The colloidal quantum dots (CQDs) phototransistor is a promising photodetector due to its low cost and substrate compatibility, but a reliable and thin dielectric film with preventive withstanding voltage and smooth surface morphology is required for the channel isolation from the silicon substrates. In this work, plasma enhanced atomic layer deposition (PEALD) has been performed to grow SiO2 on Si at a low temperature of 150 °C. The microstructure, chemical components, electrical properties and optical characterization of PEALD-SiO2 are investigated and analysized. In addition, the CQDs photodetector has been also fabricated on the SiO2/Si substrate with low leakage current. PEALD-SiO2 has a high breakdown electric field of 7 MV/cm and a low surface root-mean-square (RMS) roughness of 0.3 nm even if the thickness is down to 10 nm, indicating PEALD-SiO2 can be a reliable dielectric for CQDs phototransistors.