Interface dynamics and crystal phase switching in GaAs nanowires

Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically-grown nanowires provide an ideal system for studying the fundamental physics of phase selection, while also offering the potential for novel electronic applications based on crystal polytype engineering. Here we image GaAs nanowires during growth as they are switched between polytypes by varying growth conditions. We find striking differences between the growth dynamics of the polytypes, including differences in interface morphology, step flow, and catalyst geometry. We explain the differences, and the phase selection, through a model that relates the catalyst volume, contact angle at the trijunction, and nucleation site of each new layer. This allows us to predict the conditions under which each phase should be preferred, and use these predictions to design GaAs heterostructures. We suggest that these results may apply to phase selection in other nanowire systems.