In the recent development of the studies in iron-based superconductors, high-pressure experiments have been played an important role. Large enhancement of Tc with applying pressure and pressure-induced superconductivity were reported in LaFeAsO1-xFx. In this work, electrical, magnetic and structural measurements on 1111 type Ca(Fe1-xCox)AsF and 11 type Fe(Se1-xTex)0.92 under high pressure have been performed. For Ca(Fe1-xCox)AsF, the substitution of Co suppressed the magnetic and structural transitions and raised superconductivity. Pressure-induced superconductivity was observed for x = 0.0 and 0.05. The highest Tc was obtained in parent compound under high pressure, in contrast to LaFeAsO1-xFx. These results suggest that the substitution of Co increases carrier concentration and induces disorder in the FeAs superconducting layer. For FeTe0.92, pressure-induced superconductivity was not detected under high pressure up to 19 GPa, although the resistive anomaly due to the structural and magnetic phase transition was suppressed by applying pressure.

Processes and preparation conditions for growing epitaxial thin films of Cu-based, layered oxychalcogenides LnCuOCh (Ln = La, Ce, Pr or Nd; Ch = S1-xSex or Se1-yTey) are reported. Epitaxial thin films on MgO (001) substrates were prepared by a reactive solid-phase epitaxy method. Four-axes high-resolution x-ray diffraction measurements revealed that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO and the internal stress of the crystalline lattices in the films are relaxed during thermal-annealing process of the reactive solid-phase epitaxy. Furthermore, except for CeCuOS, systematic variations in the lattice constant by chalcogen or lanthanide ion substitutions were observed. These results demonstrated that the reactive solid-phase epitaxy is an efficient technique for fabricating LnCuOCh epitaxial films.

A new method to convert 12CaO7Al2O3 (C12A7) thin films to electronic conductor by hot Ar+ ion implantation has been developed and its mechanism is discussed. It was found that hot Ar+ ion implantation extruded free O2- ions in C12A7 films by kick-out effects at fluences higher than 1×1017 cm−2, which left electrons in the cages embedded in C12A7 crystal and produced high concentration F+-like centers (∼1.4×1021 cm−3). The resulting films show coloration and persistent electronic conduction with conductivities up to ∼1 Scm−1. On the other hand, fluences less than 1×1017 cm−2 kept the films transparent and insulating.

Electronic structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, Te) and La2CdO2Se2, were studied using ab-initio band calculations in relation to their optical and electronic properties. It was found that the dispersions of the top valence bands are much smaller in Γ-Z direction than in Γ-X direction, indicating that the electronic structure is highly two-dimensional, and that holes are confined in the CuCh or CdSe layers. The two-dimensional electronic structure is supported experimentally by staircase-like structure observed in optical absorption spectra at 10 K associated with two excitonic absorption peaks split by spin-orbit interaction of Ch ions. La2CdO2Se2has the largest bandgap due to the two-dimensional network structure of CdSe tetrahedra.

Carrier transport properties and electronic structure of an n-type transparent oxide semiconductor, InGaO3(ZnO)5, were investigated using single-crystalline thin films. Room-temperature Hall mobility strongly depends on carrier concentration, and rapidly increased from ∼ 2 cm2(Vs)-1 to > 10 cm2(Vs)-1 around the carrier concentration (Nth ∼3 × 1018 cm−3. This change is associatedwith insulator-metal transition. These results are explained by a model similar to Anderson localization, in which shallow semi-localized states are formed originating from random distribution of Ga3+ and Zn2+ ions in the intrinsic crystal structure of InGaO3(ZnO)5. The present conclusion suggests that electron densities larger than Nth are necessary to attain high performances in drift carrier devices fabricated using InGaO3(ZnO)5. It was demonstrated that transparent filed-effect transistors exhibited good performances such as a “normally-offcharacteristics”, an on/off current ratios as large as 105 and a field-effect mobility ∼80 cm2(Vs)-1when high-k material, amorphous HfOx, was used as a gate insulator.

We examined the densification of 12CaO · 7Al2O3 (C12A7), which has a microporous lattice framework with a cavity of approximately 0.4 nm. Fully densified translucent ceramics are obtained when hydrated C12A7 powders prepared as precursors are sintered in a dry oxygen atmosphere at 1300 °C. The average transmittance between 400–800 nm for 1-mm-thick samples is improved up to approximately 70% with a 48-h increase in the sintering time. Dissociation of water molecules at the grain–pore interface into grains which entraps hydroxide ions in the crystallographic cage, and the release of the hydroxide ions into the atmosphere during the sintering procedure are considered to play crucial roles in the pore-annihilation processes.

