面态,进而能够有效地优化器件的性能.采用基于密度泛函理论的第一性原理计算方法研究了表面钝化效应对GaAs纳米线电子结构性质的影响.考虑了不同的钝化材料,包括氢元素、氟元素、氯元素和溴元素.研究结果表明:具有小尺寸的GaAs裸纳米线的能带结构呈间接带隙特征,表面经过完全钝化后,转变为直接带隙特征;GaAs纳米线表面经过氢元素不同位置和不同比例钝化后,展示出不同的电学性质;表面钝化的物理机理是钝化原子与纳米线表面原子通过电荷补偿移除纳米线表面的电子态;与氢元素钝化相比,GaAs纳米线表面经过氟元素、氯元素和溴元素钝化后,带隙宽度较小,原因是氟元素、氯元素和溴元素在钝化过程中具有较小的电荷补偿能力,不能完全移除表面态.
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