Supplement Number 1

We present scanning transmission electron microscopy (STEM) data, including a low-magnification high-angle annular dark-field (HAADF) STEM image of wurtzite (WZ) nanowire (NW) ensembles, HAADF-STEM images of an additional WZ-on-ZB NW, and energy-dispersive-spectroscopy-determined Si atomic fractions (with error bars) for individual NWs. We also present the values for density-functional theory (DFT) computed differences in the WZ vs. ZB formation energies for GaN containing Group IVA (VIA) elements substituting for Ga (N).