Yasushi Hirose

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.

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.

Direct growth of transparent conducting Nb-doped anatase TiO2 polycrystalline films on glass

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...

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).

Possible ferroelectricity in perovskite oxynitride SrTaO2N epitaxial thin films

Compressively strained SrTaO2N thin films were epitaxially grown on SrTiO3 substrates using nitrogen plasma-assisted pulsed laser deposition. Piezoresponse force microscopy measurements revealed small domains (101–102 nm) that exhibited classical ferroelectricity, a behaviour not previously observed in perovskite oxynitrides. The surrounding matrix region exhibited relaxor ferroelectric-like behaviour, with remanent polarisation invoked by domain poling. First-principles calculations suggested that the small domains and the surrounding matrix had trans-type and a cis-type anion arrangements, respectively. These experiments demonstrate the promise of tailoring the functionality of perovskite oxynitrides by modifying the anion arrangements by using epitaxial strain.

High Mobility Exceeding 80 cm2 V-1 s-1 in Polycrystalline Ta-Doped SnO2 Thin Films on Glass Using Anatase TiO2 Seed Layers

High-mobility Ta-doped SnO2 (TTO) thin films were grown on glass substrates by pulsed laser deposition using a seed-layer technique. The use of 10-nm-thick polycrystalline anatase TiO2 seed layers was found to lead to the preferred growth of (200)-oriented TTO films, resulting in a 30% increase in the carrier density and a more than two times increase in mobility, compared to films grown directly on the glass substrates. The highest mobility obtained was 83 cm2 V-1 s-1 with a resistivity of 2.8×10-4 Ω cm, whereas the film with the lowest resistivity of 1.8×10-4 Ω cm had a mobility of 60 cm2 V-1 s-1.

Preparation of Layered-Rhombohedral LiCoO2 Epitaxial Thin Films Using Pulsed Laser Deposition

Epitaxial thin films of layered-rhombohedral LiCoO2 (α-NaFeO2 structure, R3m) have been successfully grown on Al2O3(0001) substrates using pulsed laser deposition. A single phase of LiCoO2 was obtained in the narrow substrate temperature range of 250–300 °C, above which secondary phases, such as Co2O3, Co3O4, and LiCo2O4, appeared. In addition, it was found that annealing of precursor films deposited at room temperature yielded atomically flat LiCoO2 films with a surface roughness of ~0.2 nm.