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

Abstract

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 Ti 1− x Nb x O 2 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 (Ti 1−0.39Nb 0.39O 2) with Nb 9.7 at.% exhibited a dramatically reduced resistivity of 9.2 × 10 −4 Ω cm, a carrier density of 6.6 × 10 21 cm −3 and a carrier mobility around 1.0 cm 2 V −1 s −1. In contrast, the mixed-phase Ti 1−0.39Nb 0.39O 2 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.