Vertically oriented gallium nitride (GaN) nanocolumns (NCs) approximately 90 + 10 nm wide and 0.75 nm tall were grown by plasma-assisted molecular beam epitaxy on Al2O3 (0001) and Si(111). The dense packing of the NCs gives them the appearance of a continuous film in surface view, but cross-sectional analysis shows them to be isolated nanostructures. Low-temperature photoluminescence measurements of NCs show excitonic emission with a dominant, narrow peak centered at 3.472 eV and FWHM of 1.26 meV. This peak is identified as the ground state of the A free exciton as confirmed by reflection measurements. Cross-sectional transmission electron microscopy identifies the NC microstructure as wurtzite GaN and that the NCs are largely free of defects. The GaN NCs are subsequently utilized as a defect-free vehicle for optical studies of Si-doped GaN; and the donor state was identified through low-temperature photoluminescence experiments. (c) 2005 Elsevier B.V. All rights reserved.