Annealing-Free Ohmic Contacts to n-Type GaN via Hydrogen Plasma-Assisted Atomic Layer Deposition of Sub-Nanometer AlO_x

A plasma-assisted atomic layer deposition (PE-ALD) process is reported for creating ohmic contacts to n-type GaN that combines native oxide reduction, near-surface doping, and encapsulation of GaN in a single processing step, thereby eliminating the need for both wet chemical etching of the native oxide before metallization and thermal annealing after contact formation. Repeated ALD cycling of trimethyl aluminum (TMA) and high-intensity hydrogen (H₂) plasma results in the deposition of a sub-nanometer-thin (≈8 Å) AlO_x layer via the partial transformation of the GaN surface oxide into AlO_x. Hydrogen plasma-induced nitrogen vacancies in the near-surface region of GaN serve as shallow donors, promoting efficient out-of-plane electrical transport. Subsequent metallization with a Ti/Al/Ti/Au stack results in low contact resistance, ohmic behavior, and smooth morphology without requiring annealing. This electrical contracting approach thus meets the thermal budget requirements for Si-based complementary metal–oxide–semiconductor structures and can facilitate the design and fabrication of advanced GaN-on-Si heterodevices.