Analysis of crystal orientation in AlN layers grown on m-plane sapphire

• Microstructure of AlN layers grown by MOVPE on m-plane sapphire was analyzed.
• A V/III ratio of 626 results in three different orientations of AlN crystallites.
• Vertical and lateral growth rates of AlN crystallites can be tuned by V/III ratio.
• A high V/III ratio of 1043 results in predominantly semipolar (112̄2) AlN.

Our study reports on the microstructure of AlN layers grown on m-plane sapphire by metal organic vapor phase epitaxy. We have found that AlN can nucleate with three different orientations on the m-plane sapphire surface: semipolar (112̄2) and (11̄03) as well as m-plane (11̄00). 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/III ratio of 626 the vertical growth rate of semipolar (112̄2) AlN regions is much lower than that of the (11̄03) and (11̄00) oriented grains, which results in an almost complete lateral overgrowth of the (112̄2) AlN oriented regions. In contrast, a high V/III ratio of 1043 leads to the formation of uniform semipolar (112̄2) AlN layers. Nevertheless, the formation of differently oriented AlN 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.