Yutaka Furubayashi

A transparent metal: Nb-doped anatase TiO2

This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti1−xNbxO2 films with x=0.002–0.2. The resistivity of films with x⩾0.03 is 2–3×10−4Ωcm at room temperature. The carrier density of Ti1−xNbxO2 can be controlled in a range of 1×1019to2×1021cm−3. The internal transmittance for films with x⩽0.03 (40nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In2−xSnxO3 and ZnO.

Fabrication of highly conductive Ti1−xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition

Nb-doped anatase TiO2 [Ti0.94Nb0.06O2 (TNO)] films with high electrical conductivity and transparency were fabricated on nonalkali glass using pulsed laser deposition and subsequent annealing in a H2 atmosphere. The amorphous films as deposited on unheated substrates were found to crystallize, forming polycrystalline films at around 350°C. The films annealed at 500°C showed resistivity down to 4.6×10−4Ωcm at room temperature and optical transmittance of 60%–80% in the visible region, which are comparable to those of epitaxial films. These results indicate that TNO films have the potential to be practical transparent conducting oxides that could replace indium tin oxide.

Ta-doped Anatase TiO2 Epitaxial Film as Transparent Conducting Oxide

We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5×10-4 Ω cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.

Electronic Band Structure of Transparent Conductor: Nb-Doped Anatase TiO2

We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.

Transport properties of d-electron-based transparent conducting oxide: Anatase Ti1−xNbxO2

The transport properties of a d-electron-based transparent conducting oxide, Nb-doped anatase Ti1−xNbxO2, were investigated as a function of the Nb content x. From optical resistivity spectra, the static effective mass was evaluated to be ∼0.4m0 in the low-carrier-concentration (ne) regime, which is approximately the same as those of conventional transparent conducting oxides (TCOs), and two orders of magnitude smaller than that reported for rutile TiO2. The Hall mobility at room temperature, which is maximized at around x=0.01 (ne∼1021cm−3), was found to be mainly dominated by optical phonon scattering unlike that of other TCOs.The transport properties of a d-electron-based transparent conducting oxide, Nb-doped anatase Ti1−xNbxO2, were investigated as a function of the Nb content x. From optical resistivity spectra, the static effective mass was evaluated to be ∼0.4m0 in the low-carrier-concentration (ne) regime, which is approximately the same as those of conventional transparent conducting oxides (TCOs), and two orders of magnitude smaller than that reported for rutile TiO2. The Hall mobility at room temperature, which is maximized at around x=0.01 (ne∼1021cm−3), was found to be mainly dominated by optical phonon scattering unlike that of other TCOs.

Fabrication of Low Resistivity Nb-doped TiO2 Transparent Conductive Polycrystalline Films on Glass by Reactive Sputtering

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.

Fabrication of TiO2-Based Transparent Conducting Oxide Films on Glass by Pulsed Laser Deposition

Nb-doped anatase TiO2 (Ti0.94Nb0.06O2) films with excellent conductivity and transparency were deposited on non-alkali glass by pulsed laser deposition. X-ray diffraction analysis and transmission electron microscopy confirmed that the obtained films were polycrystalline with anatase structure. The films deposited at a substrate temperature of 250 °C with subsequent H2 annealing at 500 °C showed a resistivity of 1.5 ×10-3 Ωcm at room temperature and an optical transmittance of 60–80% in the visible region. These results indicate that anatase Ti0.94Nb0.06O2 has great potential as a transparent conducting oxide that could replace Sn-doped In2O3 (ITO).

Carrier induced ferromagnetism in Nb doped Co:TiO2 and Fe:TiO2 epitaxial thin film

We have investigated the carrier induced magnetic properties of anatase Ti1−x−yNbxMyO2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95−xNbxCo0.05O2, the n-type carrier density could be controlled in a wide range (4.9×1017cm−3to2.7×1021cm−3) by Nb doping (x=0–0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95−xNbxCo0.05O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x<0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94−xNbxFe0.06O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3K, while room-temperature ferromagnetism appears at x=0.01.

Anatase phase stability and doping concentration dependent refractivity in codoped transparent conducting TiO2 films

Nb0.06SnxTi0.94−xO2 (x ≤ 0.3) thin films were grown by a pulsed-laser deposition method with varying Sn concentration. Through a combinatorial technique, we find that Sn concentration can reach a maximum of about x = 0.3 while maintaining the stable anatase phase and epitaxy. A doping concentration dependence of the refractivity is revealed, in which refractivity reduction at a wavelength of λ = 500 nm is estimated to be 12.4% for Nb0.06Sn0.3 Ti0.64O2 thin film. Sn doping induced band-gap blue shift can be contributed to the mixing of extended Sn 5s orbitals with the conduction band of TiO2. Low resistivity on the order of 10−4 Ω cm at room temperature and high internal transmittance of more than 95% in the visible light region are exhibited for Nb0.06Snx Ti0.94−xO2 thin films (x ≤ 0.2). Optical and transport analyses demonstrate that doping Sn into Nb0.06 Ti0.94O2 can reduce the refractivity while maintaining low resistivity and high transparency.

Heteroepitaxial Growth of Rutile TiO2 on GaN(0001) by Pulsed Laser Deposition

Rutile TiO2(100) thin films have been grown on GaN(0001) surfaces by using the pulsed laser deposition method. Reflection high-energy electron diffraction (RHEED) measurements during the deposition clearly revealed the layer-by-layer growth of TiO2 at a substrate temperature of 400°C under an oxygen pressure of 1×10-5 Torr. X-ray diffraction and atomic force microscopy confirmed that the obtained films have high crystallinity with atomically flat surfaces. Pole figure measurements revealed the epitaxial relationship between TiO2 and GaN, namely that the in-plane TiO2 axis aligns parallel to the GaN .