Density of States in Amorphous In-Ga-Zn-O Thin-Film Transistor under Negative Bias Illumination Stress

Amorphous In-Ga-Zn-O ( a -IGZO) thin-film transistors on glass substrates were fabricated, and the density of state (DOS) in TFTs after a negative bias stress (NBS) and negative bias illumination stress (NBIS) was investigated. For a NBIS of −20 V, the threshold voltage ( V th ) shifted toward the negative direction but an increase of shallow states expected as positively ionized oxygen vacancies (V o + or V o ++ ) were not observed in the DOS curves. On the other hand, for NBIS of −30 V, changes in the DOS near the conduction band edge before and after applying a bias stress were observed. This suggests that V o + or V o ++ was generated by NBIS but accumulation of holes at the a -IGZO/SiO 2 interface mainly caused the V th shifts.