Positive-Bias Stress Test on Amorphous In–Ga–Zn–O Thin Film Transistor: Annealing-Temperature Dependence

We performed constant positive gate bias stress tests on sputter-deposited amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) fabricated with various annealing conditions. We found that the time evolution of threshold voltage shift ( $\Delta V _{\rm th}$ ) during the stress test exhibits two different behaviors; a power function type and a logarithmic function type. Combining with thermal desorption spectroscopy (TDS), we found that the $\Delta V _{\rm th}$ type changes from the log-function type to the power-function type as ${{H}} _{2}$ desorption increases. The power-function type is attributed to a H-diffusion limited process. As for the log-function type, the magnitude of $\Delta V _{\rm th}$ has correlation with the hysteresis width in a first transfer curve and the desorption amount of ${{H}} _{2} {{O}}$ and ${{O}} _{2}$ species between 200 $^{\circ}{{C}}$ –300 $^{\circ}{{C}}$ . Hence, it is considered that the traps causing the log-function type $\Delta {\rm V} _{\rm th}$ are meta-stable states related to O- and/or OH-related weak bonds.