Heterojunction diodes of n-type ZnO were fabricated on a p-type Si(100) substrate using an ultra-high vacuum radio frequency magnetron sputtering method at room temperature. A short-time post-annealing process was performed to prevent inter-diffusion of Zn, dopants, and Si atoms. The post-annealing process at 600 degrees C enhanced the crystallinity of ZnO films and produced a high forward to reverse current ratio of the heterojunction diode with a barrier height of approximately 0.336 eV. A thin SiOx layer at the interface of the ZnO film and Si substrate appeared distinctly at the 600 degrees C annealing, however the post-annealing at 700 degrees C showed an a-(Zn2xSi1-xO2) structure caused by diffusion of silicon into the ZnO film. In the n-ZnO/p-Si sample annealed at 700 degrees C, a rapid change in the barrier height was considered due to the effect of the dopant segregation from the substrate and deformation of the a-SiOx structure. (C) 2012 Elsevier B.V. All rights reserved.