The crystalline structure near the substrate interface has been studied for AlGaN films grown on (0001) sapphire substrates by metalorganic chemical vapor deposition, using AlN buffer layers. Transmission electron lattice images show that the sapphire/AlN interface is coherent, with misfit dislocations separated by 2.0 nm, corresponding to relaxed bulk lattice parameters. The interface between the buffer layer and the AlGaN film is discussed. The defect structure of the epilayer near the substrate interface consists mostly of dislocations and stacking faults lying on basal planes. © 1994 American Institute of Physics.