Takayuki Ban

Tin doped indium oxide thin films: Electrical properties

Tin doped indium oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and having nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. The association of these properties in a single material explains the vast domain of its applicability and the diverse production methods which have emerged. Although the different properties of tin doped indium oxide in the film form are interdependent, this article mainly focuses on the electrical aspects. Detailed description of the conduction mechanism and the main parameters that control the conductivity is presented. On account of the large varieties and differences in the fabrication techniques, the electrical properties of ITO films are discussed and compared within each technique.

Thin Film Transistor of ZnO Fabricated by Chemical Solution Deposition.

A ZnO thin film was deposited on a Si wafer having an oxidized SiO2 layer using a chemical solution deposition process and was applied to a bottom-gate type thin film transistor (TFT). The films prepared by combined heating at 600° and 900°C exhibited typical enhancement-type TFT characteristics with electrons as carriers. The low heating temperature around 600°C degraded the insulating properties of the SiO2 layer but high temperature annealing recovered that.

Optical, structural, and electrical properties of indium oxide thin films prepared by the sol-gel method

This article reports the fabrication of thin films of In2O3 from an aqueous sol and an organic solution. The stability of the aqueous sol with respect to the concentration of indium and pH are discussed. Thermo-gravimetric and differential thermal analysis were performed on the dried sol particles as well as the gel for better understanding of thermal events occurring during the sintering process. Microstructure, phase purity, optical, and electrical properties of the thin films deposited on glass substrates by dip-coating process from aqueous sol and organic solution were studied by transmission electron microscopy, x-ray diffractometry, visible light spectrometry, and the four-probe method, respectively. The electrical properties of the films derived from the two systems are discussed based on the differences in their morphologies.

Dip-coating of ITO films

Abstract Uniform, transparent indium tin oxide (ITO) films were prepared by dip-coating process using an organic sol composed of indium acetate—diethanolamine—tin octylate-n-propanol mixture and the relationship between their electrical properties, film morphology and dip-coating conditions have been investigated. The optimum Sn-doping concentration was about 4 mol% relative to In ion. The conductivity of as-prepared ITO films increased with an increase in firing temperature. Multiple coating of the layers as thin as a few tens of nanometers accelerated the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 × 10−4 Ω cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. Through this study we conclude that the conductivity of dip-coated films was mainly controlled by the crystallite size and, hence, by carrier mobility.

Electronic transport in tin-doped indium oxide thin films prepared by sol-gel technique

Hall effect measurements have been performed on tin-doped indium oxide thin films prepared by the dipping method. The apparent variation of electron mobility with carrier concentration in these films has been quantitatively interpreted in terms of scattering at charged and neutral impurities. An electrical resistivity as low as 3.2×10−4 Ω cm was obtained for the films doped with about 6 at. % Sn. A further increase in the doping content lead to the formation of neutral defects without contributing carriers to the material.Hall effect measurements have been performed on tin-doped indium oxide thin films prepared by the dipping method. The apparent variation of electron mobility with carrier concentration in these films has been quantitatively interpreted in terms of scattering at charged and neutral impurities. An electrical resistivity as low as 3.2×10−4 Ω cm was obtained for the films doped with about 6 at. % Sn. A further increase in the doping content lead to the formation of neutral defects without contributing carriers to the material.

Mg-doped TiO2 nanorods improving open-circuit voltages of ammonium lead halide perovskite solar cells

Mg-doped TiO2 nanorods were successfully synthesized from colloidal titanate by a microwave hydrothermal reaction. Use of such TiO2 having an elevated conduction band edge as an electron extracting material for ammonium lead halide perovskite solar cells resulted in an increase of Voc by as much as 215 mV.

Development of New Modifiers for Titanium Alkoxide-Based Sol-Gel Process

The effects of several hydroxyketones such as acetol, actoin, γ-ketobutanol themselves and their combinations with monoethanolamine (MEA) or ethylenediamine (ED) on the stabilization of titanium tetraisopropoxide (TTIP) in isopropanol solution are examined. Acetoin itself and the imine derivatives of acetol and acetoin were found to show extraordinarily strong stabilizing effect for the alkoxide. The properties including the crystal modifications and refractive index of TiO2 films that were dip-coated using each stabilized solution are examined and discussed in comparison with those of the films obtained from the diethanolamine (DEA) systems. The effect of UV-light irradiation to the gel films on the crystallization of TiO2 is also examined and discussed.

Fabrication of Zinc Oxide Transparent Thin-Film Transistor with ZrO2 Insulating Layer by Sol-Gel Method

A thin-film transistor consisting of a ZnO active layer and a ZrO2 insulating layer was fabricated on a tin-doped indium oxide sputtered glass substrate as well as on a SiO2/Si wafer. The ZnO and ZrO2 layers were deposited by a sol–gel, dip-coating procedure. The resultant ZrO2 layer was about 150 nm thick and the ZnO layer 70 nm thick. The ZnO layer consisted of a single-grain thickness while the ZrO2 layer consisted of about 10 nm grains and was rather porous. The multilayered film consisting of ZnO/ZrO2/ITO/glass was transparent with 60–85% transmittance in the visible region and exhibited characteristics of a field-effect transistor. The multilayered film of the ZnO/ZrO2/SiO2/Si wafer was also examined and the behavior of the thin-film transistor was confirmed. The ZrO2 layer deposited on the SiO2/Si wafer minimized leakage through the insulating layer.

Size-controlled synthesis of anisotropic TiO2 single nanocrystals using microwave irradiation and their application for dye-sensitized solar cells

A microwave hydrothermal reaction of colloidal titanates is presented as a cost-effective synthesis to produce TiO(2) single nanocrystals. The photoelectrode consisting of anisotropic nanorods and V-shaped twins has a significant advantage for achieving an appreciable incident photon-to-current conversion efficiency of 85.6% for the dye-sensitized solar cell.

Preparation and Characterization of Titania Thin Films from Aqueous Solutions

Dip- or spin-coating and characterization of titania (TiO2) thin films from various aqueous solutions have been studied. The aqueous titanium solutions mainly used in this study were halogen- and chelate-free solutions with the concentrations up to 1.4 M derived from titanium isopropoxide (TIP) with tetramethylammonium hydroxide (TMAOH) or some alkylamines, while aqueous and alcoholic solutions containing titanium atoms stabilized chelating ligands were examined for comparison. The TiO2 films prepared from the TIP-TMAOH solution were already crystallized at 350°C to anatase form and those formed at 600°C had high transparency and refractive indices of 2.40. No carbon residue in the film prepared at 400°C was detected by XPS. The pure anatase form was sustained up to 850°C. Interestingly, it was found that the (004) preferentially oriented anatase films were obtained from TIP-lactic acid (LA) system until 700°C. The solutions containing citric acid (CA) or alkanolamines yielded anatase and rutile form fired at the temperatures equal to or higher than 600°C. Carbon residue was detected in the film fired at 400°C. The film thickness monotonically decreased from the upper to the bottom ends of the substrate. However, it was found that the thickness uniformity was drastically improved by an addition of sucrose to the aqueous solutions. The effects of the solution composition and polyhydroxy compounds on the crystal modifications of formed films and the film uniformity are discussed.