Band structures and spatial carrier confinement in GaAs/GaP core-shell nanowires: Core/shell composition and size effects

The core/shell composition and size effects on band structures and spatial confinement of carriers in zinc-blende and wurtzite GaAs/GaP core-shell nanowires are investigated by using first-principles calculations. Our results demonstrate that surface GaP shell layer in the core-shell nanowires suppresses surface states of GaAs core, resulting in an intrinsic semiconductor characteristic. The results of electronic structures suggest that band gap, band-structure characteristic, and spatial carrier confinement of core-shell nanowires strongly depend on their size, core/shell composition, and crystal phase. Unlike the direct band gap in wurtzite core-shell nanowires, there is an indirect-to-direct band-gap transition in zinc-blende core-shell nanowires with increasing composition x. The direct band-gap characteristic in zinc-blende core-shell nanowires can be maintained in a larger composition range (0.12 < x ≤ 1). The spatial charge-density distribution of band-edge states indicate that the smaller-sized GaAs/GaP core-shell nanowires ( d < 5 nm) have a pseudo-type I band alignment that creates a stronger confinement for hole carriers but there is a type-I band alignment in larger-sized nanowires where both electrons and holes are localized in the GaAs core. The findings provide a theoretical guidance for engineering electronic structures of GaAs/GaP nanowires and their applications in electronic and optoelectronic devices.