Formation and preferential orientation of Au-free Al/Ti-based ohmic contacts on different hexagonal nitride-based heterostructures

Wide-bandgap nitride semiconductors are currently in development for high-power electronic applications. Compositional layered heterostructures of such nitrides result in a high polarization field at the interface, enabling a higher electron mobility, a higher power density, and a higher conversion efficiency. Further optimization of such GaN-based high-electron-mobility transistors can be achieved by evolving from a top $AlxGa1−xN$ barrier toward AlN or even $InyAl1−yN$⁠. An ongoing challenge in using such hexagonal nitride semiconductors is the formation of a low-resistive, Au-free, ohmic contact far below $1Ωmm$⁠. In this paper, we investigate the formation of ohmic contacts by Ti–Al–TiN-based metalization as a function of different annealing temperatures (up to $950°C$⁠), Ti–Al ratios (from 15 up to 35 at. %) and nitride barrier composition (⁠$AlxGa1−xN$⁠, GaN, AlN, and $InyAl1−yN$⁠). Contacts processed on Al_xGa_1–x/GaN, and AlN/GaN heterostructures result in low contact resistance of, respectively, 0.30 and $0.55Ωmm$⁠, whereas the same contact stack on $InyAl1−yN$ results in resistance values of $1.7Ωmm$⁠. The observed solid-phase reaction of such Ti–Al–TiN stacks were found to be identical for all investigated barrier compositions (e.g., $AlxGa1−xN$ , GaN, AlN, and $InyAl1−yN$⁠), including the preferential grain alignment to the epitaxial nitride layer. The best performing ohmic contacts are formed when the bottom Ti-layer is totally consumed and when an epitaxially-aligned metal layer is present, either epitaxial Al (for a contact which is relatively Al-rich and annealed to a temperature below $660°C$⁠) or ternary $Ti2AlN$ (for a relatively Ti-rich contact annealed up to $850°C$⁠). The observation that the solid-phase reaction is identical on all investigated nitrides suggests that a further decrease of the contact resistance will be largely dependent on an optimization of the nitride barriers themselves.