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.

Amorphous semiconducting transparent oxides like InGaZnO4 (a-IGZO) have a broad distribution of metal and oxygen vacancy defects that determine thin film transistor (TFT) characteristics and impact device reliability metrics such as hysteresis. Here, we demonstrate how hydrogen modifies the density of states (DoS) through a novel on-chip method that spectrally resolves trap concentration in a-IGZO spanning the bandgap. Requiring laser energies continuously tunable from 0:26 to 3:1 eV, this method also employs difference frequency generation to access shallow states near the conduction band. We characterize the effect of hydrogen incorporation on the sub-gap peaks of the DoS of an a-IGZO TFT. Specifically, our data suggests hydrogen hybridizes with vacancy defects through metal-hydrogen (M-H) bonds that passivate oxygen vacancy sites and O-H bonds that passivate metal vacancy sites. These interactions result in a suppression of oxygen vacancy and metal vacancy- related trap states in the sub-gap and an enhancement of a metal-hydrogen bonding peak near the VBM. Temperature dependent, photon energy-dependent hysteresis, and transient defect lifetime measurements further reveal the strong impact of hydrogen concentration on a-IGZO TFT performance germane to current optical display technology.

Peroxide ligation of aqueous metal-oxo clusters provides rich speciation and structural diversity, radiation sensitivity for manipulation with light, and both broadens and shifts pH-range stability. Here we demonstrate peroxide ligation of the polyoxometalate (POM) [Ta6O19]8-. We study in detail solution speciation of the peroxide-substituted cluster, and benchmark it to the peroxide-ligated niobate analogue, [Nb6O10(OH)3(O2)6]5-, whose solid-state structure has been reported. Raman and electrospray ionization mass spectroscopy do not detect any significant differences between the two analogues. However, small and wide-angle and total X-ray scattering strongly indicate that peroxide promotes linking of the hexameric tantalate clusters, rather than terminating and capping the clusters, as observed for the niobate analogue. We used computational studies to identify Raman peak positions, determine the energetics of exchange of oxo-ligands for peroxo-ligands, and provide models to help explain the X-ray scattering data. Understanding the solution speciation of peroxide-substituted polyoxotantalates is an important step towards its use in solution processed thin film materials, as well as developing new Ta-POM chemistry.