Origin of light instability in amorphous IGZO thin-film transistors and its suppression

Radiating amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with deep ultraviolet light (λ = 175 nm) is found to induce rigid negative threshold-voltage shift, as well as a subthreshold hump and an increase in subthreshold-voltage slope. These changes are attributed to the photo creation and ionization of oxygen vacancy states (V_O), which are confined mainly to the top surface of the a-IGZO film (backchannel). Photoionization of these states generates free electrons and the transition from the neutral to the ionized V_O is accompanied by lattice relaxation, which raises the energy of the ionized V_O. This and the possibility of atomic exchange with weakly bonded hydrogen leads to metastability of the ionized V_O, consistent with the rigid threshold-voltage shift and increase in subthreshold-voltage slope. The hump is thus a manifestation of the highly conductive backchannel and its formation can be suppressed by reduction of the a-IGZO film thickness or application of a back bias after radiation. These results support photo creation and ionization of V_O as the main cause of light instability in a-IGZO TFTs and provide some insights on how to minimize the effect.