  The excited holes occupying the valence band tail states in amorphous oxide
semiconductors are found to induce formation of meta-stable O$_2^{2-}$ peroxide
defects. The valence band tail states are at least partly characterized by the
O-O pp{\sigma}* molecular orbital, and the localized-hole-mediated lattice
instability results in the formation of the peroxide defects. Along with the
O-O bond formation, the pp{\sigma}* state is heightened up into the conduction
bands, and two electrons are accordingly doped in the electronic ground state.
The energy barrier from the O$_2^{2-}$ peroxide state to the normal disorder
state is found to be 0.97 eV in hybrid density functional theory. The
hole-mediated formation of the meta-stable peroxide defects and their
meta-stability is suggested as an origin of the negative bias and/or
illumination stress instability in amorphous oxide semiconductors.
