We fabricated n-ZnO/p-Si photodiodes by ultra-high vacuum sputtering that were highly responsive to violet-green light illumination at low operating voltages. Under illumination with light at 420 nm, the maximum photoresponsivity of the photodiodes was about 0.55 A/W at -8 V. As the ZnO film thickness increased from 150 nm to 500 nm, the photoresponsivity of the diodes to the wavelength range of 370-560 nm increased markedly, while the absorption bandwidth decreased. In particular, a slow photo-response was observed under light between the wavelengths of 540-560 nm. We speculate that the high photoresponsivity and slow photoresponse of the synthesized diodes are due to shallow and deep acceptor states originating from zinc vacancy defects in the ZnO thin film, respectively. (C) 2013 Elsevier B.V. All rights reserved.