Nonalloyed transparent Ti/indium tin oxide (ITO) and Ti-only contacts on n-type GaN have been investigated. The ITO/Ti/n-GaN and Ti/n-GaN samples are shown to have very low specific contact resistances of 3.2 x 10(-6) Omega cm(2) and 8.7 x 10(-7) Omega cm(2), respectively. Plasma treatment prior to Ti deposition is a possible cause to render these low contact resistances, in which plasma damage increases defect density and causes nitrogen vacancies near GaN surface. The nitrogen vacancies act as donors, heavily doping the GaN surface and, hence, allowing electron tunneling through the junction of Ti and n-GaN. Plasma treatments were performed by using a sputtering system at a substrate temperature of 25 degrees C in Ar gas and 30 W plasma power. Additionally, surface Auger electron spectroscopy (AES) was investigated to analyze the atom species on the GaN surface before and after plasma treatment. More than 90% optical transmittance is found in our ITO films, thus our ITO/Ti/n-GaN and Ti/n-GaN samples are suitable for applications in optoelectronic devices. (c) 2005 Elsevier Ltd. All rights reserved.