A systematic study of light dependency of persistent photoconductivity in a-InGaZnO thin-film transistors

Persistent photoconductivity (PPC) effect and its light-intensity dependence of both enhancement and depletion (E-/D-) mode amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) are systematically investigated. Density of oxygen vacancy (V_O) defects of E-mode TFTs is relatively small, in which formation of the photo-induced metastable defects is thermally activated, and the activation energy (E_a) decreases continuously with increasing light-intensity. Density of V_O defects of D-mode TFTs is much larger, in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated. Furthermore, for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects. Under low light-exposure below the threshold, only a low PPC barrier is formed and the PPC recovery is fast. With increasing the light-exposure to the threshold, the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies (\rmV_\rmO^2+) is fully developed, and the PPC barrier increases to ∼ 0.25 eV, which remains basically unchanged under higher light-exposure. Based on the density of V_O defects in the channel and the condition of light illumination, a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations.