Impurity incorporation is studied as a function of metalorganic vapor phase epitaxy growth conditions. The same GaN growth conditions were used initially, resulting in films with approximately the same dislocation density, after which a single growth parameter was varied and the impurity concentrations measured using SIMS. The C concentrations were found to decrease with increasing growth temperature, pressure, and ammonia flow, and to increase with increasing H-2 carrier and trimethylgallium flow. The Si concentrations for both unintentionally doped (UID) and intentionally doped (ID) films increased with increasing growth pressure. The UID and ID Si concentrations varied inversely with the GaN growth rate, suggesting an independent source for UID Si within the reactor. Moreover, the NH3 flow rate influenced the Si-doping concentration, even though the GaN growth rate remained constant. A H-2/NH3 etching mechanism is proposed to explain the growth parameter influence on the observed C and Si concentrations. The reduction in the ID Si concentrations at high NH3 flows is explained by NH3 site blocking, similar to that proposed for increased Ga vacancies at high NH3 flows. (C) 2002 Elsevier Science B.V. All rights reserved.