  Under varying growth and device processing conditions, ultrabroadband
photoconduction (UBPC) reveals strongly evolving trends in the defect density
of states (DoS) for amorphous oxide semiconductor thin-film transistors (TFTs).
Spanning the wide bandgap of amorphous InGaZnO$_x$ (a-IGZO), UBPC identifies
seven oxygen-deep donor vacancy peaks that are independently confirmed by
energetically matching to photoluminescence emission peaks. The sub-gap DoS
from 15 different types of a-IGZO TFTs all yield similar DoS, except only
back-channel etch TFTs can have a deep acceptor peak seen at 2.2 eV below the
conduction band mobility edge. This deep acceptor is likely a zinc vacancy,
evidenced by trap density which becomes 5-6x larger when TFT wet-etch methods
are employed. Certain DoS peaks are strongly enhanced for TFTs with active
channel processing damage caused by plasma exposure. While Ar implantation and
He plasma processing damage are similar, Ar plasma yields more disorder showing
a 2x larger valence-band Urbach energy and two orders of magnitude increase in
the deep oxygen vacancy trap density. Changing the growth conditions of a-IGZO
also impacts the DoS, with zinc-rich TFTs showing much poorer electrical
performance compared to 1:1:1 molar ratio a-IGZO TFTs owing to the former
having a ~10xlarger oxygen vacancy trap density. Finally, hydrogen is found to
behave as a donor in amorphous indium tin gallium zinc oxide TFTs.
