A model for the growth of aluminum nitride (AlN) films on Si(100) and Si(111) substrates by plasma-assisted molecular beam epitaxy is proposed. AlN thin films were grown in a c-axis preferred orientation irrespective of Si substrates. AlN films grown on Si(100) substrates, having no lattice coherency with AlN, showed a columnar structure and each column was rotated by a uniform angle in a direction lateral to the substrate. AlN films grown on Si(111) having lattice coherency showed an epitaxial relationship for AlN[0001]// Si[111] in a lateral direction to the substrate. Based on a comparison between experimental results and a schematic lattice diagram of AlN/Si, the model for the growth of AlN on Si is proposed. (C) 2002 The Electrochemical Society.