Crystallinity engineering of stoichiometric TiO 2 : transition from insulator to semiconductor

Over decades, the electrical properties of metal oxides have been a myth, stimulating rigorous debate and research. It is commonly accepted that the oxygen vacancy, working as shallow donors, gives rise to the n-type semiconductor behavior in these metal oxides. However, the function of the oxygen vacancy has been doubted, the energy level of which is reported to be far below conduction band to be responsible for the n-type conductivity [1] . In addition, metal oxides with oxygen deficiency in nature is susceptible to the spurious oxidation from the environment, resulting in stability and reliability issue in electronic device. In this work, we observe the conductivity transition from insulator to semiconductor of the stoichiometric TiO 2 films via judicious control of the crystallinity, highlighting the important role of crystallinity in the electrical properties of metal oxide. Based on this observation, high-performance thin film transistors (TFTs) with polycrystalline TiO 2 (poly-TiO 2 ) as the active channel and InAlN/GaN metal-insulator-semiconductor high-mobility-electron-transistors (MISHEMTs) with amorphous TiO 2 (a-TiO 2 ) as the gate dielectrics are demonstrated, indicating the full potential of stoichiometric TiO 2 films as functional electronic materials in device applications.