An aqueous process based on a unique flow-reactor design was developed for the preparation of gallium-doped ZnO (ZnO:Ga) epitaxial films with a low electrical resistivity. In this process, a ZnO: Ga film was grown on a ZnO-seeded sapphire substrate heated at 80 degrees C under a constant flow of a reaction solution. The Ga content of the resulting films was found to increase in relation to the concentration of GaCl3 used-0 to 9 mM GaCl3-resulting in epitaxial growth of ZnO containing 0-5% Ga, whereas a polycrystalline ZnO film was produced with 10 mM GaCl3. The electrical resistivity of the as-grown ZnO: Ga films varied from 0.2 to 2 Omega cm, but was reduced by two to three orders of magnitude after the films were annealed in air at 300 degrees C. Thus, the lowest resistivity of 7 x 10(-4) Omega cm was obtained with an annealed film containing 2.5% Ga, whose carrier concentration and mobility were 7 x 10(20) cm(-3) and 13 cm(2) V-1 s(-1), respectively. Furthermore, even though the non-doped ZnO film was rendered translucent by annealing, ZnO: Ga films with 1.8-4.0% Ga still exhibited transmittance as high as similar to 80% in the visible spectrum. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.