Electronic Structure of Oxygen Interstitial Defects in Amorphous In-Ga-Zn-O Semiconductors and Implications for Device Behavior

We investigate the atomic and electronic properties of O interstitial defects in amorphous In-Ga-Zn-O (a-IGZO) through density-functional calculations. We find that an O interstitial forms a dimer with a host O atom in its neutral state, but the O - O dimer bond is easily broken upon electron capture. When bond-breaking relaxations take place, the antibonding defect level of the dimer is significantly lowered from the conduction band toward the valence-band edge, making it energetically more favorable for the dimer to capture two electrons. With the hybrid functional for the exchange-correlation energy, the agreement of the energy barrier for two-electron capture with an experiment is greatly improved. The implication of the results is that O interstitials act as electron traps for the Fermi level close to the conduction-band edge; thus, excess O atoms can be the origin of positive shifts of threshold voltage observed under positive-bias stress in a-IGZO thin-film transistors. On the other hand, under light-illumination or negative-bias stress, which lower the Fermi level to the valence-band edge, the original dimer configuration is recovered by capturing hole carriers without any energy barrier, and the stability of current-voltage characteristics is restored.