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Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy

We have examined the influence of Ga surface saturation on gallium nitride (GaN) nanowire (NW) polytype selection during molecular-beam epitaxy. The Ga surface saturation in the absence and presence of nitrogen determines the GaN polytype and morphology (i.e., films vs NW) selection, respectively. We discuss the interplay between surface and step-edge diffusion barriers governing the NW-to-film-transition and the influence of SixNy interlayer formation on zinc blende (ZB) vs wurtzite (WZ) polytype selection of GaN. In addition, distinct exciton emissions associated with ZB and WZ GaN are observed, suggesting a type-I WZ/ZB GaN band-offset. This work provides a crucial step toward the realization of polarization-free, CMOS-compatible GaN-based optoelectronics.