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 ( Dit and Nss , respectively) are Dit = 4.8 × 10 11 cm −2 /eV and Nss = 7.0×10 16 cm −3 /eV for the unannealed TFT, which increased to 5.2×10 11 cm −2 /eV and 9.8×10 16 cm −3 /eV, respectively, by the PB stress test. These are reduced significantly to Dit = 0.82×10 11 cm −2 /eV and Nss = 3.2×10 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.