Transparent conducting oxide films of heavily Nb-doped titania by reactive co-sputtering

Niobium-doped titania (TNO) films of various Nb content were deposited on glass and silicon substrates by reactive co-sputtering of Ti and Nb metal targets. Nb content in the TNO films was varied from 0 to ∼13 at.% (atomic percent), corresponding to Ti1−xNbxO2 with x = 0–0.52, by modulating the Nb target power from 0 to 150 W (Watts). The influence of ion bombardment on the TNO films was investigated by applying an RF substrate bias from 0 to 25 W. The as-deposited TNO films were all amorphous and insulating, but after annealing at 600 °C for 1 h in hydrogen, they became crystalline and conductive. The annealed films crystallized into either pure anatase or mixed anatase and rutile structures. The as-deposited and the annealed films were transparent, with an average transmittance above 70%. Anatase TNO film (Ti1−0.39Nb0.39O2) with Nb 9.7 at.% exhibited a dramatically reduced resistivity of 9.2 × 10−4 Ω cm, a carrier density of 6.6 × 1021 cm−3 and a carrier mobility around 1.0 cm2 V−1 s−1. In contrast, the mixed-phase Ti1−0.39Nb0.39O2 showed a higher resistivity of 1.2 × 10−1 Ω cm. This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films.

► Nb-doped titania (TNO) films were deposited on glass and silicon substrates by reactive co-sputtering of Ti and Nb metal targets.
► Nb content in the TNO films was varied from 0 to ∼13 at.% (atomic percent), corresponding to Ti1−xNbxO2 with x = 0–0.52, by modulating the Nb target power from 0 to 150 W.
► The influence of ion bombardment on the TNO films was investigated by applying an RF substrate bias from 0 to 25 W.
► Anatase Ti1−0.39Nb0.39O2 possesses a reduced resistivity of 9.2 × 10−4 Ω cm, while the mixed-phase Ti1−0.39Nb0.39O2 shows a higher resistivity of 1.2 × 10−1 Ω cm.
► This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films.