The effects of surface treatment using Cl2/BCl3 and Ar inductive coupled plasmas on the Ohmic characteristics of Ti/Al/Ti/Au contacts to n-type AlxGa1-xN (x=0–0.5) were investigated. Plasma treatment significantly increased the surface conductivity of GaN and Al0.1Ga0.9N, leading to improved Ohmic behaviors of the contacts. However, it reduced the surface doping level in AlxGa1-xN (x≥0.3) and degraded the contact properties. Following a 900–1000 °C anneal, the Ti/Al/Ti/Au contacts to AlxGa1-xN (x=0–0.3) became truly Ohmic, with specific contact resistances of (3–7)×10-5 Ω cm2, whereas the contact to Al0.5Ga0.5N 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 Al0.5Ga0.5N. This study emphasizes the need to mitigate plasma damage introduced during the mesa etch step for AlGa- - N-based deep-UV emitters and detectors.