The Anomalous Effect of Oxygen Ratio on the Mobility and Photobias Stability of Sputtered Zinc–Tin–Oxide Transistors

The oxygen ratio-dependent device performance and reliability of zinc tin oxide (ZTO) thin-film transistors (TFTs) were examined. ZTO TFTs fabricated under pure Ar conditions exhibited a high saturation mobility of 21.5 cm²/Vs, low subthreshold gate swing of 0.34 V/decade, and high I_ON/OFF ratio (10⁹), whereas modest mobility of 7.3 cm²/Vs was obtained for the ZTO TFTs prepared at an oxygen ratio [R = O₂/(Ar + O₂)] of 0.3. The photobias stability (AV_th ≈-2.0 V) of the ZTO TFTs under the Ar only condition was much better than that (AV_th ≈-5.9 V) of the device at R of 0.3, even though their channel layers contained the larger density of oxygen vacancies. This abnormal behavior was attributed to the compressive stress of ZTO films retarding the phototransition of oxygen vacancies.