We investigated the relationship between the composition of AlGaN and the gas phase ratio of group-III metalorganic precursors at various TMAl flow rates. The composition of AlGaN significantly deviated from the linear relationship with the gas phase ratio. To elucidate the origin of the deviation, we performed the thermodynamic analysis. The effect of the growth parameters (ammonia decomposition ratio, growth pressure, and growth temperature) on the composition of AlGaN was examined. At 90% decomposition of ammonia, the calculated composition was in accordance with the experimental composition. The positive deviation of AlGaN composition is attributed to high ammonia decomposition ratio, low growth pressure, and high growth temperature. (c) 2006 Elsevier B.V. All rights reserved.