Electronic structure of oxygen-vacancy defects in amorphous In-Ga-Zn-O semiconductors

We perform first-principles density functional calculations to investigate the atomic and electronic properties of various O-vacancy (${V}_{\mathrm{O}}$) defects in amorphous indium gallium zinc oxides ($a$-IGZO). The formation energies of ${V}_{\mathrm{O}}$ have a tendency to increase with increasing number of neighboring Ga atoms, whereas they are generally low in the environment surrounded with In atoms. Thus, adding Ga atoms suppresses the formation of O-deficiency defects, which are considered as the origin of device instability in $a$-IGZO-based thin film transistors. The conduction band edge state is characterized by the In $s$ orbital and insensitive to disorder, in good agreement with the experimental finding that increasing the In content enhances the carrier density and mobility. In $a$-IGZO, while most ${V}_{\mathrm{O}}$ defects are deep donors, some of the defects act as shallow donors due to local environments different from those in crystalline oxides. As ionized O vacancies can capture electrons, it is suggested that these defects are responsible for positive shifts of the threshold voltage observed under positive gate bias stress. Under light illumination stress, ${V}_{\mathrm{O}}$ defects can be ionized, becoming ${V}_{\mathrm{O}}^{2+}$ defects due to the negative-$U$ behavior. When electrons are captured by applying a negative bias voltage, ionized ${V}_{\mathrm{O}}^{2+}$ defects return to the original neutral charge state. Through molecular dynamics simulations, we find that the initial neutral state is restored by annealing, in good agreement with experiments, although the annealing temperature depends on the local environment. Our calculations show that ${V}_{\mathrm{O}}$ defects play an important role in the instability of $a$-IGZO-based devices.