The electrical and optical properties of undoped ZnO films deposited by electron cyclotron resonance (ECR) plasma sputtering at room temperature were characterized. The lowest resistivity we achieved was 2.6 x 10(-3) Omega cm with optical transmittance at visible wavelengths higher than 85%. The X-ray diffraction (002) peak was weak and the rocking curve was asymmetrical, indicating that oxygen vacancies prevented large crystalline domains from forming. At low argon-sputtering-gas pressure, carrier concentration and Hall mobility increased with increasing argon pressure. When the optimum pressure (40 mPa) was exceeded, however, Hall mobility and optical transmittance were severely reduced, which indicated that excess Zn atoms were populated at the interstitials of the network. Admitting only 0.67 mPa of O(2) gas during deposition deteriorated resistivity over 1 M Omega cm due to high excitation efficiency in the ECR plasma. Deposition under a higher magnetic field produced lower resistivities. (C) 2009 Elsevier B.V. All rights reserved.