Hidefumi Odaka

Photocatalytic TiO2 thin film deposited onto glass by DC magnetron sputtering

A high performance photocatalytic TiO2 thin film was successfully obtained by reactive DC magnetron sputtering. The film was deposited onto SiO2-coated glass at a substrate temperature of 220°C using a titanium metal target in O2 100% atmosphere. The film showed good uniformity of thickness in a large area with the optical transmittance of ∼80% in the visible region. The decomposition ability of acetaldehyde (CH3CHO) of the film under UV irradiation was almost the same as that of the sol–gel-derived TiO2 thin film but the sputtered film showed a much higher mechanical durability. The characterization of the films was carried out using XRD, SEM, AFM, XPS and SIMS, and the electronic structures of the films were calculated using a first-principle calculation method based on the density functional theory. It was found that the amount of incorporated 18O into the film was larger for the films with lower photocatalytic activity when the films were annealed in 18O2/N2 atmosphere. This result indicates that the amount of oxygen vacancies, which were occupied by incorporated 18O, was larger for the films with lower photocatalytic activity. Furthermore, the introduction of structural defects associated with oxygen vacancies was found to create some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holes and electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated with oxygen vacancies is important for improving the photocatalytic activity of the films.

Control of magnetic-field effect on electroluminescence in Alq3-based organic light emitting diodes

The magnetic-field effect on electroluminescence (EL) has been investigated for the tris-(8-hydroxyquinolino) aluminum (Alq3)-based organic light emitting diode. The EL intensity sharply increases up to 8% with increasing magnetic field to 500Oe at room temperature. The magnetic field effect on EL depends on the interface structure between a hole transporting and a light emitting layers, indicating the importance of the spin-state dynamics of the electron-hole pairs at the interface.

Electronic Structures and Optical Properties of ZnO, SnO2 and In2O3

Electronic structures and optical properties of ZnO, SnO2 and In2O3 are investigated by using a first-principles calculation method based on the density functional theory. The dielectric functions dominated by electron interband transitions are analyzed in terms of the calculated electronic band structures and charge density distributions are analyzed to clarify the chemical bonding and electrical conduction characteristics. The calculated results elucidate the similarities and disparities among these materials and also provide a guideline for manufacturing optoelectronic devices with as large a transparent region as possible.

Electrical properties of heteroepitaxial grown tin‐doped indium oxide films

Oriented thin‐film tin‐doped indium oxide (ITO) was heteroepitaxially grown on optically polished (100) or (111) planes of single‐crystalline yttria‐stabilized zirconia (YSZ) substrates using e‐beam evaporation or dc magnetron sputtering techniques. Pole figure x‐ray diffraction analyses revealed that the heteroepitaxial relations were (001)ITO∥(001)YSZ, [100]ITO∥[100]YSZ, and (111)ITO∥(111)YSZ, [110]ITO∥[110]YSZ, respectively. X‐ray rocking curve analyses and Rutherford backscattering spectrometry revealed that the e‐beam evaporated heteroepitaxial ITO films had much higher crystallinity than the one deposited by dc magnetron sputtering. Both carrier density and Hall mobility of the e‐beam evaporated heteroepitaxial films showed steady increases in a wide temperature range, which could be interpreted in terms of the increasing Sn‐doping efficiency caused by the improvement of the crystallinity of In2O3 host lattice, and hence the decreasing Sn‐based neutral scattering centers.

Electronic Structure Analyses of Sn-doped In2O3

Electronic structures of Sn-doped In2O3 (ITO) have been investigated for the first time by using a first-principles calculation method based on the density functional theory. Calculated partial density of states (PDOS) analyses showed that a Sn atom substituted for an indium one formed three impurity bands with s-like symmetry, the second band of the three bands overlapped the conduction band of In2O3, and the Fermi energy of ITO was captured in this impurity band. The PDOS analyses also revealed that the substitution of a Sn atom did not significantly destroy the shape of density of states around the bottom of the conduction band, which gave a physical foundation for the Burstein-Moss shift model used up to now. Carrier generation mechanism and past experimental results, such as those of X-ray photoelectron spectroscopy, temperature dependency of electrical conductivity and carrier-concentration dependency of optical effective mass of ITO, are discussed based on the present theoretical calculation results.

