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

Naomi Pieczulewski; Kathleen T Smith; Corey M Efaw; Arjan Singh; Cameron A Gorsak; Joshua T Buontempo; Jesse Wensel; Kathy Azizie; Katie Gann; Michael O Thompson; Darrell G Schlom; Farhan Rana; Hari P Nair; Steven M Hues; Elton Graugnard; Paul H Davis; Debdeep Jena; Huili Grace Xing; David A Muller

doi:10.1063/5.0276786

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

APL Mater. 2025 Jun;13(6):061122. doi: 10.1063/5.0276786.

Authors

Naomi Pieczulewski¹, Kathleen T Smith², Corey M Efaw³, Arjan Singh⁴, Cameron A Gorsak¹, Joshua T Buontempo¹, Jesse Wensel⁵, Kathy Azizie¹, Katie Gann¹, Michael O Thompson¹, Darrell G Schlom, Farhan Rana⁴, Hari P Nair¹, Steven M Hues³, Elton Graugnard³, Paul H Davis³, Debdeep Jena, Huili Grace Xing, David A Muller

Affiliations

¹ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA.
² School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
³ Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725, USA.
⁴ School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA.
⁵ Micron Technology, 8000 S Federal Way, Boise, Idaho 83707, USA.

Abstract

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.