Enhanced Ti-Based Ohmic Contacts to AlGaN/GaN High Electron Mobility Transistors through SiN_<i>x</i>-Assisted TiN_<i>x</i> Formation

SiNx 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 SiNx in proximity to the metal stacks yields the lowest contact resistance (Rc) of 0.23 ohm·mm (ρc ≈ 1.38 × 10–4 ohm·mm2) after rapid thermal annealing. Cubic TiN, ζ-Ti4N3–x, and η-Ti3N2–x were found to form near the interface between Ti and AlGaN. These findings were confirmed by comparing XRD data, which showed more pronounced TiNx signals for the SiNx-optimized sample. Elemental mapping by energy-dispersive X-ray spectroscopy conducted under transmission electron microscopy revealed more extended TiNx formation and Ti–Al interdiffusion when annealed with a layer of SiNx near the metal stacking. The formation of TiNx 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 SiNx 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.