Effect of N2O plasma treatment on the improvement of instability under light illumination for InGaZnO thin-film transistors

This paper investigates the impact of N2O plasma treatment on the light-induced instability of InGaZnO thin film transistors with a SiO2 passivation layer deposited by plasma-enhanced-chemical-vapor-deposition (PECVD). For the untreated device, because the deposition of the SiO2 passivation layer by PECVD causes extra trap states, the anomalous subthreshold leakage current can be attributed to a lowering of the source side barrier due to trap-assisted photogenerated holes. In contrast, the N2O plasma treatment applied to both the gate insulator and the active layer effectively suppresses the device instability under illumination. In order to clarify the influence of the N2O plasma treatment, this study investigates a device with treatment of only the gate insulator. This device shows a slight decrease of light-induced subthreshold leakage current. This demonstrates that N2O plasma treatment on IGZO active layer after its deposition is critical in preventing damage from the subsequent SiO2 passivation deposition process. In addition, the instability of threshold voltage (VT) under negative bias illumination stress (NBIS) is significantly improved by the N2O plasma treatment. Furthermore, a different dark recovery rate follows NBIS for untreated and N2O plasma-treated devices, indicating different hole-trapping levels exist in the energy band.