Our study reports on the microstructure of AIN layers grown on m-plane sapphire by metal organic vapor phase epitaxy. We have found that AIN can nucleate with three different orientations on the m-plane sapphire surface: semipolar (11 (2) over bar2) and (1 (1) over bar 03) as well as m-plane (1 (1) over bar 00). Depending on the growth conditions, i.e. V/III ratio, the differently oriented crystallites exhibit different lateral and vertical growth rates. At a low V/Ill ratio of 626 the vertical growth rate of semipolar (11 (2) over bar2) AIN regions is much lower than that of the (1 (1) over bar 03) and (1 (1) over bar 00) oriented grains, which results in an almost complete lateral overgrowth of the (11 (2) over bar2) AIN oriented regions. In contrast, a high V/III ratio of 1043 leads to the formation of uniform semipolar (11 (2) over bar2) AIN layers. Nevertheless, the formation of differently oriented AIN crystallites could not be suppressed completely. These randomly appearing crystallites still show a high vertical growth rate and lead to a deterioration of the surface morphology. (C) 2014 Elsevier B.V. All rights reserved.