Influence of transition metal doping on the structural, optical, and magnetic properties of TiO₂ films deposited on Si substrates by a sol-gel process

Transition metal (TM)-doped TiO² films (TM = Co, Ni, and Fe) were deposited on Si(100) substrates by a sol-gel method. With the same dopant content, Co dopants catalyze the anatase-to-rutile transformation (ART) more obviously than Ni and Fe doping. This is attributed to the different strain energy induced by the different dopants. The optical properties of TM-doped TiO² films were studied with spectroscopic ellipsometry data. With increasing dopant content, the optical band gap (E_OBG) shifts to lower energy. With the same dopant content, the E_OBG of Co-doped TiO² film is the smallest and that of Fe-doped TiO² film is the largest. The results are related to electric disorder due to the ART. Ferromagnetic behaviors were clearly observed for TM-doped TiO² films except the undoped TiO² film which is weakly magnetic. Additionally, it is found that the magnetizations of the TM-doped TiO² films decrease with increasing dopant content.