AlN nucleation layers are being investigated for growth of GaN on Si. The microstructures of high-temperature AlN nucleation layers on Si (111) substrates grown by metalorganic chemical vapor deposition with trimethylaluminum (TMA) pretreatments have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM results show that with TMA pretreatment, AlN nucleates under a pseudo-two dimensional growth mode due to the enhanced lateral growth rate and improved wetting property of AlN on silicon. In addition, with TMA pretreatment, absence of amorphous SiNx layer at the interface demonstrates good crystalline quality. Transmission electron diffraction patterns reveal epitaxial crystallographic orientation: AlN[0001]parallel toSi[111] and AlN[11 (2) over bar0]parallel toSi[110]. High-resolution TEM measurements also indicate a 5:4 lattice matching relationship for AlN and Si along the Si[110] direction. (C) 2004 Elsevier B.V. All rights reserved.