Nb-doped anatase TiO2 (TNO) polycrystalline films with excellent conductivity and transparency were successfully fabricated by reactive sputtering combined with post annealing in H2 gas. The H2 annealing of as-deposited amorphous films caused an abrupt decrease in resistivity (ρ), which was accompanied by crystallization into the anatase structure. A film deposited on an unheated glass substrate with subsequent H2 annealing at 600 °C exhibited a resistivity of 9.5×10-4 Ω cm and an average optical transmittance of ∼75% in the visible region. This ρ value is of the same order as that of epitaxial TNO films, which indicates that sputtering is a promising technique for obtaining large-area TNO films.

We present a low-temperature (∼300 °C) process for preparing transparent conducting anatase Nb-doped TiO2 (TNO) polycrystalline films by sputtering. We first deposited amorphous films composed of an oxygen-rich bottom layer and oxygen-deficient top layer at room temperature. These films were then crystallized in a reducing atmosphere. The oxygen-rich bottom layer behaved as a seed layer during crystallization of the top layer, resulting in significant improvement of crystallinity and reduction of crystallization temperature. We obtained TNO polycrystalline films showing a resistivity of 6.4×10-4 Ω cm and absorption below 10% in the visible region by post-deposition annealing at 400 °C. The developed low-temperature process was applied to fabricating TNO films on plastics and glass with low glass-transition temperature.

This paper proposes a novel sputter-based method for the direct growth of transparent conducting Ti1−xNbxO2 (TNO) polycrystalline films on glass, without the need for any postdeposition treatments, by the use of an initial seed-layer. Anatase TNO epitaxial films grown on LaAlO3 (100) substrates under a reducing atmosphere exhibited a low resistivity (ρ) of (3–6)×10−4Ωcm. On glass, however, highly resistive rutile phase polycrystalline films (ρ∼100Ωcm) formed preferentially under the same conditions. These results suggest that epitaxial stabilization of the oxygen-deficient anatase phase occurs on lattice-matched substrates. To produce a similar effect on a glass surface, we deposited a seed-layer of anatase TNO with excellent crystallinity under an increased oxygen atmosphere. As a result, anatase phase TNO polycrystalline films could be grown even under heavily reducing atmospheres. An optimized film exhibited ρ=1.1×10−3Ωcm and optical absorption lower than 10% in the visible region. This ρ value is more than one order of magnitude lower than values reported for directly deposited TNO polycrystalline films. This indicates that the seed-layer method has considerable potential for producing transparent conducting TNO polycrystalline films on glass.