Role of Invisible Oxygen in the Trilayer Laminates of Ultrathin a-IGZO/SiO_x/a-IGZO Films

In this study, ultrathin multilayered films of IGZO/SiO_x/a-IGZO were fabricated via radio frequency (RF) magnetron cosputtering, with the SiO_x layer thickness systematically varied between 1 and 7 nm while maintaining a constant a-IGZO layer thickness. The effect of the SiO_x thickness on the electrical properties of the films was thoroughly investigated. A significant deterioration in electrical performance was observed for SiO_x layers up to 3 nm; however, an improvement was noted as the SiO_x thickness increased to 7 nm. X-ray photoelectron spectroscopy (XPS) analysis revealed that the oxygen structure and chemical composition within the multilayers remained unchanged. However, it confirmed that the ultrathin 2 nm thick SiO_x (x ∼ 1.5) layer exhibited nonstoichiometric configurations. The contribution of Fowler-Nordheim (FN) tunneling was observed in multilayer films with varying thicknesses of SiO_x. The presence of oxygen was found to play a critical role in modulating electron trap states within the SiO_x layer, thereby mitigating the reduction in the charge carrier concentration in the films. By optimizing oxygen flow during deposition, we successfully eliminated the charge carrier drop in a-IGZO_{20 nm}/SiO_{x(2 nm)}/a-IGZO_{10 nm} and a-IGZO_{20 nm}/SiO_{x(3 nm)}/a-IGZO_{10 nm} films. Notably, the ultrathin SiO_x layers in the a-IGZO/SiO_x/a-IGZO films functioned as highly effective carrier suppressor layers, presenting a promising alternative to conventional doping approaches for controlling electrical performance.