Millisecond Stretched Exponential Recovery of Threshold Voltage for Mechanically Stressed Flexible a-IGZO Thin-Film Transistors

We report the recovery kinetics of drain currents for flexible amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) on polyimide (PI) substrate after mechanical bending stress (MBS) of 10⁴ cycles. The drain currents increase and threshold voltage shifts of -3.5 V by 1.65% tensile MBS. These shifts could be recovered by applying high drain/high gate voltages for high current. The recovery of V_TH can be fitted by ΔV_TH(t) = AV_TH(ε, n) · [1 - exp - (t/τ)^β] with time constant τ and stretch parameter β. Positive bias recovery (PBR) under 30 V [V_G = V_D = 30 V] completely returns to the pristine TFT performance within 10^-3 s with τ of 5 × 10^-5 s, whereas, positive bias temperature stress [PBTS, V_G = 30 V] at 60 °C exhibits a partial recovery after 1 h, with τ of 1.75 × 10³ s. The time dependent reduction of defect states generated by MBS can be fitted well by the stretched exponential equation. The rise in temperature by Joule heating during PBR anneals out the generated defects/trapped charges.