GaAs nanowires: A new place to explore polytype physics

Recently, polytypism has been observed in nanowires in materials (e.g. GaAs) for which normally only one crystal structure is stable. Here we provide band structure parameters for wurzite and 4H GaAs and use them for modeling the nanowire electronic states. The band gap, crystal field splitting, and its strain dependence are calculated using the quasiparticle self-consistent GW method. The nanowire electronic states are obtained in the envelope function approximation within a simplified cylindrical model. The crystal field splitting of the wurtzite GaAs valence band is found to be 180 meV while in 4H-GaAs it is 69 meV, suggesting a downward bowing as a function of hexagonality. The conduction band minimum at Γ changes symmetry character under strain. We discuss the consequences for nanowires and determine the conditions under which a polarization reversal of photoluminescence can occur from mostly perpendicular to parallel to the wire.