Local Structure Properties of Hydrogenated and Nonhydrogenated Amorphous In–Ga–Zn–O Thin Films Using XAFS and High-Energy XRD

The strong effect of impurity hydrogen (H) on subgap electronic states in amorphous In–Ga–Zn–O thin-film transistors (a-IGZO TFTs) was confirmed using standard (STD) and ultrahigh-vacuum (UHV) sputtering systems with different base pressures of 10^–4 and 10^–7 Pa, respectively. However, comprehensive studies of the atomic-scale structure have yet to be reported. We investigated the correlations between the atomic-scale structure, the electronic state, and the impurity hydrogen content in a-IGZO by high-energy X-ray diffraction (HEXRD) coupled with reverse Monte Carlo (RMC) modeling and X-ray absorption fine structure (XAFS) spectroscopy. XAFS probed the distribution of unoccupied electronic states above the Fermi level and the local coordination structure around the In, Ga, and Zn atoms. A possible contribution for H from voids in the a-IGZO films was observed by the HEXRD and RMC configuration models. In contrast, the STD 3% film has many voids, which are occupied by impurity H. The proportion of lower coordinated M-O (M = Zn, Ga, In) structures is increased because of the H in voids. We revealed that this electronic and atomic-scale structure of the a-IGZO TFTs, which results in enhanced TFT characteristics, can be stabilized by the hydrogen-passivated defects resulting from STD sputtering with an optimum oxygen flow rate ratio of 3%.