Optimization of Two-Dimensional Channel Thickness in Nanometer-Thick SnO₂-Based Top-Gated Thin-Film Transistors Using Electric Field Thermopower Modulation: Implications for Flat-Panel Displays

Transparent amorphous oxide semiconductor (TAOS) based thin-film transistors (TFTs) are essential as the backplane for developing advanced flat panel displays. Among many TAOSs, amorphous (a-) SnO2 is promising active material due to its abundance compared with the state-of-the-art a-InGaZnO4. However, practical application of a-SnO2-based TFTs has not been realized because of its unstable transistor characteristics coming from the high residual carrier concentration. Precise optimization of the two-dimensional channel thickness is required to stabilize the transistor characteristics of a-SnO2-based TFTs. Here we use electric field thermopower modulation analyses to show that the two-dimensional channel thickness of a-SnO2 for TFT can be optimized at ∼2 nm. After the optimization of the channel thickness, we reduced the thickness of the HfO2 gate insulator film to further improve the transistor characteristics. The resultant TFT exhibited excellent transistor characteristics: on-to-off current ratio of ∼105, normally off behavior (Vth ≈ +0.65 V), small subthreshold swing of ∼230 mV/decade, high mobility (∼10 cm2 V–1 s–1), and stability in changing oxygen atmospheres. The present results would bring further possibilities for the development of next-generation low-cost and low-power electronic devices.