Vibrational and optical properties of GaN nanowires synthesized by Ni-assisted catalytic growth

GaN nanowires synthesized by Ni-assisted catalytic vapour-liquid-solid growth at different temperatures were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-Raman spectroscopy, and photoluminescence spectroscopy. The nanowires exhibit low defect density. The growth direction of the nanowires is []. For nanowires grown at 800°C, Raman scattering is consistent with the presence of point defects and subsequently yellow luminescence dominating their photoluminescence properties. A low free carrier concentration of less than 10¹⁷ cm^-3 is present in the nanowires. In contrast, for nanowires grown at 900°C, strong phonon-plasmon coupling was evidenced, suggesting a free carrier concentration in excess of the mid-10¹⁸ cm^-3 region. Photoluminescence spectra show strong near-band-edge luminescence and negligible yellow luminescence. A Ni-related luminescence peak was observed at 3.436 eV at 80 K. Raman and photoluminescence results obtained from individual nanowires demonstrate that the nanowire crystalline quality improves not only with increasing growth temperature, but also along the nanowire growth direction.