The effect of VI/II ratio on crystalline quality and surface flatness in ZnO films is investigated by molecular beam epitaxy (MBE) with a radio-frequency(RF)-plasma oxygen source. The minimum residual electron concentration of 2.4 x 10(17) cm(-3) has been obtained for the sample grown at stoichiometry. The sample grown under oxygen-enriched condition, however, exhibits an excellent surface flatness. To combine these desirable characteristics at the same time, we use flux-modulation method. In this method, deposition sequence is composed of the repetition of oxygen- and zinc-enriched growth durations. By using this method, we succeeded in growing high-quality ZnO epitaxial films with low-residual electron concentration of 9.7 x 10(16) cm(-3) and improved surface flatness. (c) 2007 Elsevier B.V. All rights reserved.