Study on Electronic Structure and Optoelectronic Properties of Indium Oxide by First-Principles Calculations

The electronic structure of In2O3 has been studied for the first time using a first-principles calculation method based on the density functional theory. Although the complexity of the crystal structure of In2O3 which contained 40 atoms in its unit cell had prevented studies of its electronic structure, we were able to study it using the characteristic of minimum basis sets of the linear muffin-tin orbital method with atomic sphere approximation. The calculated partial density of states (PDOS) showed that the valence bands were composed mainly of oxygen 2p-like states and the conduction bands consisted mainly of indium 5s-like states with free-electron-like character. The results of PDOS analysis were used to analyze the spectra from X-ray photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy. Calculated results were also used to interpret optoelectronic properties of tin-doped indium oxide.

Ion beam modification of transparent conducting indium-tin-oxide thin films

Abstract We have examined the effects of ion implantation of various chemical species on the electrical properties of transparent, conducting indium-tin-oxide (ITO) polycrystalline films with resistivities less than 200 μΩ cm and optical transmission greater than 90%. We report on implantations of N + , O + , F + , Ne + and In + under a variety of conditions. At low to moderate doses, damage effects dominate and reduce the conductivity slightly before saturating at doses of ∼ 10 14 /cm 2 . At higher doses, when the implanted concentration becomes comparable to the free-carrier concentration (∼ 10 21 /cm 3 ), some species (e.g., In + ) can improve the conduction slightly, while other species (e.g., O + ) can reduce the conduction, in some cases by several orders of magnitude. We also describe preliminary results of some new experiments begun with single-crystal ITO films to permit better characterization of the damage effects.

Improving Mobility of F-Doped SnO2 Thin Films by Introducing Temperature Gradient during Low-Pressure Chemical Vapor Deposition

High mobility is required to suppress free-carrier absorption in the near-infrared (NIR) region. Toward this end, we investigated the properties of a F-doped SnO2 (FTO) film deposited using low-pressure chemical vapor deposition (LPCVD) and found that the optimum deposition temperature varied with film thickness. On the basis of this result, we introduced a temperature gradient into LPCVD, which resulted in an improvement in the mobility of F-doped SnO2 on glass to 77.5 cm2 V-1 s-1.

Elastic-Constant Measurement in Oxide and Semiconductor Thin Films by Brillouin Oscillations Excited by Picosecond Ultrasound

In this study, an elastic-stiffness evaluation in transparent or translucent thin films using Brillouin oscillations detected by picosecond ultrasound is conducted. An ultrahigh-frequency (\gtrsim50 GHz) strain pulse is generated using femtosecond light pulse in specimens and propagates in the film-thickness direction. The time-delayed probe light pulse enters the specimen, which is diffracted by the strain pulse, causing oscillations in the reflectivity change of the probe light pulse. The oscillation frequency gives the elastic modulus with ellipsometry for refractive index. The theoretical calculation predicts the accuracy of stiffness measurement. The methodology is applied to the study of amorphous silica, amorphous tantalum oxide, diamond thin films, and silicon wafers.

Fabrication of selenized/sulfurized Cu(In,Ga)(Se,S) 2 solar cells based on high temperature process using high strain point glass substrate

A new high strain point glass substrate for selenized/sulfurized Cu(In,Ga)(Se,S)2 (CIGS) solar cells was developed. The developed glass has advantages not only higher strain point but also lighter weight and higher mechanical strength than the glass for Plasma Display Panels. CIGS solar cells were fabricated by a selenization/sulfurization 2-step process at high temperature using the developed glass and Soda Lime Glass (SLG) substrates. The SLG needed SiO2 alkali barrier layer to fabricate CIGS absorber thin film, otherwise delamination occurred. The CIGS fabricated under the high temperatures conditions had better crystal properties, which resulting in higher conversion efficiency than those fabricated at a low temperature of 520 degrees which is commonly used for CIGS solar cells fabrication taking into account the strain point of SLG. The CIGS solar cell fabricated on the developed glass substrate showed higher PV performance than on the SLG with SiO2 substrate processed on 580 degrees. As a reason of the difference, over 15μm warpage was observed in 30mm length of the SLG substrate, while less than 1μm of warpage occurred in the developed substrate. Our best cell so far is a 0.538-cm2 aperture-area efficiency 17.5% using the developed substrate adapting 580 degrees of process temperature.