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− 4 Ωcm), 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.