Oxide thin-film transistors (TFTs) should be manufactured with high mobility and stability based on a selfaligned top-gate structure to drive high-end displays. In this study, the effect of oxygen plasma time over one cycle of plasma-enhanced atomic layer deposition (PEALD) SiO2 on the properties of top-gate oxide TFTs was investigated systemically. The subsurface reaction of oxygen plasma causes a difference in oxygen vacancy (Vo). In addition, hydrogen incorporation also differs according to plasma time. Considering Vo and hydrogen are donors, tendency of electric properties could be explained. These surface reactions and atomic incorporation also induce differences in the positive bias temperature stress (PBTS) stability. Based on oxygen plasma time of 2.0 s, a positive shift in threshold voltage (Vth) due to interfacial degradation was observed when the plasma was longer, while an abnormal negative shift due to H+ drift was observed when it was shorter. When the oxygen plasma time is 2.0 s, the TFT was free from the deterioration of the interface and SiO2. Based on this condition, a self-aligned TFT with superior performance including a high mobility of 31.1 cm2/V s, positive Vth and high stability of 0.016 V shifting during the PBTS was fabricated successfully. (c) 2021 Published by Elsevier B.V.