Defect structures were investigated by transmission electron microscopy for AlN/sapphire (0001) epilayers grown by high temperature hydride vapor phase epitaxy using a growth mode modification process. The defect structures, including threading dislocations, inversion domains, and voids, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. AlN film growth was initiated at 1450 degrees C with high V/III ratio for 8 min. This was followed by low V/III ratio growth for 12 min. The near-interfacial region shows a high density of threading dislocations and inversion domains. Most of these dislocations have Burgers vector b = 1/3 < 1120 > and were reduced with the formation of dislocation loops. In the middle range 400 nm < h < 2 mu m, dislocations gradually aggregated and reduced to similar to 10(9) cm (2). The inversion domains have a shuttle-like shape with staggered boundaries that deviate by similar to +/- 5 degrees from the c axis. Above 2 mu m thickness, the film consists of isolated threading dislocations with a total density of 8 x 10(8) cm (2). Most of threading dislocations are either pure edge or mixed dislocations. The threading dislocation reduction in these films is associated with dislocation loops formation and dislocation aggregation-interaction during island growth with high V/III ratio. (C) 2017 Elsevier B.V. All rights reserved.