This study examined the effect of oxygen plasma treatment on light-enhanced bias instability in Zn-Sn-O (ZTO) thin film transistors (TFTs). The treated ZTO TFT exhibited only a threshold voltage (V(th)) shift of -2.05 V under negative bias illumination stress (NBIS) conditions, whereas the pristine device suffered from a negative V(th) shift of 3.76 V under identical conditions. X-ray photoelectron spectroscopic analysis revealed that the oxygen vacancy defect density was diminished via the oxygen plasma treatment. This suggests the V(th) degradation under NBIS is due to photo-transition of oxygen vacancy defects. (C) 2011 American Institute of Physics. [doi:10.1063/1.3634053]