As vacancies, Ga antisites, and Au impurities in zinc blende and wurtzite GaAs nanowire segments from first principles

In this paper some specific issues related to point defects in GaAs nanowires are addressed with the help of density functional theory calculations. These issues mainly arise from the growth of nanowires under conditions different from those used for thin film or bulk GaAs, such as the coexistence of zinc blende and wurtzite polytypes, the use of gold particles as catalyst, and the arsenic-limited growth regime. Hence, we carry out density functional calculations for As vacancies, GaAs antisites, and Au impurities in zinc blende (ZB) and wurtzite (WZ) GaAs crystals. Our results show that As vacancies can diffuse within a ZB GaAs crystal with migration barriers of similar to 1.9 eV. Within WZ Ga-As, As vacancy diffusion is found to be anisotropic, with low barriers of 1.60 up to 1.79 eV (depending on doping conditions) in the ab plane, while there are higher barriers of 2.07 to 2.44 eV to diffuse along the c axis. The formation energy of Au impurities is found to be generally much lower than those of arsenic vacancies or GaAs antisites. Thus, Au impurities will be the dominant defects formed in Au-catalyzed nanowire growth. Moreover, we find that it is energetically more favorable by 1 to 2 eV for a Au impurity to replace a lattice Ga atom than a lattice As atom in GaAs. A Au substitutional defect for a lattice Ga atom in ZB GaAs is found to create a charge transfer level in the lower half of the band gap. While our calculations locate this level at E-v + 0.22 eV, taking into account the inaccuracy of the density functional that ought to be corrected by a downshift of E-v by about 0.2 eV, results in good agreement with the experimental result of E-v + 0.4 eV. DOI: 10.1103/PhysRevB.87.075308