Preheated ammonia was introduced during GaN growth on Si by compound-source molecular-beam epitaxy (CS-MBE) at a low temperature. The use of preheated ammonia increased the surface smoothness and the intensity of cathodoluminescence (CL). Although the introduction of ammonia leads to the limited migration of the Ga surface atoms during the growth, the thermal energy supplied by the preheated ammonia assisted the surface migration. Furthermore, it was demonstrated that the introduction of preheated ammonia during the growth was effective for improving the CL intensity of the GaN layers at low temperatures. This is due to the reduction of the hydrogen concentration in the layers. (c) 2007 Elsevier B.V. All rights reserved.