Influence of temperature, voltage and hydrogen on the reversible transition of electrical conductivity in sol–gel grown nanocrystalline TiO2 thin film

Abstract

Undoped nanocrystalline p-type TiO2 thin film was deposited by sol–gel method on a thermally oxidized p-Si (2–5 Ω 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 TiO2 thin film to study the electrical conductivity. A clear p- to n-type transition was observed at 240 °C 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 °C and 0.1 V. However, the reversal of the type of conductivity from n- to p-type was observed below 240 °C during lowering the temperature. The creation of oxygen vacancy and the diffusion of lattice oxygen to the surface of TiO2 thin film might be the most possible mechanism of such transitions. Presence of hydrogen enhanced the process.