Catalyst-directed crystallographic orientation control of GaN nanowire growth.

In this work, we demonstrate that catalyst composition can be used to direct the crystallographic growth axis of GaN nanowires. By adjusting the ratio of gold to nickel in a bimetallic catalyst, we achieved selective growth of dense, uniform nanowire arrays along two nonpolar directions. A gold-rich catalyst resulted in single-crystalline nanowire growth along the ⟨11̅00⟩ or m axis, whereas a nickel-rich catalyst resulted in nanowire growth along the ⟨112̅0⟩ or a axis. The same growth control was demonstrated on two different epitaxial substrates. Using proper conditions, many of the nanowires were observed to switch direction midgrowth, resulting in monolithic single-crystal structures with segments of two distinct orientations. Cathodoluminescence spectra revealed significant differences in the optical properties of these nanowire segments, which we attribute to the electronic structures of their semipolar {112̅2} or {11̅01} sidewalls.