An amorphous p-type conductive oxide semiconductor was created based on a mother crystalline material, a p-type conductive ZnRh2O4 spinel. The amorphous film of ZnRh2O4 was deposited by an rf sputtering method. Seebeck coefficient was positive, +78 μVK-1, indicating that major carrier is a positive hole. A moderate electrical conductivity (2 S cm-1 at room temperature) for a p-type semiconductor was observed. Optical band gap was estimated to be 2.1 eV. P-n junction diodes with a structure of Au / a-ZnRh2O4 / a-InGaZnO4 / ITO fabricated on glass substrates, operated with a good rectifying characteristics, a rectification current ratio at ± 5V of ∼103. The threshold voltage was 2.1 eV, which corresponds to the band gap energy of the amorphous ZnRh2O4. This is the first discovery of a p-type amorphous oxide and the demonstration of p-n junction all composed of amorphous oxide semiconductors.

We report the first electronic conduction in main group metal oxides (MGOs) or light-metal oxides, which are represented by alkaline-earth oxides, alumina, and silica. They are believed to be never converted to an electronic conductor. One of the MGOs, 12CaO·7Al2O3 (C12A7), has optical transparency, but electrical insulation limits intrinsic form. It is characterized by sub-nanometer sized cages in the lattice framework. Hydride ion, H- was incorporated into the cage by a thermal treatment of C12A7 single crystals or ceramics in hydrogen atmosphere. The product, C12A7:H, was colorless, transparent and a good insulator having electronic conductivity less than 10-10 S·cm-1. We found that the C12A7:H exhibits a coloration of yellowish green corresponding to optical absorptions at 2.8 and 0.4 eV with simultaneous conversion into an electronic conductor with 0.3 S·cm-1 at 300 K upon a irradiation of ultraviolet light. The conductive state continued even after the irradiation was stopped. Inversion to the insulator occurred when heated more than ∼320°C accompanying with rapid decay of the optical absorptions. When temperature rose above 550°C, H2 gas was released from the sample and the photosensitivity was lost. We consider that high concentration of F+-like centers are created by photo-released electrons from the H- anions being captured by empty cages. Further, a migration of the electrons at the F+-like centers may be responsible for the conduction. The visible light absorption loss is estimated to be only 1% for a 200 nm thick conductive C12A7:H films. The present properties provides novel applications such as direct optical writing of conducting wires on insulating transparent media.

Epitaxial films of LaCuOS, a wide gap p-type semiconductor, were grown on yittria- stabi-lized-zirconia (YSZ) (001) or MgO (001) substrates by a reactive solid phase epitaxy (R-SPE) method. Crystal quality, electrical and optical properties on the epitaxial films on each substrate are examined in this paper. Achievement of the heteroepitaxial growth of LaCuOS on the MgO (001) substrate improves optical properties of LaCuOS such as spectral bandwidths and emission intensity, suggesting that the MgO (001) substrate is more preferable than the YSZ (001) for epitaxial growth substrate for LaCuOS.

Several Cu(I)-containing layered oxysulfides were selected as candidates for wide-gap p-type semiconductors by extending a concept of a materials design for transparent p-type conducting oxides. The electrical and optical properties of the selected oxysulfides were investigated, and their electronic structures were analyzed by energy band calculations. LaCuOS, Sr2Cu2ZnO2S2 and Sr2CuGaO3S were found to be wide-gap p-type semiconductors, and LaCuOS showed the largest energy gap (Eg=3.1eV) among these layered oxysulfides. It was also found that LaCuOS shows band edge emission under uv excitation at room temperature, which is consistent with the results of the energy band calculations that LaCuOS has a direct-allowed-type energy gap at Γ point. In further materials research, analogous layered oxychalcogenides such as LaCuOSe and LnCuOS (Ln=Pr, Nd) were found to show similar optical and electrical properties to those of LaCuOS. Therefore, it is considered that the layered crystal structure and the electronic structure are responsible for the wide-gap p-type conductive properties in these materials.

