As vacancies, Ga antisites and Au impurities in Zincblende and Wurtzite GaAs nanowire segments from first principles

TL;DR

This study uses density functional theory to investigate point defects like vacancies, antisites, and impurities in GaAs nanowires, revealing defect formation energies, diffusion barriers, and electronic levels relevant for nanowire growth.

Contribution

First-principles calculations of defect energetics and diffusion in GaAs nanowires under growth conditions with zincblende and wurtzite structures, including Au impurity effects.

Findings

Au impurities are the most energetically favorable defects.

As vacancies diffuse anisotropically in WZ GaAs.

Au substitution introduces a charge transfer level near the band gap.

Abstract

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 films or bulk GaAs, such as the co-existence of zincblende 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, Ga_As antisites, and Au impurities in ZB and WZ GaAs crystals. Our results show that As vacancies can diffuse within in a ZB GaAs crystal with migration barriers of ~1.9 eV. Within WZ GaAs, 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 Ga_As 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 an Au impurity to replace a lattice Ga atom than a lattice As atom in GaAs. An 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 + 0.22eV, taking into account the inaccuracy of the density functional that ought to be corrected by a downshift of E_v by about 0.2eV results in good agreement with the experimental result of E_v + 0.4eV.