The initial stage of AlN film growth on 6H‐SiC(0001) substrates by plasma‐assisted, gas source molecular beam epitaxy (PAGSMBE) has been investigated in terms of growth mode and interface defects. Cross‐sectional high resolution transmission electron microscopy (HRTEM) was used to observe the microstructure of the deposited films and the AlN/SiC interfaces. Surface morphologies and interface atomic structures were compared between films grown on vicinal and on‐axis surfaces. Essentially atomically flat AlN surfaces were obtained using on‐axis substrates. This is indicative of two‐dimensional growth to a thickness of ∼15 A˚. Islandlike features were observed on the vicinal surface. The coalescence of these features at steps gave rise to double positioning boundaries (DPBs) as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of DPBs formed at the outset of growth. ©1995 American Institute of Physics.