Elemental compositions of GaN surfaces treated by N-2 and BCl3/Cl-2 plasmas have been investigated via AES and XPS analyses. The AES results indicate a low N/Ga ratio of 0.1 for N-2 plasma-treated GaN surface and a relatively high N/Ga ratio of 0.5 for BCl3/Cl-2 plasma-treated GaN surface. Although the N/Ga ratio has often been used as an indicator for N-vacancies (V-N) formation, ohmic contacts fabricated on plasma treated n-GaN surfaces with a lower N/Ga ratio do not necessarily have a correspondingly lower specific contact resistance (rho(c)). Ohmic contacts on N-2 plasma treated n-GaN (Sisimilar to2.6 x 10(18) cm(-3)) yield a higher rho(c) of 6.7 x 10(-5) Omega cm(2), despite a lower N/Ga ratio, than similar samples with BCl3/Cl-2 plasma treatment, which is 6.1 x 10(-6) Omega cm(2). XPS analysis of the Ga 3d peak shows a high degree of oxidation on N-2 plasma treated GaN. Hence, its low N/Ga ratio is not indicative of V-N abundance at the GaN surface but a high concentration of Ga in the form of an oxide (GaxOy), thus translating to a higher rho(c). (C) 2004 Elsevier B.V. All rights reserved.