Origination of the direct-indirect band gap transition in strained wurtzite and zinc-blende GaAs nanowires: A first principles study

Recent work has demonstrated that uniaxial strain applied to wurtzite (WZ) GaAs nanowires leads to an interesting direct-indirect band gap transition. Here, we explored the potential of strain engineering on electronic structures of one-dimensional WZ and zinc-blende (ZB) GaAs nanowires along the [0001] and [111] directions, respectively. The studied strain includes uniaxial strain, radial strain, and strain along zigzag and armchair directions in the cross section of the nanowires and shear strains. It was found that the WZ and ZB GaAs nanowires with a diameter of ∼2 nm have an indirect band gap, whereas, bulk GaAs has a direct gap. The near-gap states (valence-band maximum/conduction- band minimum) are dominated by s or p orbitals of Ga or As atoms. The energies of these states respond very differently to the applied strains. For example, the energy increases with a positive uniaxial expansion while decreasing with a negative uniaxial compression for a state dominated by bonding s orbitals. However, for a state dominated by antibonding s orbitals, an opposite trend of its energy response to strain is observed. Furthermore, the energy response to strain of bonding p orbitals was found to be different from that of bonding s orbitals. Due to the different responses of the near-gap state energies with respect to strain, the direct-indirect band gap transition was produced. It was further found that whether a strain can trigger the direct-indirect band gap transition in the GaAs nanowires depends strongly on the type of applied strain. It requires less strain energy to convert the indirect gap to be direct in both the WZ and the ZB nanowires through applying a radial strain in the cross section, compared with applying a strain in the zigzag direction.