Stability and high-frequency operation of amorphous In-Ga-Zn-O thin-film transistors with various passivation layers

The stability of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated focusing on the effects of passivation layer materials (Y2O3, Al2O3, HfO2, and SiO2) and thermal annealing. Positive bias constant current stress (CCS), negative bias stress without light illumination (NBS), and negative bias light illumination stress (NBLS) were examined. It was found that Y2O3 was the best passivation layer material in this study in terms of all the stability tests if the channel was annealed prior to the passivation formation (post-deposition annealing) and the passivation layer was annealed at >= 250 degrees C (post-fabrication annealing). Post-fabrication thermal annealing of the Y2O3 passivation layer produced very stable TFTs against the CCS and NBS stresses and eliminated subgap photoresponse up to the photon energy of 2.9 eV. Even for NBLS with 2.7 eV photons, the threshold voltage shift is suppressed well to -4.4 V after 3 h of test. These results provide the following information: (i) passivation removes the surface deep subgap defects in a-IGZO and eliminates the subgap photoresponse, but (ii) the bulk defects in a-IGZO should be removed prior to the passivation process. The Y2O3-passivated TFT is not only stable for these stress conditions, but is also compatible with high-frequency operation with the current gain cut-off frequency of 91 kHz, which is consistent with the static characteristics. (C) 2011 Elsevier B.V. All rights reserved.