Vibrational-Electrical Properties Relationship in Donor-Doped TiO2 by Raman Spectroscopy

Transparent conducting TiO2, obtained by Nb or Ta doping of the anatase structure, is gaining increasing attention for the development of transparent electrodes. Usually, regardless of the deposition technique, a crystallization process in a reducing atmosphere is necessary to achieve large mobility; in addition, electrical and optical properties are also strongly sensitive to the oxygen deposition pressure. These facts reveal that the defect chemistry of donor-doped TiO2 is not trivial and involves a strict interplay among extrinsic dopant atoms, oxygen vacancies, and electron killer defects such as Ti vacancies and O interstitials. We here present a Raman characterization of donor-doped TiO2 films synthesized under several deposition and postannealing conditions, employing different doping levels and dopant elements (i.e., Ta and Nb). Correlations between structure, crystallinity, shift and width of Raman peaks, and electrical properties are shown and discussed. In particular, a clear relationship between the shift of the E-g(1) anatase Raman mode and the charge carrier density is found, while the B-1g(1) mode connected to Ti Ti vibrations is significantly affected by the extrinsic doping level. In this complex framework, Raman spectroscopy can provide an invaluable contribution toward understanding the material structure and its influence on the functional properties.