Bulk growth by direct synthesis method was carried out. Thick GaN films with smooth surfaces were obtained on metalorganic chemical vapor deposition (MOCVD) GaN films. Surface became worse when bare sapphire was used as a substrate. A buffer layer was introduced and a GaN film with the thickness of about 70 mum was obtained on sapphire in 1-hour growth. The crystallinity of the grown layer on sapphire with buffer is comparable to that on a MOCVD film. (C) 2002 Elsevier Science B.V. All rights reserved.