Low-Resistance Nonalloyed Ti/Al Ohmic Contacts on N-Face n-Type GaN via an $\hbox {O}_{2}$ Plasma Treatment

The authors report improved electrical properties of nonalloyed Ti/Al ohmic contact to N-face n-type GaN via oxygen (O₂) plasma treatment. The contact resistivity of Ti (50 nm)/ Al (35 nm) electrodes is reduced significantly from 4.3 × 10^-1 Ω · cm² to 2.53 × 10^-5 Ω · cm² by applying O₂ plasma to the GaN surface before the Ti/Al deposition. In this process, Ti-N bonds are expected to form, while the Ga-N bonds are broken by the O₂ plasma, which eventually increases the nitrogen vacancies as well as the Ti-N phases at the GaN surface. This suggestion has been verified by X-ray photoelectron spectroscopy and transmission electron microscopy analyses.