The effects Of SiCl4 reactive-ion-etching (RIE) plasma treatment on n-GaN and n-Al0.20Ga0.80N surfaces, and the subsequent cleaning of the surfaces using ammonium hydroxide (NH4OH), hydrochloric acid (HCl), and buffered oxide etch (BOE) solutions, have been investigated using x-ray photoelectron spectroscopy and Auger electron spectroscopy measurements. Of these cleaning schemes, BOE was found to be the most effective treatment to remove oxides from the surfaces of the SiCl4 plasma treated samples. The SiCl4 plasma treatment of GaN and AlGaN resulted in the blueshift of Ga-N (Ga3d) peaks to higher binding energies corresponding to a shift of the Fermi level (E-F) toward the conduction band edge at the surface. It has been reported that this type of shift is caused by the creation of N vacancies, which act as n-type dopant [D. W. Jenkins and J. D. Dow, Phys. Rev. B. 39,3317 (1989); M. E. Lin, Z. F. Fan, Z. Ma, L. It. Allen. and H. Morkoc, Appl. Phys. Lett. 64, 887 (1994); A. T. Ping, Q. Chen, J. W. Yang, M. A, Khan, and I. Adesida. J. Electron. Mater. 27, 261 (1998)] on the surface due to SiCl4 plasma treatment. This corresponds to an increase in n-type dopant density on the surface. Thus, SiCl4 plasma treatment in a RIE system thins the Schottky barrier heights of n-GaN and n-AlGaN and aids in the formation of ohmic contacts on such surfaces. (c) 2005 American Vacuum Society.