Non-polar (10 (1) over bar0) ZnO thin films were epitaxially grown on m-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The homobuffer thickness effect on the conduction type of undoped ZnO thin films is carefully investigated. With a relatively thicker buffer layer, weak p-type conductivity with a hole concentration of 1.6 x 10(16) cm(-3), a Hall mobility of 0.33 cm(2) V-1 s(-1), and a resistivity of 1.2 x 10(3) Omega cm are achieved for the film. By careful analysis of results from low temperature photoluminescence and transmission electron microscopy measurements, a correlation of the 3.32-eV emission to the p-type conductivity in the undoped non-polar ZnO films is revealed and discussed. The results are important to help deepen understanding of the origin of p-type behavior in ZnO-based materials. (C) 2014 Elsevier B.V. All rights reserved.