We investigated the threshold pressure necessary for heterogeneous nucleation of GaN using pure nitrogen gas and nitrogen gas mixed with ammonia in the Na-flux method. In our investigation, we found the appropriate pressure region for nucleating GaN and growing GaN seed crystals without additional heterogeneous nucleation. This study revealed that the threshold pressure to nucleate GaN decreases as the ratio of ammonia increases. We also succeeded in reducing the nucleation threshold pressure from 30 to 10 atm by using nitrogen gas with 25-40% ammonia gas instead of pure nitrogen gas. We confirmed that seed growth without additional nucleation is possible in both pure nitrogen and nitrogen gas mixed with ammonia at pressures below those required for nucleation. (C) 2003 Elsevier Science B.V. All rights reserved.