Comprehensive studies of the degradation mechanism in amorphous InGaZnO transistors by the negative bias illumination stress

In this paper an examination is presented of the effect of the gate dielectric materials and bilayer stack on the negative bias illumination stress (NBIS) instability of InGaZnO (IGZO) TFTs. The threshold voltage (Vth) movement by NBIS was greatly dependent on the valence band off-set and the effective vertical electrical field, suggesting that the trapping or injection of photo-generated hole carriers into the gate insulator plays an important role. The process of hole carrier creation by photon irradiation onto the IGZO channel layer is shown by independent experiments to involve the oxygen vacancy defects. The reduction in the oxygen vacancy of IGZO semiconductor by the high pressure O2 annealing resulted in the strong increase in the Vth instability induced by NBIS, indicating that the generation of hole carriers can result from the photo-induced transition mechanism from [VO] to [VO2+].

Instability of IGZO TFTs by the application of negative bias illumination stress and the suggested degradation mechanism. The Vth value of IGZO TFTs is negatively shifted by the application of NBIS. The hole carriers can be created either via (i) the direct band-to-band transition by photo-excitation or (ii) the oxygen vacancy defects as a carrier generation center. Thus, the reduction in the oxygen vacancy concentration is of importance for the photo-bias stability of IGZO TFTs.Figure optionsDownload as PowerPoint slideHighlights
► Application of NBIS on IGZO TFTs causes the negative shift of Vth.
► The hole carrier trapping is responsible for this degradation.
► The oxygen vacancy [VO] defects are acting as the hole creation centers.
► The reduction of [VO] is essential for the NBIS stability of IGZO TFTs.