Toshikazu Nishide

Surface, structural and optical properties of sol-gel derived HfO2 films

Abstract The surface, structural and optical properties of sol-gel derived HfO2 films have been investigated. The water droplet contact angles reached constant values between 80 and 100° after the films were left for 1–4 days in air at ambient temperature, indicating that the film surface exhibited hydrophobicity. Hydrophobicity was maintained even after the films were treated with organic solvents. Scanning electron microscopy and atomic force microscopy observation showed that the film surface was smooth and uniform. The HfO2 in the films crystallized to the monoclinic phase at firing temperatures between 500 and 550°C and no other phases were observed to 900°C. The refractive index and optical gap of the films were 1.88–1.93 and 5.1 eV, respectively.

Crystal structures and optical properties of tungsten oxide films prepared by a complexing-agent-assisted sol-gel process

Abstract Tungsten oxide (WO3) films were prepared by the sol-gel process, using 2,4-pentanedione (PTN) as an organic ligand. The effect of the ligand on the crystallization and crystal structure of the WO3 films was examined by Raman, IR and X-ray diffraction spectroscopies, and their optical properties were investigated in relation to the refractive index. Tungsten oxides prepared with PTN on a quartz glass substrate and a silicon wafer, and those prepared without PTN on a silicon wafer are amorphous when fired at 300 °C. The oxides crystallized when fired at temperatures between 300 and 500 °C, and the amounts of crystalline WO3 increased when the films were fired at 700 °C. Only cubic crystals of WO3 were formed selectively on the quartz glass substrates when the films prepared with PTN were fired at 500 and 700 °C. However, without PTN, a mixture of cubic and monoclinic crystals was formed under the same firing conditions. The WO3 film prepared with PTN and fired at 500 °C showed a lower value for the real part of the complex refractive index than that for the corresponding film prepared without PTN. The WO3 film prepared with PTN did not densify immediately after being fired, resulting in a lower value for the real part of the refractive index.

Indium–tin-oxide thin films prepared by dip-coating of indium diacetate monohydroxide and tin dichloride

Abstract Tin-doped In 2 O 3 (ITO) films were prepared by the dip-coating method using an ethanol solution of indium diacetate monohydroxide, In(OH)(CH 3 COO) 2 , and tin dichloride, SnCl 2 ·2H 2 O, with 2-aminoethanol (monoethanolamine), H 2 NC 2 H 4 OH. The influence of the tin concentration was investigated between 0 and 20 at.% Sn. The composition of the films (thickness ∼90 nm) approximately agreed with that of the solution when the dipping and the heating at 600°C in air for 30 min were repeated three times. The lattice constant of the ITO films increased by the tin addition and annealing at 600°C for 1 h in a nitrogen flow. The films were highly transparent in the visible range. At 6.4 at.% Sn, the maximum carrier concentration (6.2×10 20 cm −3 ) and minimum resistivity (5.8×10 −4 Ω cm) were obtained after annealing in nitrogen (oxygen partial pressure ∼2.0 Pa). The mobilities of the ITO films (∼10 cm 2 V −1 s −1 ) were almost independent of the film composition and the annealing. The mobilities of the undoped In 2 O 3 films were ∼40 cm 2 V −1 s −1 .

Crystal structure and optical property of TiO2 gels and films prepared from Ti-edta complexes as titania precursors

Titania gel and films were prepared from Ti-edta complex solutions as new ceramic precursors, and their crystal structures and optical properties were investigated. Anatase appeared in the gel at firing temperatures between 450 and 500 °C, and was transformed to rutile at temperatures between 550 and 600 °C. This low transformation temperature is presumed to be due to the small crystalline size and micro pores in the gel. Refractive indices of the films were higher than those of the films prepared by a sol-gel process at firing temperatures above 700 °C. These high values were caused by the rutile phase in the TiO2 films.

Fabrication of indium–tin-oxide films by dip coating process using ethanol solution of chlorides and surfactants

Highly-conductive ITO transparent films were deposited by the dip coating process. An ethanol solution of indium (III) chloride, InCl3·3.5H2O, and tin (II) chloride, SnCl2·2H2O [Sn/(In+Sn), 5 at.%] was successfully dip coated on a Corning #7059 glass substrate by the addition of a surfactant [Sorbon T-80 (non-ionic type); polyoxyethylene sorbitan fatty acids esters]. The dip coating (withdrawal rate, 10 or 28 cm/min) and the heating in air at 600 °C for 1 h were repeated 10 times to fabricate the ITO film with a resistivity of approximately 2×10−3 Ω·cm. Post-deposition annealing in a N2–0.1% H2 atmosphere drastically lowered the resistivity to approximately 2.5×10−4 Ω·cm. This value is lower than most of the reported minimums (4×10−4 Ω·cm) for the thicker ITO films fabricated by the dip coating process.

