Ngoc Lam Huong Hoang

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...

Low-temperature Fabrication of Transparent Conducting Anatase Nb-doped TiO2 Films by Sputtering

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.

Properties of TiO2-based transparent conducting oxide thin films on GaN(0001) surfaces

Anatase Nb-doped TiO2 transparent conducting oxide has been formed on GaN(0001) surfaces using a sputtering method. Amorphous films deposited at room temperature were annealed at a substrate temperature of 500 °C in vacuum to form single-phase anatase films. Films with a thickness of 170 nm exhibited a resistivity of 8×10−4 Ω cm with absorptance less than 5% at a wavelength of 460 nm. Furthermore, the refractive index of the Nb-doped TiO2 was well matched to that of GaN. These findings indicate that Nb-doped TiO2 is a promising material for use as transparent electrodes in GaN-based light emitting diodes (LEDs), particularly since reflection at the electrode/GaN boundary can be suppressed, enhancing the external quantum efficiency of blue LEDs.

Metal-induced solid-phase crystallization of amorphous TiO2 thin films

Metal-induced solid-phase crystallization of amorphous TiO2 thin films was investigated by introducing metal contact layers such as Ni or Cu between the TiO2 film and substrate. The crystallization temperature of TiO2 was found to be lowered by 30 °C (from ∼250 to ∼220 °C) with the use of a Ni contact layer. Based on the fact that part of the Ni atoms diffused to the surface of the crystallized TiO2 film, we proposed a reaction-assisted crystallization model in which an intermediate complex containing Ti-O and Ni-O bonds decomposes to crystallize TiO2 at a relatively low temperature.

Enhanced Carrier Transport in Uniaxially (001)-Oriented Anatase Ti0.94Nb0.06O2 Films Grown on Nanosheet Seed Layers

Uniaxially (001)-oriented anatase Ti0.94Nb0.06O2 films were sputtered on a seed layer of glass covered with Ca2Nb3O10 nanosheets (CNO-NS). The films had an effective electron mass of 0.7m0 (m0 denotes the free electron mass), which was smaller than that of randomly oriented films, i.e., 0.9m0. The as-grown films exhibited a low resistivity of 7.6×10-4 Ω cm. Postdeposition annealing under H2 atmosphere further reduced the resistivity to 4.0×10-4 Ω cm. This value is comparable with that of conventional transparent conducting oxides. Results indicate that sputtering using CNO-NS is promising for the fabrication of highly conductive Ti1-xNbxO2 electrodes on glass.

Crystallization Kinetics of Amorphous Sputtered Nb-Doped TiO2 Thin Films

The crystallization kinetics of amorphous sputtered Nb-doped TiO2 (TNO) thin films during isothermal annealing was examined using in situ X-ray diffraction (XRD) measurements. A Johnson–Mehl–Avrami analysis yielded Avrami exponents in the range of 2.0 to 2.7 for TNO films with various oxygen contents, indicating that the crystallization of amorphous TNO films is essentially two-dimensional. The two-dimensional crystal growth is also confirmed by ex situ polarized-light optical microscopic observations of grains, which have much larger lateral sizes than the film thickness.