Analyzing the effects of ambient dependence for InGaZnO TFTs under illuminated bias stress

Mechanism of the light-induced instability for InGaZnO thin film transistors has been investigated in this paper. The obvious ambient dependence under light illumination in vacuum and oxygen illustrates that the gas adsorption/desorption dominate the light-induced instability. Under illumination, the photo-generated holes would react with oxygen ions (O2−) and restore into oxygen molecules (O2) resulting in an apparent negative threshold voltage (Vt) shift. In order to verify the oxygen adsorption/desorption mechanism, the dark recovery characteristics were performed in vacuum and oxygen ambient follows 1000 s light illumination and the fast recovery rate in oxygen ambient indicates that the oxygen molecules could be readsorpted. Furthermore, the instability of the gate bias stress under light illumination was investigated. For positive bias illumination stress (PBIS), the competition of the light-induced oxygen desorption and electron trapping mechanisms caused an unusual negative Vt shift comparing with gate bias stress in dark. On the contrary, the apparent Vt shift under negative bias illumination stress (NBIS) in different environments suggests that the light-induced oxygen adsorption will accelerate the degradation phenomenon.