Tuning the orientation of the top-facets of GaN nanowires in molecular beam epitaxy by thermal decomposition

In this work, we demonstrate the possibility of controlling the orientation of the top-facets of GaN nanowires grown by plasma-assisted molecular beam epitaxy by means of an in situ thermal treatment at temperatures in the 800--910 ${}^{\ensuremath{\circ}}\text{C}$ range, at which the material decomposes with a non-negligible rate. Depending on whether the process is carried out under vacuum or active nitrogen exposure, the nanowires can develop either ${1\overline{1}0\overline{2}}$ or ${1\overline{1}0\overline{8}}$ semipolar top-facets. The shape transformation is reversible; namely, the original $(000\overline{1})$ polar facets of as-grown nanowires can be recovered after further GaN growth. Using reflection high-energy electron diffraction, the reshaping process is monitored in vivo by tracking the formation and subsequent evolution of chevrons in the diffraction pattern. This analysis reveals that the reshaping takes place after a certain delay time that can last up to several tens of minutes. It also evidences that the shape transformation is not abrupt; instead there is a continuous evolution from high- to low-index semipolar facets. We observe energy barriers higher than 3 eV along the reshaping path toward the ${1\overline{1}0\overline{2}}$ top-facets. The formation of ${1\overline{1}0\overline{8}}$ top-facets is assigned to an intermediate energy minimum along the reshaping path where the system freezes in the presence of active nitrogen. A change in the Ga chemical potential at the nanowire tip is proposed as the trigger for the reshaping, a process that could be either thermodynamically or kinetically driven. The formation of semipolar facets after thermal decomposition is also observed for $\mathrm{GaN}(000\overline{1})$ layers, demonstrating that the reshaping process is not related to the peculiar nanowire morphology. This unprecedented ability to form semipolar facets at the tip of N-polar GaN nanowires may pave the way for the fabrication of semipolar axial nanowire heterostructures along the $[000\overline{1}]$ direction.