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Trap states of a-ZnO thin film transistors

We have investigated the stability of ZnO thin film transistors after application of positive gate bias stress and light illumination. We have observed a positive shift of the transfer curve after positive gate bias stress and a negative shift after light illumination. The recovery characteristics were observed by isochronal annealing after stress. After isochronal annealing, we could categorize the recovery into two types; recovery below 100°C, and that at around 125°C. The low temperature recovery was attributed to the trap charges and is consistent with the absence of variation of the sub-threshold slope with annealing temperature. The high temperature recovery at around 125°C was attributed to the presence of trap states mediated by hydrogen diffusion and is consistent with the observation of a sub-threshold slope increase with increasing annealing temperature. We conducted isochronal annealing analysis on oxide TFTs for the first time, and identified two kinds of stabilities: one is charge trapping and the other is trap states creation which are annealed out at around 125°C.