High-electron-mobility ZnO epilayers are gown on c-plane sapphire with ZnO/MgO double-buffer layers by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction and transmission electron microscopy analysis showed the growth mode of ZnO buffer layers (LT-ZnO) grown at low temperature significantly affected the structural properties of the ZnO epilayers grown at high temperature. thereby affecting the electrical properties of the epilayers. When LT-ZnO was gown Lit a high-growth-rate. three-dimensional growth dominated and threading dislocation (TD) density was as high as ca. 1 x 10(10) cm(-2). By using the low growth rate of LT-ZnO, two-dimensional growth dominated and TD density was reduced by one order of magnitude. down to ca. 2 x 10(9) cm(-2), yielding significantly improved electrical properties of the ZnO epilayers. The highest electron mobility in as-grown undoped ZnO film, 145 cm(2) V-1 s(-1). was achieved at room temperature. comparable to the mobility previously reported for high-quality bulk ZnO. (C) 2004 Elsevier B.V. All rights reserved.