Onset of uncontrolled polytypism during the Au-catalyzed growth of wurtzite GaAs nanowires

The optoelectronic properties of a semiconductor are determined by the combination of its elemental composition and the crystal structure. The vapor-liquid-solid nanowire growth mechanism offers the controlled epitaxy of the zinc-blende and wurtzite polytype for a number of semiconductors. Long, thin, and phase-pure wurtzite GaAs nanowires are desirable as epitaxial templates for the growth of hexagonal SiGe shells, but the growth of such nanowires remains a challenge. Here, we study the growth of wurtzite GaAs nanowires and find a diameter dependent critical length beyond which the crystal phase becomes mixed. The onset of uncontrolled polytypism is modeled with a small contribution of As diffusion during growth. Due to this increased supply of As during prolonged growth, Ga is depleted from the liquid catalyst, thereby decreasing the contact angle. We investigate possible pathways of As through diffusion on the facets and edges of the nanowire, and from the scaling of the critical length we deduce that edge diffusion has an important contribution. This study offers new insights for realizing long, phase-pure wurtzite GaAs nanowires with high aspect ratio.