Enhanced Ti-Based Ohmic Contacts to AlGaN/GaN High Electron Mobility Transistors through SiN_x-Assisted TiN_x Formation

SiN x is found to contribute to optimizing the ohmicity of Ti/Al/Ti/Au metal contacts for AlGaN/GaN high electron mobility transistors (HEMTs). The presence of a nominally 20 nm-thick SiN x in proximity to the metal stacks yields the lowest contact resistance ( R c ) of 0.23 ohm·mm (ρ c ≈ 1.38 × 10 –4 ohm·mm 2 ) after rapid thermal annealing. Cubic TiN, ζ-Ti 4 N 3– x, and η-Ti 3 N 2– x were found to form near the interface between Ti and AlGaN. These findings were confirmed by comparing XRD data, which showed more pronounced TiN x signals for the SiN x -optimized sample. Elemental mapping by energy-dispersive X-ray spectroscopy conducted under transmission electron microscopy revealed more extended TiN x formation and Ti–Al interdiffusion when annealed with a layer of SiN x near the metal stacking. The formation of TiN x is believed to have altered electronic band structures that exhibit higher density of states near the Fermi energy and lighter effective mass of the charge carriers as judged by the dispersive nature of conduction electrons reported in the literature. The results establish the use of SiN x as a viable enhancer of ohmic behaviors for GaN-based HEMT devices, which have extensive applications in high-voltage power electronics and RF communication devices.