Independent control of phases and defects in TiO₂ thin films for functional transistor channels

TiO₂ has various inherent advantages in practical devices: chemical stability, non-toxicity, and abundance on earth, as highlighted in a range of applications of TiO₂ surfaces in photocatalysis/optoelectronics. However, the application of TiO₂ to three-terminal devices has been limited; for example, TiO₂-channel transistors, which may potentially modulate the TiO₂ surface properties by electrostatic back gating, has suffered from low field-effect mobility (μ_FE < 1 cm² V⁻¹ s⁻¹) irrespective of fabrication methods. The major challenge is to control phases (rutile/anatase) and defects (oxygen vacancy) simultaneously in TiO₂ thin films. Here, we achieved μ_FE ∼ 10 cm² V⁻¹ s⁻¹ in TiO₂-channel transistors, one order higher than before and even comparable to InGaZnO channels. The major improvement is the independent control of phases and defects in TiO₂ thin films; we formed short-range order of anatase phase in the amorphous deposition, and minimized the number of defects in the subsequent thermal treatment. Besides, we showed this independent control of phases and defects are underpinned by excellent thermal stability of TiO₂/SiO₂ interfaces. These results demonstrate TiO₂ can be a transistor channel with a reasonable μ_FE and ON/OFF ratio (>6 orders), not only changing the conventional image of this material, but also promising new functionalities based on unique properties of TiO₂.