High quality ZnO epilayer with background electron concentration as low as 2.6 x 10(14) cm(-3) was obtained by plasma-assisted MBE on c-sapphire using a thin Mg film as the buffer layer. High-resolution XRD measurement shows a sharp (0 0 2) peak with full width at half maximum (FWHM) of only 0.029 degrees. Photoluminescence spectroscopy presents a weak defect-related near-edge emission. A metal-semiconductor-metal (MSM) typed photodetector based on the material demonstrates a response of similar to 43 A/W under the bias of 1 V and an ON/OFF ratio of 10(4). This un-doped ZnO with ultra-low background electron concentration could be a promising starting material for p-type doping. (c) 2012 Elsevier Ltd. All rights reserved.