ZnO thin films were grown on r-plane sapphire substrates using electron cyclotron resonance (ECR) plasma-assisted molecular-beam epitaxy. The effect of the oxygen ECR plasma power on the growth rate, structural, electrical, and optical properties of the ZnO thin films were studied. It was found that larger ECR power leads to higher growth rate, better crystallinity, lower electron carrier concentration, larger resistivity, and smaller density of non-radiative luminescence centers in the ZnO thin films. (C) 2008 Elsevier B. V. All rights reserved.