Employing a GaAs substrate, GaN nanowires encapsulated in carbon nanotubes are synthesized by microwave-plasma-enhanced chemical vapor deposition. Almost 100% of the carbon nanotubes are filled with GaN. Both high-resolution transmission electron microscopy and selected area electron diffraction pattern reveal high crystallization in the GaN nanowires. The formation mechanism of the GaN-core/C-shell structure is discussed, emphasizing chemistry's influence on the structure produced, particularly the role of Ga from the substrate. Photoluminescence measurements reveal an ultraviolet band located at 3.35 eV and a yellow band located at 2.20 eV. The redshift of the ultraviolet band is attributed to N vacancies that result from the Ga-rich conditions of growth. (C) 2003 Elsevier B.V. All rights reserved.