This study focus on the nature of different Si-based sensitizers for Er3+ ions in Silicon- Rich Silicon oxide thin films. The samples were first analyzed by Cathodoluminescence technique to probe all emitting centers in the films. Some of these centers were found to be potential sensitizers for Er3+ ions, such as Silicon Oxygen Deficient Center and Non-Bridging Oxygen Hole Center, in addition to the well-known Silicon-nanoclusters (Si-nc). The influence of the thickness was subsequently examined, revealing that the formation of Si-nc is inhibited for films thinner than 100 nm and this led to less sensitization of the Er3+ ions. We demonstrate that the introduction of a SiO2 buffer layer can overcome this issue and increase the luminescence of Er3+ ions by a factor of five for films thinner than 50 nm that are usually used for electrically-driven photonic devices.