Interface and bulk effects for bias-light-illumination instability in amorphous-In-Ga-Zn-O thin-film transistors

Positive-current-bias (PB) instability and negative-bias-light-illumination (NBL) instability in amorphous-In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) have been examined. The channel-thickness dependence indicated that the V-th instability caused by the PB stress is primarily attributed to defects in the bulk a-IGZO region for unannealed TFTs and to those in the channel-gate-insulator interface for wet-annealed TFTs. The interface and bulk defect densities (D-it and N-ss, respectively) are D-it = 4.8x10(11) cm(-2)/eV and N-ss = 7.0x10(16) cm(-3)/eV for the unannealed TFT, which increased to 5.2x10(11) cm(-2)/eV and 9.8x10(16) cm(-3)/eV, respectively, by the PB stress test. These are reduced significantly to D-it = 0.82x10(11) cm(-2)/eV and N-ss = 3.2x10(16) cm(-3)/eV for the wet-annealed TFTs and are unchanged by the PB stress test. It was also found that the photo-response of a-IGZO TFTs begins at 2.3 eV of photon excitation, which corresponds to subgap states observed by photoemission spectroscopy. The origin of the NBL instability for the wet-annealed TFTs is attributed to interface effects and considered to be a trap of holes at the channel-gate-insulator interface where migration of the holes is enhanced by the electric field formed by the negative gate bias.