Instability of Amorphous Oxide Semiconductors via Carrier-Mediated Structural Transition between Disorder and Peroxide State

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.