The details of epitaxial growth and microstrictural characteristics of AlN films grown on sapphire (0001), (101¯2) and Si (100), (111) substrates were investigated using plan‐view and cross‐sectional high‐resolution transmission electron microscopy and x‐ray diffraction techniques. The films were grown by metalorganic chemical vapor deposition using TMA1+NH3+N2gas mixtures. Different degrees of epitaxy were observed for the films grown on &agr;‐Al2O3and Si substrates in different orientations. The epitaxial relationship for (0001) sapphire was found to be (0001)AlN∥(0001)sapwith in‐plane orientation relationship of [011¯0]AlN∥[1¯21¯0]sap. This is equivalent to a 30° rotation in the basal (0001) plane. For (101¯2) sapphire substrates, the epitaxial relationship was determined to be (112¯0)AlN∥(101¯2)sapwith the in‐plane alignment of [0001]AlN∥[1¯011]sap. The AlN films on (0001) &agr;‐Al2O3were found to contain inverted domain boundaries anda/3⟨112¯0⟩ threading dislocations with the estimated density of 1010cm−2. The density of planar defects (stacking faults) found in AlN films was considerably higher in the case of (101¯2) compared to (0001) substrates. Films on Si substrates were found to be highly texturedcaxis oriented when grown on (111) Si, andcaxis textured with random in‐plane orientation on (100) Si. The role of thin‐film defects and interfaces on device fabrication is discussed. ©1996 American Institute of Physics.