  The effects of hole injection in amorphous-IGZO is analyzed by means of
first-principles calculations. The injection of holes in the valence band tail
states leads to their capture as a polaron, with high self-trapping energies
(from 0.44 to 1.15 eV). Once formed, they mediate the formation of peroxides
and remain localized close to the hole injection source due to the presence of
a large diffusion energy barrier (of at least 0.6eV). Their diffusion mechanism
can be mediated by the presence of hydrogen. The capture of these holes is
correlated with the low off-current observed for a-IGZO transistors, as well
as, with the difficulty to obtain a p-type conductivity. The results further
support the formation of peroxides as being the root cause of Negative bias
illumination stress (NBIS). The strong self-trapping substantially reduces the
injection of holes from the contact and limits the creation of peroxides from a
direct hole injection. In presence of light, the concentration of holes
substantially rises and mediates the creation of peroxides, responsible for
NBIS.
