Metal-like conductivity in undoped TiO2-x: Understanding an unconventional transparent conducting oxide.

Abstract Substoichiometric TiO 2 films have been prepared by reactive magnetron sputtering. Employing an oxygen feedback loop has enabled a precise control of the sub-stoichiometry in the growing TiO 2-x film as well as a high level of sample reproducibility. TiO 2-x was deposited on unheated substrates and subsequently annealed at 300 °C. These films have been characterized by a variety of techniques to unravel the interplay between stoichiometry and optical as well as electronic properties. Capping the films by a Si 3 N 4 layer was sufficient to prepare films with resistivities as low as 8 mΩcm showing a high mobility of up to 15 cm2/Vs. At the same time, a high transparency above 80% could be realized for these samples. The results verify that metal-like conductivity and high transparency can be realized simultaneously in undoped TiO 2-x. Upon variation of the film stoichiometry a metal-insulator transition has been observed in temperature dependent electrical measurements performed down to 2 K. Highlights • High conductivity and transparency in undoped TiO2 films was achieved. • Annealing the reactively sputtered films at 300 °C in Ar-atmosphere was sufficient. • Influence of oxygen content of the slight sub-stoichiometric films was investigated. • Temperature dependent Hall measurements prove metal-like transport. • Important differences in comparison to conventional TCOs are shown and explained. [ABSTRACT FROM AUTHOR]