The effects of surface treatment using Cl 2/BCl 3 and Ar inductive coupled plasmas on the Ohmic characteristics of Ti/Al/Ti/Au contacts to n-type Al xGa 1-xN (x=0-0.5) were investigated. Plasma treatment significantly increased the surface conductivity of GaN and Al 0.1Ga 0.9N, leading to improved Ohmic behaviors of the contacts. However, it reduced the surface doping level in Al xGa 1-xN (x≥0.3) and degraded the contact properties. Following a 900-1000 °C anneal, the Ti/Al/Ti/Au contacts to Al xGa 1-xN (x=0-0.3) became truly Ohmic, with specific contact resistances of (3-7) × 10 -5 Ωcm 2, whereas the contact to Al 0.5Ga 0.5N remained rectifying even without the plasma treatment. X-ray photoelectron spectroscopy measurements confirmed that the Fermi level moved toward the conduction band in GaN after the plasma treatment, but it was pinned by plasma-induced deep-level states in Al 0.5Ga 0.5N. This study emphasizes the need to mitigate plasma damage introduced during the mesa etch step for AlGaN-based deep-UV emitters and detectors.