Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited Y₂O₃ with Ozone

We report the effect of Y₂O₃ passivation by atomic layer deposition (ALD) using various oxidants, such as H₂O, O₂ plasma, and O₃, on In-Ga-Zn-O thin-film transistors (IGZO TFTs). A large negative shift in the threshold voltage (V_th) was observed in the case of the TFT subjected to the H₂O-ALD Y₂O₃ process; this shift was caused by a donor effect of negatively charged chemisorbed H₂O molecules. In addition, degradation of the IGZO TFT device performance after the O₂ plasma-ALD Y₂O₃ process (field-effect mobility (μ) = 8.7 cm²/(V·s), subthreshold swing (SS) = 0.77 V/dec, and V_th = 3.7 V) was observed, which was attributed to plasma damage on the IGZO surface adversely affecting the stability of the TFT under light illumination. In contrast, the O₃-ALD Y₂O₃ process led to enhanced device stability under light illumination (ΔV_th = -1 V after 3 h of illumination) by passivating the subgap defect states in the IGZO surface region. In addition, TFTs with a thicker IGZO film (55 nm, which was the optimum thickness under the current investigation) showed more stable device performance than TFTs with a thinner IGZO film (30 nm) (ΔV_th = -0.4 V after 3 h of light illumination) by triggering the recombination of holes diffusing from the IGZO surface to the insulator-channel interface. Therefore, we envisioned that the O₃-ALD Y₂O₃ passivation layer suggested in this paper can improve the photostability of TFTs under light illumination.