AlN nucleation layers are being investigated for growth of GaN on Si. The microstructures of high-temperature AlN nucleation layers grown by MOCVD on Si (111) substrates with trimethylaluminium pre-treatments have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM results show that with TMA pre-treatments, AlN grows in a pseudo-2-dimensional mode because the lateral growth rate of AlN is increased, and the wetting property of the AlN on silicon is improved. Also, no amorphous SiNx layer was observed at the interface with TMA pre-treatments and AlN films with good epitaxial crystalline quality were obtained. Transmission electron diffraction patterns revealed that the AlN and Si have the crystallographic orientation relationship AlN [0001]â Si[111] and AlN[11 2 0] â Si[110]. High resolution transmission electron microscopy indicates a 5:4 lattice matching relationship for AlN and Si along the Si [110] direction. Based on this observation, a lattice matching model is proposed.Singapore-MIT Alliance (SMA)