Structural, magnetic and optical properties of Ni-doped TiO₂ thin films deposited on silicon(100) substrates by sol-gel process

Ni-doped TiO₂ films were deposited on Si(100) substrates by sol-gel technique. X-ray photoelectron spectroscopy analysis suggests that the valence of Ni ion is in +2 and oxygen vacancies increase with increasing Ni content. X-ray diffraction measurements indicate that Ni doping catalyzes the anataseto- rutile transformation (ART) of TiO₂ films, which is due to the decrease of the ART activation energy. The reversible ferromagnetism of the samples with Ni fraction is found, which is due to an anatase-torutile junction destroying an F-center bound magnetic polaron. Optical properties of Ni-doped TiO₂ films were studied by the ellipsometric spectra. With increasing Ni content, the optical band gap of TiO₂ films is decreased from 3.56 to 3.34 eV, which may be related to phase composition and impurity band.