The microstructural and optical properties of a-plane GaN grown by selective area lateral epitaxy have been studied. Lateral overgrowth along the +c and -c directions exhibits very different properties. For +c, the growth is almost free of stacking faults; whereas the opposite growth direction induces a high density of stacking faults bounded by partial dislocations. Cathodoluminescence characterization indicates that the defective region induces a distinct emission at ∼363nm at 10K. In addition, localized emission spectra indicate that materials grown on different facets exhibit distinct luminescence properties, suggesting that impurity incorporation depends on the nature of the growth surface. © 2005 American Institute of Physics.