Using high resolution transmission electron microscopy, a thin pseudomorphic AlN layer (2–3 nm) has been observed at the metal/GaN interfaces of Ti/Al(35/115 nm)and Pd/Al(25/125 nm)ohmic contacts ton-type GaN annealed in Ar at 600 °C for 15 and 30 s, respectively. The interfacial layer has ac-plane (002) lattice spacing of2.48±.03 Åand ana-plane (100) spacing matching that of GaN (2.76 Å), and the identification of this layer as AlN is consistent with chemical analysis by energy dispersive x-ray spectroscopy. The formation of this interfacial AlN layer coincides with the onset of ohmic behavior in Ti/Al and Pd/Al contacts annealed at 600 °C and may be the cause. ©1997 American Institute of Physics.