AlGaN epilayers on GaN/sapphire were successfully grown under various growth conditions using a thermally precracked ion-supplied metalorganic chemical vapor deposition. The At composition in the solid was affected by the gas-phase parasitic reaction between NH3 and trimethylaluminum (TMAl). As the operating pressure decreased, the At composition in the solid increased over the ideal incorporation efficiency. This is due to a scavenging effect and a site-blocking effect. As the TMAl flow rate increased with fixed flow rates of NH3 and trimethylgallium (TMGa), the At concentration in the solid increased but started to saturate. As the TMGa flow rate decreased, the solid Al composition increased linearly, which means different parasitic reactions between TMGa:NH3 and TMAl:NH3. In addition, we found that the separating plate that was inserted to the reactor in front of the heated susceptor to separate ammonia gas flow from MO source input played an important role in the AlGaN growth. Particularly, the separating plate was more attractive under high operating pressure. When it was inserted, a white crystalline solid formed by the adduct (TMAl:NH3) of parasitic reaction in the gas phase disappeared. It also increased the At concentration in the solid. SEM images of AlGaN epilayer's surface showed many small islands due to the lack of surface mobility of adatoms. (C) 2002 Elsevier Science B.V. All rights reserved.