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 $\text{(35/115\hspace{0.17em}}\mathrm{nm})$ and Pd/Al $\text{(25/125\hspace{0.17em}}\mathrm{nm})$ ohmic contacts to $n$ -type GaN annealed in Ar at 600 °C for 15 and 30 s, respectively. The interfacial layer has a $c$ -plane (002) lattice spacing of $\text{2.48±.03\hspace{0.17em}Å}$ and an $a$ -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.