Several N-polar GaN/AlGaN/GaN heterostructures intended for high electron mobility transistor (HEMT) devices were grown by molecular beam epitaxy using either SiC or freestanding GaN substrates. The microstructure of these heterostructures has been compared using transmission electron microscopy and associated analytical techniques. Considerable 1:1 ordering was observed within the AlxGa1-xN layers (where x similar to 0.34-0.37) for both substrate types. Threading dislocations were the most common defects observed in structures grown on SiC substrates, whereas interfacial misfit dislocations and surface pits were the primary defects observed in samples grown on the freestanding GaN substrates. (C) 2012 Elsevier B.V. All rights reserved.