The origin of surface hillocks (also known as pancake defects) on GaN-on-Si wafers grown by MOVPE has been investigated. FIB/TEM observations confirmed that the appearance of the hillocks is due to the formation of Ga-rich precipitates within the AlGaN buffer layer. XRD (002) FWHM measurements also show that the surface hillocks are associated with a high degree of crystal tilt in the AIN nucleation layer. Two factors are considered to be the cause of such a phase separation: (1) a high density of surface steps associated with the regions of large crystal tilt which act as nucleation centers and (2) a lower mobility of Al adatoms at the growth surface compared with Ga, leading to a preferential incorporation of Ga in the precipitates. The impact of these precipitates on the wafer bow of the structures is considered. (C) 2015 Elsevier B.V. All rights reserved.