The material and optical properties of the silicon rich oxide (SRO) films prepared by PECVD and sputtering were comprehensively studied. The optical properties were found strongly depend on the material properties of the SRO films. The photocurrent of the photodiodes using PECVD deposited SRO was similar to 3 order higher than that prepared by the sputtering, while the leakage current of the later was much lower than the former, leading the light/dark current ratio of the sputtered photodiodes was higher than the PECVD prepared ones when the bias voltage exceeded similar to 1.6 V. The transmission electron microscopy and the X-ray photoelectron spectroscopy were used to investigate the variations of the constitutive Si nanocrystals and the oxidation states. After annealing, size of the nanocrystals decreased and the oxidation states changed, changing the optical properties of the photodiodes. (C) 2011 Elsevier B.V. All rights reserved.