Nitrogen-doped ZnO layers were grown on sapphire substrates by radical source molecular beam epitaxy by simultaneously introducing O-2 and N-2 via a RF radical source. Reflection high-energy electron diffraction and X-ray diffraction measurements revealed that high N-2/O-2 flow ratios induced growth ta ins into the ZnO layer. A nitrogen-doped ZnO fabricated using a N-2/O-2 flow ratio of 10% was found to have a chemical nitrogen concentration of 1 x 10(19) cm(-3). However, type conversion from n-type to p-type did not occur while large nitrogen incorporation was observed to induce extended defects.