Understanding the zincblende to wurtzite polytype transition during epitaxy of gallium nitride nanowires

Due to reduced strain-induced polarization in nanowires (NWs) and reduced spontaneous polarization for zincblende (ZB) polytypes, ZB GaN NWs are ideal candidates for high efficiency visible optoelectronics. To date, a predictive framework for selection of ZB vs. wurtzite (WZ) polytype NWs has yet to be established. Recently, we examined the influence of Ga supersaturation on polytype selection during molecular-beam epitaxy of GaN NW ensembles. Using real-time reflection high-energy diffraction, in combination with post-growth low-incident-beam-angle x-ray diffraction, we identified conditions for the nucleation of ZB NWs and films. We hypothesize that high Ga-flux during Ga pre-deposition hinders SixNy formation, allowing epitaxy of ZB GaN on silicon. Interestingly, ZB NWs and films transform to the WZ polytype after ~ 1 hour of growth. Therefore, the transformation is likely not due to a random event but instead likely due to heating at the growth surface, suggesting opportunities for tailoring the ZB-WZ polytype transformations. We will discuss investigations of individual NWs using high resolution transmission electron microscopy, in particular searching for the stacking faults in the vicinity of the ZB-WZ polytype transformation.