Low-Resistance Ta/Al/Ni/Au Ohmic Contact and Formation Mechanism on AlN/GaN HEMT

In this article, we systematically investigated the Ta/Al/Ni/Au and Ti/Al/Ni/Au ohmic contacts, and contact formation mechanism on ultrawide bandgap (UWBG) AlN barrier heterostructure without using the source–drain regrowth technique. The excellent ohmic contact performance of the Ta-based scheme was observed. The transmission line model (TLM) results depicted an ultralow contact resistance of $0.08 \Omega \cdot $ mm and a specific contact resistance of $1.06\times 10^{-{7}} \,\, \Omega \cdot $ cm2. Atomic force microscope (AFM) shows that the Ta/Al/Ni/Au sample presents a surface morphology improvement compared with the Ti/Al/Ni/Au sample. Transmission electron microscope (TEM) illustrated that the dominant contact mechanism for ultralow resistance is direct contact through TaxAlyAuz alloy penetration. Meanwhile, the difficulty of contact formation for the Ti-based sample was also discussed through the microstructural analysis. These results demonstrate that the proposed Ta/Al/Ni/Au metal scheme is a high-performance and cost-effective ohmic contact technique well suited for AlN/GaN HEMT fabrication process.