Mechanism Analysis of Ultralow Leakage and Abnormal Instability in InGaZnO Thin-Film Transistor Toward DRAM

In this article, mechanisms of extremely low OFF-state current and abnormal negative bias stress (NBS) are systematically investigated by the varying process that contains various gas ratios and gas flows during In–Ga–Zn–O (IGZO) sputtering, and different annealing conditions. One model is proposed to indicate the level of ultralow leakage in IGZO thin-film transistor (TFT) dominated by the trap-limited conduction (TLC) along with different internal chemical states of the film, which is highly consistent with the results acquired by X-ray photoelectron spectroscopy (XPS) data. Some special leakage behaviors of devices with low oxygen-deficient defects or with high oxygen-related states are well matched the present model. Moreover, all devices show an abnormal positive threshold voltage shift ( $\Delta {V}_{\text {TH}}$ ) under NBS, which are also discussed using the similar mechanism. Following the proposed optimization strategy based on mechanism analysis, the IGZO TFT exhibits $ > 100\times $ reduction in leakage, $0.55\times $ decrease in subthreshold (SS), and $1.24\times $ increase in mobility compared with the pristine device, achieving an ultralow leakage of <10 ⁻¹⁶ A/ $\mu \text{m}$ , promising mobility of 20.2 cm ² /V·s, and good NBS stability with $\Delta {V}_{\text {TH}} < {0.38}$ V, simultaneously. Those results provide useful guidance to design IGZO-based high-density dynamic random access memory (DRAM) with long retention time, fast access time, and excellent stability under a low thermal budget.