F2 excimer laser (157 nm) irradiation was applied to modify sol–gel-derived amorphous Al2O3 thin films at ambient temperature. The surface morphology and density of the film were significantly altered by the laser irradiation (power: 2 mW/cm2/pulse). The surface properties of the film were also changed from hydrophilic to hydrophobic. These alterations were not observed when using ArF excimer laser (193 nm) irradiation at the same laser power as that of the F2 laser. It was found that the changes induced by F2 laser irradiation mainly arose from the direct photoexcitation of C = O groups in ethylacetoacetate, which was added as a chelating agent of aluminum-alkoxides. Consequently, photochemical reactions of the Norrish-type occur, resulting in the formation of hydrocarbon or olefin and the elimination of carbon monoxide (CO) or decomposition products. The elimination of CO is considered to cause the marked change in structure and surface properties of the film. Patterning of the gel films was successfully performed by using these findings.

Indium-tin-oxide films were grown hetero-epitaxially on YSZ surface at a substrate temperature of 900 °C, and their surface microstructures were observed by using atomic force microscopy. ITO films grown on (111) surface of YSZ exhibited very high crystal quality; full width at half maximum of out-of-plane rocking curve was 54 second. The ITO was grown spirally, with flat terraces and steps corresponding to (222) plane spacing of 0.29 nm. Oxygen pressure during film growth is another key factor to obtain atomically flat surfaced ITO thin film.

A transparent oxide semiconductor with delafossite structure, CuInO2, was found to exhibit both p-type and n-type conduction by doping of an appropriate impurity and tuning of proper film-deposition conditions. Thin films of Ca-doped or Sn-doped CuInO2 were prepared on -Al2O3 (001) single crystal substrates by pulsed laser deposition method. The films were deposited at 723 K in O2 atmosphere of 1.0 Pa for the Ca-doped films or 1.5 Pa for the Sn-doped films. The positive sign of the Seebeck coefficient demonstrated p-type conduction in the Ca-doped films, while the Seebeck coefficient of the Sn-doped films was negative indicating n-type conductivity. The electrical conductivities of Ca-doped and Sn-doped CuInO2 thin films were 2.8×10−3 S·cm−1 and 3.8×10−3 S·cm−1, respectively, at 300 K. The optical band gap of each film was estimated to be ∼3.9 eV. Since CuInO2 exhibited bipolarity in electrical conduction, transparent p-n homojunctions based on CuInO2 were fabricated on (111) surface of yttria-stabilized zirconia single-crystal substrates. The structure of the diode was In2O3:Sn / n-CuInO2:Sn / p-CuInO2:Ca / In2O3:Sn electrode on the substrate. The contact between the n-and p-type CuInO2 semiconducting oxides was found to be rectifying. The turn-on voltage was ∼1.8 V.

Deep ultraviolet transparent and electrical conductive β-Ga2O3 film was prepared on SiO2 glass substrate. The film had electric conductivity of 1 Scm−1, carrier density of 1.4 × 1019cm−3 and mobility of 0.44 cm2V−1s−1. Internal transmittance at wavelength of 248 nm exceeded 50 %. Conductivity was enhanced at an oxygen pressure less than 10−4 Pa, and transparency appeared at a substrate temperature over 800 °C.

High quality ITO thin films were grown hetero-epitaxially on extremely flat substrate of (001) YSZ by a pulsed laser deposition technique at a substrate temperature of 600°C. The crystal orientation relationship between the film and YSZ were confirmed as ITO (001) // YSZ (001) and ITO (010) // YSZ (010), respectively, by HR-XRD and HR-TEM. The carrier densities of the films were almost equal to Sn02 concentration in the films. That is, almost all the doped Sn4+ ions were activated to release electrons to the conduction band. The carrier densities of the films were enhanced up to 1.9×1021cm−3, while the Hall mobility showed a slight, almost linear, decrease from 55 to 40cm2V−1s−1 with increasing SnO2 concentration. The low resistivity is due to larger electron mobility, which most likely resulted from good crystal quality of the films. The optical transmissivity of the film exceeded 85% at wavelengths from 340 to 780nm.

Room temperature operation of UV LED is realized for the first time using a hetero p-n junction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Ni/SrCu2O2/ZnO/ITO multi-layered film was epitaxially grown on an extremely flat YSZ (111) surface by a PLD. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n junction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 1.5V. A relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO.