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 (ΔV
_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 ΔV
_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 ΔV
_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
^°
C-300
^°
C. Hence, it is considered that the traps causing the log-function type ΔV
_th
are meta-stable states related to O- and/or OH-related weak bonds.