Yasutaka Takahashi

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.

Photoconductivity of Ultrathin Zinc Oxide Films

Electrical and photoelectrical properties of nondoped and doped zinc oxide films coated on glass plates by the dip-coating method are investigated at room temperature in various ambient atmospheres. The dark conductivity of the nondoped films exponentially decreased with decreasing film thickness while the conductivity under illumination of 350 nm light was almost constant at 100 Scm-1 irrespective of the film thickness. Consequently thinner films showed larger photoresponse than thicker films. This thickness dependence is explained by the variation of ZnO particle size with the film thickness (fine particle model) and the additional effect of the Schottky barrier generated between the film and gold electrodes.

Dip-coating of TiO2 films using a sol derived from Ti(O-i-Pr)4-diethanolamine-H2O-i-PrOH system

Diethanolamine (DEA) can suppress the precipitation of oxides from the alcoholic titanium isopropoxide solution in its hydrolysis so that much water can be added to the Ti(O-i-Pr)4-diethanolamine-i-propanol the solution to give a clear solution. Addition of excess water converted the solution to the gel. The conditions for the formation of the clear solution and gel are examined. Uniform transparent TiO2 films can be prepared by dip-coating with the clear solution. The limit of the thickness of uniform films was 200 to 240 nm. Films thicker than 1 μm can be prepared by repeating the coating cycle. These films were well densified. Diethanolamine has some positive effect on the densification of the TiO2 crystals.

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.

Low temperature deposition of metal nitrides by thermal decomposition of organometallic compounds

Decomposition reaction of dialkylamides of boron, silicon, tin, titanium, zirconium, niobium, and tantalum was investigated. The amides of transition metals decomposed to the corresponding nitrides at 300~176 whereas those of boron, silicon, and tin yielded elemental deposits at higher temperatures. In the deposition of titanium nitride from titanium tetrakis(dimethylamide), two optimum temperatures at 400 ~ and at 800~ in nitrogen or hydrogen atmosphere were found, but in argon only low temperature deposition was possible. The low and high temperature processes are discussed in relation to mass spectral analysis of the exhaust gases formed at various decomposition temperatures of titanium amide. Metal nitrides are well known to have outstanding physical and chemical properties and have been prepared by the reactions of metal halides or hydrides with nitrogen + hydrogen or ammonia, or of metals with nitrogen (1). These reactions usually require temperatures higher than 1000~ which have limited the application of the nitrides as the useful coatings on many materials. Therefore, their preparation at lower temperature, where deformation of substrate materials can be avoided, should be important.

Dip‐Coating of Sb‐Doped SnO2 Films by Ethanolamine‐Alkoxide Method

The resistivity of the film depended slightly on film thickness: it was 0.025 Ω•cm below 1000 A and decreased to a constant value of 0.005 Ω•cm above 2000 A. Average transmittance of visible light was higher than 95% for undoped films and higher than 80% for Sb-doped films. The coated thin films were very uniform and dense, whereas an OMCVD process formed porous films with an island texture. The advantages and disadvantages of the dipcoating process are discussed in comparison with the OMCVD process

Preparation of transparent, electrically conducting ZnO film from zinc acetate and alkoxide

Very uniform and transparent zinc oxide thin films doped with aluminium and indium were fabricated by the dip-coating technique using solutions prepared by the ethanolamine method. As starting materials, zinc acetate and zinc n-propoxide were used. Zinc acetate and propoxide are soluble in PriOH in the presence of diethanolamine, although they are hardly soluble without the amine. The prepared solutions were very stable and suitable for dip-coating. Zinc oxide was crystallized by heating above 500 °C, and doping of aluminium and indium retarded the crystallization. The electrical resistivity of the film was decreased by doping with aluminium and indium. The lowest resistivity of 2 × 10−2 Ωcm was obtained by post-coating treatment in a nitrogen atmosphere.

Dip-coating conditions and modifications of lead titanate and lead zirconate titanate films

The modifications of dip-coated lead titanate (PT) and lead zirconate titanate (PZT) films strongly depend on the film thickness and the substrate in addition to the heat-treatment temperature. At 500 to 600 ° C, metastable paraelectric pyrochlore grew on glass plates (amorphous plates) when the thickness of the coated films produced by one coating cycle was below 100 nm, while ferroelectric perovskite formed on crystalline substrates or when thick films were coated on amorphous plates. This tendency is discussed in terms of an inhomogeneous reaction and the epitaxial effect. The perovskite PT films coated on single-crystal SrTiO3 plate at 700 ° C were strongly oriented to thec-axis.

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.