Zr/Al and Zr/Al/Mo/Au Ohmic Contacts to AlN-Rich AlGaN

We investigated Zr-based contacts to n-Al_xGa_1-xN with x = 0.66–0.83 using the circular transfer length method. Preparing Zr/Al stacks, we found that Al-rich contacts (with a 1:4 atomic ratio of Zr:Al) became ohmic on Al_xGa_1-xN with x = 0.67 after annealing at 750–800 °C, while Zr-rich contacts did not become ohmic even after annealing as high as 1150 °C, highlighting the importance of Al in the contacts to Al_xGa_1-xN with high x. The thickness of Au in the capping layer in Zr/Al/Mo/Au contacts also played a role in both the contact morphology and the required temperature to form ohmic contacts Al_xGa_1-xN with x = 0.66–0.67. An annealing temperature of 700 °C was optimal for the Au-rich contact, while a higher temperature of 950 °C was needed when the Au cap was thin, despite both contacts having nearly the same specific contact resistance once they became ohmic (3-4 × 10^-4 Ω -cm²). Based on an examination of the contact morphology and relevant phase diagrams, we hypothesize that contacts that undergo partial melting can be prepared using much lower annealing temperatures. The best contact in this study to date was the Zr/Al contact with layer thicknesses of 39/111 nm on Al_xGa_1-xN with x = 0.67, which yielded a specific contact resistance of 1.5 × 10^-4 Ω-cm².