Ultrabroadband density of states of amorphous In-Ga-Zn-O

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 (h nu > 2.0 eV) 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.