GaN epitaxial layers were grown by metalorganic chemical vapor deposition with preheated ammonia as a group V source. The growth rates of GaN epilayers were little affected when ammonia preheater temperature was varied from room temperature to 1000 degreesC, however, their electrical properties as well as source materials utilization efficiency was significantly improved by using preheated ammonia. GaN epilayers grown with preheated ammonia showed excellent structural and optical properties. (C) 2002 Elsevier Science B.V. All rights reserved.