Analysis of a thin AlN interfacial layer in Ti/Al and Pd/Al ohmic contacts to n-type GaN

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 to n-type GaN annealed in Ar at 600 degrees C for 15 and 30 s, respectively. The interfacial layer has a c-plane (002) lattice spacing of 2.48+/-.03 Angstrom and an a-plant (100) spacing matching that of GaN (2.76 Angstrom), 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 degrees C and may be the cause. (C) 1997 American Institute of Physics. [S0003-6951(97)02552-7].