Dual Effect of Defect-Free AlO x F y Layer for Suppressing Hydrogen Influence in Indium–Gallium–Zinc Oxide Thin-Film Transistors

It has been demonstrated that mixing atomic-layer deposition subcycles of aluminum oxide (Al₂O₃) and aluminum fluoride (AlF₃) at an optimal ratio yields an aluminum oxyfluoride (AlO_xF_y) compound with virtually no internal defects. By systematically analyzing the defect structures formed in AlO_xF_y thin films as a function of the Al₂O₃ to AlF₃ subcycle ratio, the optimal composition was identified. Because hydrogen migrates either through intrinsic defects in the film or via repeated bonding and dissociation with oxygen, a defect-free AlO_xF_y film in which AlF₃ sublayers providing no hydrogen-diffusion pathways are periodically repeated represents an exceptionally effective hydrogen barrier. The barrier performance of AlO_xF_y was experimentally validated for indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs). During thermal annealing, the fluorine (F) from the AlO_xF_y film diffuses into the IGZO layer, further enhancing its resistance to hydrogen-related degradation. Even in IGZO TFTs incorporating a silicon nitride dielectric with a high hydrogen content, the introduction of an AlO_xF_y barrier effectively suppresses hydrogen-induced threshold voltage (V_th) shifts. In addition, the F doping induced by the AlO_xF_y layer conferred extra stability, maintaining minimal V_th variation even under prolonged positive-bias stress and negative-bias stress. This work identified a defect-free AlO_xF_y thin film as a highly effective hydrogen diffusion barrier and demonstrated its capability to significantly improve the hydrogen stability of IGZO TFTs. The enhancement in hydrogen tolerance is attributed not only to the superior barrier properties of AlO_xF_y but also to the beneficial F-doping effect imparted to the IGZO channel layer.