Persistent photoconductivity in a-IGZO thin films induced by trapped electrons and metastable donors

Amorphous In–Ga–Zn–O (a-IGZO) thin films are prepared by pulsed laser deposition and fabricated into thin film transistors (TFTs). The concentration of oxygen vacancies in a-IGZO thin films is determined by the deposition oxygen pressure, as characterized by in situ x-ray photoelectron spectroscopy. The oxygen vacancies could induce persistent photoconductivity (PPC) and thus the negative shift of threshold voltage of the TFTs under illumination. The PPC in a-IGZO is quantitatively described by the time constant (τ) of decay current. The continuous illumination could cause a fast decay (τ ∼ 0.1 s) and a slow decay (τ ∼ 100 s); however, the pulsed laser (20 ns duration) just results in the fast decay (τ ∼ 0.1 s). The fast decay is temperature independent and can be ascribed to the transition of trapped electrons at 0.035–0.75 eV below the conduction band. The slow decay occurs at 210 K or above, resulting from the generation of metastable donors at 0.9 and 19.3 meV below the conduction band. The thermal activation energies required for the generation and annihilation of the metastable donors are 2.2 and 375 meV, respectively. The spectrum response of photocurrent (600–300 nm) and density functional theory calculation illustrate that the metastable donors are transformed from the neutral oxygen vacancies at a highly localized valence band tail.