Sputter deposition and computational study of M-TiO₂ (M=Nb, Ta) transparent conducting oxide films

Titanium dioxide (TiO₂) of the anatase phase has recently attracted much attention as a novel transparent conducting oxide (TCO) due to its rich availability, high refractive index with low absorption in the solar spectrum. While it has been found that Nb is a dopant to obtain low resistivity (~10^-4ωcm), other metals such as Ta, W etc., are also considered as potential effective dopants. In this paper, we carried out a parallel study on Nb- and Ta-doped TiO₂ anatase films both theoretically by first principles calculation and experimentally by sputtering deposition and optical/electrical characterizations. The Nb-TiO₂ films deposited on glass by co-sputtering at room temperature were amorphous, and the films crystallized into an anatase structure after vacuum-annealing, with the measured resistivity values comparative to the reported. The Ta-TiO₂ films were deposited similarly, and the structure and properties were compared with the Nb-doped ones. Results showed that better performance was found in Nb-TiO₂ films than that in Ta-TiO₂ films. Theoretical calculations indicate that the larger lattice distortion by substitution of Ta for Ti is the dominating factor to suppress crystal growth and weaken the ability of electron mobility.