Influence of temperature,voltage and hydrogen on the reversible transition of electrical conductivity in sol-gel grown nanocrystalline TiO_2 thin film

摘要

Undoped nanocrystalline p-type TiO_2 thin film was deposited by sol-gel method on a thermally oxidized p-Si (2-5Ω cm resistivity and 〈100〉 orientation) substrate. The thin film was characterized by two-dimensional X-ray Diffraction (2D-XRD) and Field Emission Scanning Electron Microscopy (FESEM) to confirm the formation of stable nano crystalline anatase titania and to determine the grain size (～ 10 nm). Optical absorption spectroscopy was carried out to ascertain the band gap of the material. Two lateral Pd contacts were used as the metal electrodes to TiO_2 thin film to study the electrical conductivity. A clear p- to n-type transition was observed at 240 ℃ and a bias voltage of 0.83 V and the effect was enhanced on exposure to H2 gas. The thin film showed fully n-type conductivity at 275 ℃ and 0.1 V. However, the reversal of the type of conductivity from n- to p-type was observed below 240℃ during lowering the temperature. The creation of oxygen vacancy and the diffusion of lattice oxygen to the surface of TiO_2 thin film might be the most possible mechanism of such transitions. Presence of hydrogen enhanced the process.