59.9 mV·dec Subthreshold Swing Achieved in Zinc Tin Oxide TFTs With In Situ Atomic Layer Deposited AlO Gate Insulator

Here, by depositing both the zinc tin oxide (ZTO) channel and Al2O3 gate dielectric layer using atomic layer deposition (ALD) without breaking vacuum, we made TFTs with a steep subthreshold swing ( $\textit {SS}$ ) of 59.9mV $\cdot $ dec $^{-{1}}$ , near the room temperature Boltzmann limit. An extremely low interface trap density of 9.59 $\times 10^{{9}}$ cm $^{-{2}}$ eV $^{-{1}}$ was extracted from the measured $\textit {SS}$ value, and was corroborated by the high-low frequency capacitance method. Comparison with other TFT processes shows that both the higher- ${k}$ gate dielectric and the in situ ALD process are required to obtain the low $\textit {SS}$ value. The device made with in situ dielectric deposition exhibits a maximum linear mobility of 19.2 cm $^{{2}}\text{V}^{-{1}}\text{s}^{-{1}}$ , an ON/OFF current ratio > $10^{{8}}$ , and a threshold voltage of 1.3 V. The sharp $\textit {SS}$ achieved here will enable low voltage electronics using this scalable ALD technology.