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We investigated the electronic states in amorphous In–Ga–Zn–O films with high carrier concentrations by optical absorption and hard x-ray photoelectron spectroscopy (HX-PES). Films having different Hall mobilities were prepared and their annealing effects were examined. All HX-PES spectra showed Fermi edge structures and extra subgap densities of states (DOSs). Tail-like structures observed in the optical spectra originate from subgap DOSs (⪢1020cm−3) near valence band maximas (VBMs). Subgap DOSs near VBMs provide a reason why In–Ga–Zn–O thin film transistors show hard saturation in off states and are difficult to operate in an inversion p-channel mode.
Nomura et al. (Mon,) studied this question.