Role of high-k gate insulators for oxide thin film transistors

Abstract In conventional TFTs, SiO2 or SiNx have been used as gate insulators. But they could not induce the high on-current due to their low-capacitance. Since they have low-capacitance that originated from low dielectric constant, on-current of TFTs with low-k insulated are limited by low-capacitance. We have investigated high-k materials, such as HfO2, ZrO2 and modified structures for the use of gate insulators in oxide thin film transistors. ZrO2 and HfO2 are the most attractive materials with their superior properties, such as high breakdown field intensity (~ 15 MV/cm), high dielectric constant (~ 25), and the capability of room-temperature process. Since they have high-capacitance due to high dielectric constant, it can be easily expected to result in high on-current. In this work, we demonstrated the comparison of oxide thin film transistors with HfO2, ZrO2 and SiO2 and the roles of gate insulators are analyzed. In the result, oxide thin film transistors with SiO2, HfO2 and ZrO2 have on-currents of ~100 μA, ~500 μA, and ~3 mA, respectively. Especially oxide thin film transistor with ZrO2 has larger on-current than oxide thin film transistor with HfO2. The result means that ZrO2 is more suitable than HfO2 for the gate-dielectric material which can be fabricated at room temperature.