We examine the effects of the oxygen plasma pre-treatments on the material properties of n-ZnO grown on p-Si and characterize the electrical properties of n-ZnO/p-Si heterojunction diodes. The lattice spacing of ZnO becomes larger when the ZnO thin film is grown on the oxygen plasma pre-treated Si substrate. This might be relevant to the growth of (101) ZnO onto the ultra-thin SiO2 interfacial layer, which is formed during the oxygen plasma pre-treatment onto the Si substrate. The formation of SiO2 gives rise to the increase in the donor-like defect Zn interstitial, and the increased grain size improves the carrier mobility. Because of all the above, the differential conductance at the on-state is increased for the n-ZnO/p-Si heterojunction diode. (C) 2014 Elsevier B. V. All rights reserved.