| Summary: | The subgap density of states of amorphous indium gallium zinc oxide (a-IGZO) is obtained using the ultrabroadband photoconduction response of thin-film transistors (TFTs). Density-functional theory simulations classify the origin of the measured subgap density of states peaks as a series of donorlike oxygen vacancy states and acceptorlike Zn vacancy states. Donor peaks are found both near the conduction band and deep in the subgap, with peak densities of 10^{17}−10^{18}cm^{−3}eV^{−1}. Two deep acceptorlike peaks lie adjacent to the valance-band Urbach tail region at 2.0–2.5 eV below the conduction-band edge, with peak densities in the range of 10^{18}cm^{−3}eV^{−1}. By applying detailed charge balance, we show that increasing the deep acceptor density strongly shifts the a-IGZO TFT threshold voltage to more positive values. Photoionization (hν>2.0eV) of deep acceptors is one cause of transfer curve hysteresis in a-IGZO TFTs, owing to longer recombination lifetimes as electrons are captured into acceptorlike vacancies.
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