Achieving 0.05 Ω-mm contact resistance in non-alloyed Ti/Au ohmics to β-Ga₂O₃ by removing surface carbon

Preserving a contamination-free metal-semiconductor interface in β-Ga₂O₃ is critical to achieve consistently low resistance ( < 1 Ω-mm) ohmic contacts. Here, we report a scanning transmission electron microscopy study on the variation in Ti/Au ohmic contact quality to (010) β-Ga₂O₃ in a conventional lift-off vs a metal-first process. We observe a thin ∼1 nm carbon barrier between the Ti and Ga₂O₃ in a non-conductive contact fabricated by a conventional lift-off process, which we attribute to photoresist residue, not previously detected by x-ray photoelectron spectroscopy due to the thinness and patchy coverage of the carbon layer, as well as roughness of the Ga₂O₃ surface. This thin carbon barrier is confirmed by electron energy loss spectroscopy and atomic force microscopy-infrared spectroscopy. We believe that the presence of the thin and patchy carbon layer leads to the highly inconsistent contact behavior in previous reports on non-alloyed contacts. Adventitious carbon is also observed in a conductive ohmic contact metal-first processing on an as-grown sample. We find that a five minute active oxygen descum is sufficient to remove this carbon on as-grown samples, further improving the ohmic behavior and reducing the contact resistance R_c to 0.06 Ω-mm. We also show that an hour long UV-ozone treatment of the Ga₂O₃ surface can eliminate carbon residue from the lift-off processing, resulting in a low R_c of 0.05 Ω-mm.