A spatial separated source delivery (SSSD) method has been proposed to alleviate parasitic reactions between group III precursors and ammonia (NH3) in gas phase using the MOCVD. AIGaN alloys and GaN films grown on sapphire has been prepared with this method. The influence of parasitic reactions on the qualities and properties of AIGaN and GaN films is investigated in detail. High-resolution X-ray diffraction (HRXRD) measurements show that the aluminum composition in materials rises up close to the ratio of trimethylaluminum (TMAI) supply when the supply ratio of TMAI is 0.2, 0.3 and 0.4 respectively, which demonstrates that the parasitic reactions are suppressed mostly. In-situ monitor reflectometer spectra for GaN show that the growth rate gets a 20% increase in comparison with materials grown without the SSSD method. Subsequently, the relationship between parasitic reactions and other reaction types are elucidated. The effect of the SSSD method on parasitic reactions is also systematically studied. Reactions are controlled to change from combination reactions to pyrolysis reactions and surface reactions with the SSSD method. A 2-inch GaN/AIGaN heterostructure is also prepared to evaluate the uniform electrical properties grown with this method. The result indicates that the SSSD method provides a better way to prepare high-quality III-nitride materials, especially for AIN and related alloys. (C) 2017 Elsevier B.V. All rights reserved.