Self-assembled nanometer-scale GaN quantum dots were fabricated on 6H-SiC(0001) substrates via the formation of Ga liquid droplets and their subsequent nitridation with a supersonic gas source seeded with NH3 molecules. The entire process was observed and controlled in situ and in real time in a low-energy electron microscope. The microstructure of the quantum dots was studied by high-resolution cross-sectional transmission electron microscopy illustrating the perfectly coherent wurtzite structure of GaN quantum dots with 5 nm base width. Spatially resolved cathodoluminescence spectra yield the characteristic band edge emission near 3.48 eV for larger size GaN dots. © 2002 American Institute of Physics.