Role of oxygen vacancies on the bias illumination stress stability of solution-processed zinc tin oxide thin film transistors

Solution-processed ultra-thin (∼3 nm) zinc tin oxide (ZTO) thin film transistors (TFTs) with a mobility of 8 cm²/Vs are obtained with post spin-coating annealing at only 350 °C. The effect of light illumination (at wavelengths of 405 nm or 532 nm) on the stability of TFT transfer characteristics under various gate bias stress conditions (zero, positive, and negative) is investigated. It is found that the ΔV_th (V _th^{stress 3400 s - stress 0 s}) window is significantly positive when ZTO TFTs are under positive bias stress (PBS, ΔV _th = 9.98 V) and positive bias illumination stress (λ = 405 nm and ΔV_th = 6.96 V), but ΔV_th is slightly negative under only light illumination stress (λ = 405 nm and ΔV_th = -2.02 V) or negative bias stress (ΔV_th = -2.27 V). However, the ΔV_th of ZTO TFT under negative bias illumination stress is substantial, and it will efficiently recover the ΔV_th caused by PBS. The result is attributed to the photo-ionization and subsequent transition of electronic states of oxygen vacancies (i.e., V_o, V_o⁺, and V _o⁺⁺) in ZTO. A detailed mechanism is discussed to better understand the bias stress stability of solution processed ZTO TFTs.