CMOS-Compatible Au-Free Ti/Al/Ni/TiC Ohmic Contact for Undoped AlGaN/GaN HEMTs

We propose a novel Au-free Ti/Al/Ni/TiC ( 1.68 × 10 − 5 Ω ⋅ cm 2 , 0.94 Ω ⋅ mm ) metal stack as Ohmic contact electrodes for AlGaN/GaN high electron mobility transistors (HEMTs), demonstrating performance comparable to conventional Ti/Al/Ni/Au ( 2.73 × 10 − 5 Ω ⋅ cm 2 , 1.12 Ω ⋅ mm ) scheme. Transmission electron microscopy results reveal that Ohmic contact formation in the Ti/Al/Ni/TiC stack primarily depends on a solid-phase reaction between metals and the heterostructure, forming a TiN layer at the AlGaN interface without a direct connection to the two-dimensional electron gas (2DEG). Temperature-dependent electrical measurements confirm that carrier transport in this stack is dominated by thermionic-field emission. Due to the absence of a metal-2DEG direct connection, the contact resistance of the proposed Ti/Al/Ni/TiC Ohmic contacts remains stable even at 225 °C. Based on this property, HEMTs with Ti/Al/Ni/TiC electrodes exhibit significantly lower on-resistance degradation at elevated temperatures compared to those with Ti/Al/Ni/Au electrodes. Additionally, the proposed Ti/Al/Ni/TiC electrodes show excellent surface morphology, with a root-mean-square roughness of 4.363 nm, significantly lower than that of Ti/Al/Ni/Au (122.9 nm). Scanning electron microscopy and electron dispersive X-ray spectroscopy results indicate that the TiC cap layer effectively suppresses the upward diffusion of the underlying metals during annealing, without forming island-like or groove-like morphology.