Impact of ZrO2 Dielectrics Thickness on Electrical Performance of TiO2 Thin Film Transistors with Sub-2 V Operation

In the present work, the impact of ZrO2 gate dielectric thickness on the electrical performance of TiO2 thin film transistors (TFTs) is systematically investigated. Exhaustive electrical measurements on TFTs, metal-insulator-metal, and metal-oxide-semiconductor capacitors are carried out with varying ZrO2 dielectrics of different thicknesses. It is found that the ZrO2 possesses an outstanding thickness scalability, providing reliable dielectric properties under an ultrathin physical thickness of 5 nm, while further reduction in ZrO2 thickness would lead to a breakdown of the TFTs, indicating the physical limitations for the ultrathin ZrO2 dielectrics. The TiO2 TFTs with the 5 nm-thick ZrO2 dielectric exhibit an enhanced electrical performance, including a high on/off current ratio (I-on/I-off) of 7.7 X 10(8), a nearly ideal subthreshold (SS) of 72 mV/dec, and a high electron mobility (mu(eff)) of 5.74 cm(2).V-1.s(-1) under an ultralow voltage of 2 V. Such prominent electrical characteristics with a battery-drivable low-voltage operation prove the suitability of the TiO2 TFTs for Internet of Things applications. This comprehensive study, revealing the behaviors of leakage current, oxide capacitance, oxide charges, and interface traps with respect to the ZrO2 thickness, sheds light on the quality and scalability of the ZrO2 dielectrics that can be extended to other channel materials, and also offers a framework for evaluating the material quality of other dielectrics. Furthermore, from a material point of view, the underlying physical reasons for the excellent ZrO2 thickness scalability are believed to be (1) the smooth surfaces of TiO2 and ZrO2, (2) the well-structured ZrO2 without detectable oxygen-related defects, and (3) the large conduction band offset between ZrO2 and TiO2 . Overall, this study not only shows that the TiO2 TFTs hold great potential to empower future IoT applications, but also provides important guidance regarding dielectric scaling and quality evaluation in the nanometer scale.