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 similar 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 degrees 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 x 10(-4) Omega cm, a carrier density of 6.6 x 10(21) cm(-3) and a carrier mobility around 1.0 cm(2) V-1 s(-1). In contrast, the mixed-phase Ti1-0.39Nb0.39O2 showed a higher resistivity of 1.2 x 10(-1) Omega cm. This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films. (C) 2011 Elsevier B.V. All rights reserved.