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-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-th type changes fromthe log-function type to the power-function type as H 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-th has correlation with the hysteresis width in a first transfer curve and the desorption amount of H2O and O-2 species between 200 degrees C-300 degrees C. Hence, it is considered that the traps causing the log-function type Delta V-th are meta-stable states related to O- and/or OH-related weak bonds.