Hydrogen incorporation into amorphous indium gallium zinc oxide thin-film transistors

Abstract

An experimental study is conducted in which the subgap trap density of states (DoS) is measured by ultrabroadband photoconduction (UBPC) to examine hydrogen incorporation into the channel layer of top-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). UBPC reveals that hydrogen incorporation leads to the creation of a spectrally broad (∼0.3 eV FWHM) distribution of electronic states in the bandgap centered at 0.4 eV above the valence band mobility edge and to an increase in valence band tail state density. Concomitantly, drain current–gate voltage transfer curves demonstrate that hydrogen incorporation results in a negative shift in the turn-on voltage. Quantitatively, electronic state densities estimated by UBPC and the turn-on voltage shift are identical. These experimental findings imply that hydrogen acts as a donor in a-IGZO, but that its donor ionization energy is extraordinarily large, i.e., ∼2.8 eV, inconsistent with that of a normal donor. It is proposed that this anomalous donor behavior is a consequence of the negative-U property of hydrogen in a-IGZO in which hydrogen ionization precedes its incorporation into the lattice network.