Fumihiko Hirose

Sodium adsorption on nanometer-thick-TiO2-channel thin-film transistors (TFTs) are examined for enhancing the drain current. In the TFTs, the channel thickness of TiO2 is set as thin as ∼16 nm. The TiO2 film is deposited by atomic layer deposition using plasma excited humified Ar, followed by crystallization into the anatase phase by thermal annealing at 500 °C in air. The gate oxide is a 300 nm thick SiO2 film, which is grown on a highly doped n+ Si substrate. The n+ Si substrate is used as the gate electrode. The drain and source electrodes of Ti are deposited by an electron beam evaporation at room temperature. The TiO2 channel is covered with multiple layers of aluminum silicate and SiO2 films to enhance the Na sorptivity. The multiple films consist of combinations of 1 nm thick SiO2 and 0.16 nm thick aluminum silicate. The channel length and width are 60 and 1000 μm, respectively. The TFT without the Na adsorption exhibits a field effect mobility of ∼0.5 cm2/V s, where the drain current is recorded around 30 μA with a gate voltage of 10 V. With immersion of the TFT in a 10 mM NaCl solution, the drain current is enhanced to the order of mA. The simulation with an equivalent circuit with source and drain resistances suggests that the field effect mobility is enhanced to ∼30 cm2/V s with the adsorption of Na. In this paper, we discuss the operation mechanism of the Na adsorbed TiO2 TFT and its applicability as TFT-based high current switch devices and sensors.