Subgap states in transparent amorphous oxide semiconductor, In-Ga-Zn-O, observed by bulk sensitive x-ray photoelectron spectroscopy

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 (>> 10(20) cm(-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. (C) 2008 American Institute of Physics.