Effect of ligands on crystal structures and optical properties of TiO2 prepared by sol-gel processes

Abstract The refractive index and crystal structures of TiO 2 films formed on soda lime glass substrates by sol–gel processes with and without organic ligands were investigated to establish the effect of the ligands on the optical property and crystal structure. The TiO 2 prepared with 2-(2-methoxyethoxy)ethanol (MEE) ligand crystallized to anatase at firing temperatures between 350 and 400°C. The film with 2-metyhl-2,4-pentanediol (MPD) ligand, however, crystallized between 400 and 500°C. The refractive indices of the films with MEE were the highest among the three types of films at all the firing temperatures, but those of the films with MPD were the lowest. The MEE ligand aggregates grains of titania gels and accelerates the crystallization of the film, resulting in the densest film and the highest refractive index. The MPD ligand, however, does not aggregates the titania grains and is still present up to high firing temperature of 500°C, resulting in the lowest refractive index. In this sol–gel process, the MEE acts as a kind of a binder in ceramics extrusion processes.

A water-resistant precursor in a wet process for TiO2 thin film formation

Deposition of anatase TiO2 thin films on soda-lime glass has been achieved by firing an adhered precursor titanium(IV) complex of ethylenediamine-N,N,N′,N′-tetraacetic acid (H4edta) between 450 and 550 °C in air. The coating solution was prepared by the reaction of a neutral [Ti(H2O)(edta)] complex, obtained from TiCl3 and H4edta in an air-oxidation process, with dipropylamine in ethanol. The crystal structures of the oxide films on glass substrates were examined by XRD, and some of their optical properties and electronic structure were investigated by XPS. It was shown that TiO2 film formation was attainable employing a water-resistant precursor derived from a stable metal complex. The thermal properties of the initial [Ti(H2O)(edta)] complex and the facile preparation of the precursor ethanol solution are also reported.

Novel route to Co3O4 thin films on glass substrates via N-alkyl substituted amine salt of Co(III)-EDTA complex

Deposition of Co3O4 thin films on a glass substrate was attained by firing at 500°C in air an adhered precursory Co(III)-edta complex with tributylammonium cation, {(C4H9)3NH}[Co(C10H12N2O8)]·0.5 H2O. The crystal structure of the oxide film on a glass substrate was examined by XRD, together with some of its optical properties and electronic structure investigated by XPS. A facile preparation for the precursory complex and its characterization are also reported.

Material Microcharacterization of Sol–Gel Derived HfO2 Thin Films on Silicon Wafers

Upon sintering a sol–gel derived HfO2 film at 700°C, on the basis of high-resolution transmission electron microscope measurement combined with electron beam nanodiffraction, the HfO2 film was found to be crystallized in a monoclinic fcc (face centered cubic) structure. The measured interplanar spacing of the crystalline HfO2 on the (111) plane was determined to be 0.314 nm, which was close to the spacing of Si (111) planes, implying the possibility of the epitaxial growth of HfO2 films on Si (111).

Characterization of Sol-Gel Derived and Crystallized ZrO2 Thin Films

Sol–gel-derived zirconium dioxide (ZrO2) films on silicon (Si) substrates fired in air at 350 and 450 °C, using Zr(OH)4 sol based on formic acid (HCOOH) solution, are amorphous and approximately 9–10 nm thick. Crystallization occurs at first at 550 °C as amorphous/tetragonal (011), and finally, at 700 °C, the ZrO2 film crystallizes into tetragonal (011)/monoclinic (111) and (111) structures. The temperature-programmed desorption curve is separated into five distinct H2O desorption components caused by physisorbed H2O, chemisorbed OH, and Zr–OH bonds in the ZrO2 film, and a model is proposed to explain the mechanism of H2O desorption. Thus, H2O desorption and crystallization processes in the ZrO2 film are clarified: consequently the relationships between the film packing density and electrical characteristics are optimized. On the basis of capacitance–voltage characteristics, the dielectric constant (relative permittivity; eZrO2 ) of the sol–gel-derived ZrO2 film fired at 550 °C was calculated to be 12, which is much higher than that of silicon dioxide (SiO2; 3.9).