HVPE growth of AlGaN ternary alloy using AlCl3-GaCl-NH3 system is reported. AlGaN alloy was grown directly on sapphire without any buffer layers. It was found that the solid composition could be controlled in all ranges from GaN to AIN by changing the input ratio of the group III precursors under the low partial pressure of H-2 (< 0.1 atm) in the carrier gas. The typical growth rate was around 30 mu m/h at 1100 degrees C. It was found that the solid composition and the growth rate of AlGaN were strongly influenced by the partial pressure of H, in the carrier gas. In addition, results of thermodynamic analysis are described for the purposes of understanding the driving force and the vapor-solid relation for the AlGaN deposition in HVPE. (c) 2007 Elsevier B.V. All rights reserved.