In this paper. the effect of NH3 decomposition rate on the composition cond
itions necessary to grow a single phase GaN film has been studied thermodyn
amically with trimethyl gallium (Ga(CH3)(3)) and ammonia (NH3) as the sourc
e gases and hydrogen (H-2) as the carrier gas for the MOVPE process. The re
sults indicated that the Gas+Nitride two phase region essential for the gro
wth of GaN epitaxy layer becomes narrower with increase of the NH3 decompos
ition rate. This explained the high value of V/III ratio during the practic
al growth process. This analysis also implied that low NH3 decomposition ra
te might possibly facilitate the MOVPE growth of GaN epitaxy layer.