  We find that O-vacancy (Vo) acts as a hole trap and play a role in negative
bias illumination stress instability in amorphous In-Ga-Zn-O thin film
transistors. Photo-excited holes drifted toward the channel/dielectric
interface due to small potential barriers and can be captured by Vo in the
dielectrics. While Vo(+2) defects are very stable at room temperature, their
original deep states are recovered via electron capture upon annealing. We also
find that Vo(+2) can diffuse in amorphous phase, including hole accumulation
near the interface under negative gate bias.
