Showing papers by "Masahiro Orita published in 2002"

Natural-superlattice homologous single crystal thin film, method for preparation thereof, and device using said single crystal thin film

Abstract: A natural superlattice homologous single crystal thin film, characterized in that it comprises a composite oxide which is represented by the formula M1M2O3(ZnO)m, wherein M1 is at least one of Ga, Fe, Sc, In, Lu, Yb, Tm, Er, Ho and Y, M2 is at least one of Mn, Fe, Ga, In and Al, and m is a natural number of 1 or more, and has been grown epitaxially on an epitaxial thin film formed on a single crystal substrate, or on said single crystal substrate from which said epitaxial thin film has disappeared, or on a ZnO single crystal; a method for preparing the natural superlattice thin film which comprises depositing the composite oxide, and diffusing the resultant laminated film by heating it. The natural superlattice homologous single crystal thin film is suitably used in an optical device, an electronic device, an X-ray optical device and the like.

Surface morphology and crystal quality of low resistive indium tin oxide grown on yittria-stabilized zirconia

Abstract: Highly electrically conductive (resistivity: 1×10−4 Ω cm) indium tin oxide (ITO) thin films were grown heteroepitaxially on (111) and (100) surfaces of yttria-stabilized zirconia (YSZ) at a growth temperature of 900 °C using a pulsed-laser deposition technique, and their crystal quality and surface morphology were evaluated by using high-resolution x-ray diffraction and an atomic force microscope. Higher growth temperature, the use of a cleaved surface as a substrate, and an increase in oxygen pressure are essential to fabricate an ITO thin film with an atomically flat surface. The full width at half maximum of out-of-plane x-ray rocking curves of ITO (222) grown on YSZ (111) and of ITO (400) on YSZ (100) were 54 and 720 arcseconds, respectively, which indicates that (111)-oriented ITO films exhibited higher crystal quality than (100)-oriented ITO films. The ITO film was grown on YSZ (111) by the three-dimensional spiral growth mode, with flat terraces and steps corresponding to (222) plane spacing of 0.2...

Heteroepitaxial growth of a wide-gap p-type semiconductor, LaCuOS

Abstract: High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 °C results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films.

Electronic structure and transport properties in the transparent amorphous oxide semiconductor 2 CdO ⋅ GeO 2

Abstract: An amorphous $2\mathrm{CdO}\ensuremath{\cdot}{\mathrm{GeO}}_{2}$ thin film with a band gap of 3.4 eV can be converted from an insulator (conductivity $\ensuremath{\sim}{10}^{\ensuremath{-}9}{\mathrm{S}\mathrm{}\mathrm{cm}}^{\ensuremath{-}1})$ into a degenerate semiconductor $(\ensuremath{\sim}{10}^{2}{\mathrm{S}\mathrm{}\mathrm{cm}}^{\ensuremath{-}1})$ by carrier doping with ion implantation without significant loss in visible transparency. An interesting feature of this material's transport property is its Hall mobility: a pn sign anomaly, which is commonly observed for amorphous semiconductors, is not seen. The estimated Hall mobility is \ensuremath{\sim}10 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which is larger by several orders of magnitude than that of conventional amorphous semiconductors, and is comparable to that in the polycrystalline form. The electronic structure was investigated to understand these features through direct observation of the density of states (DOS) of the conduction band by inverse-photoelectron spectroscopy and molecular orbital calculation of the DOS for a model cluster justified by x-ray structural analysis combined with molecular dynamics and reverse Monte Carlo simulations. Although x-ray structural analysis revealed that disorder in the correlation between Cd-Cd ions was distinctly seen in the amorphous state, the DOS of the conduction band bottom was almost the same in the crystalline and amorphous states. Cluster calculations demonstrated that the bottom of the conduction band is primarily composed of Cd $5s$ orbitals. The characteristic transport properties in this material, such as large electron mobility, may be understood by considering that the magnitude of overlap between the $5s$ orbitals of neighboring ${\mathrm{Cd}}^{2+}$ is large and comparable to that in the crystal and insensitive to the structural randomness inherent to amorphous states.

Electronic structure and optical properties of SrCu2O2

Abstract: The electronic structure of SrCu2O2, a wide gap (∼3.3 eV) p-type oxide semiconductor, was examined by photoelectron and optical spectroscopy. The spectroscopy results were compared with the energy band structure calculated by the local density approximation method to clarify the origins of p-type conductivity in this material. Despite the significant difference in band gap energy, the basic electronic structure around the band gap region was found to be quite similar to that of Cu2O. Thus, the 3d−4sp orbital of Cu+ ion is hybridized with the 2p orbital of ligand O2− ions due to the covalency of Cu–O bonds, to form states near the valence band maximum; the conduction band minimum is predominantly composed of the hybridized orbital of Cu 4sp and O 2p, forming the direct band gap at Γ point. A sharp absorption band observed near the fundamental absorption edge is likely attributable to an exciton. Although the corresponding exciton emission was not observed near the absorption edge, a blue-green emission ban...

Electrical conductivity control in transparent p-type (LaO)CuS thin films prepared by rf sputtering

Abstract: Thin films of wide-gap p-type (LaO)CuS were prepared by rf sputtering followed by postannealing. Undoped (LaO)CuS films are electrically insulating and emit ultraviolet (UV) light arising from excitons at room temperature upon photoexcitation. The hole carrier concentration in the film was controlled in the range ∼1015 to ∼1020 cm−3 by doping Sr2+ ions to replace La3+. The hole concentration reached a maximum of 2.7×1020 cm−3 at a doping level of 3 at. % Sr2+ ions, and then the carrier generation efficiency was ∼50%. These results demonstrate that the (La1−xSrxO)CuS films are potentially applicable to optoelectronic devices operating at short wavelengths because a transparent p electrode, as well as UV-emitting layers, may be fabricated using this material by controlling the doping level.

NbO2F: An oxyfluoride phase with wide band gap and electrochromic properties

Abstract: It was demonstrated that the oxyfluoride NbO2F with an ReO3-type structure is a promising electrochromic material with wide band gap. Measurement of diffuse reflectance spectra revealed the optical band gap of NbO2F (∼3.1 eV) to be wider than that of WO3 (∼2.6 eV). It was rendered electron conductive by heating at 770 K in the presence of Pt powder in H2. Electrical conductivity for the blue-colored sintered sample of NbO2F obtained was 6×10−3 S cm−1 at 300 K, and a temperature dependence showed the semiconductive behaviors. A reversible electrochromism between pale-blue and deep-blue-color was found in H2SO4 and Na2SO4 aqueous solution.

Epitaxial growth of transparent conductive oxides

Abstract: Transparent conductive oxides of ITO, ZnO, β-Ga2O3, and CuGaO2 and SrCu2O2 were grown on single crystal substrates of α-Al2O3 and YSZ by pulsed-laser deposition, and their crystallinity was evaluated by using high-resolution X-ray diffraction and electron microscope Heteroepitaxial growth was observed in spite of relatively large lattice mismatches between film and substrate A few disordered layers were generated automatically at film/substrate boundary, played buffer layer to suppress propagation of edge dislocations

Carrier doping into MgIn2O4 epitaxial thin films by proton implantation

Abstract: Epitaxial thin films of MgIn2O4 were grown on MgO(100) single-crystal substrates through the pulsed laser deposition technique X-ray diffraction measurements revealed film orientations MgIn2O4(h00)//MgO(h00) and MgIn2O4(0k0)//MgO(0k0), respectively Proton implantation was applied to generate carrier electrons in the films The electrical conductivity of the as-deposited films is below ∼10−7 S cm−1 at room temperature The maximum conductivity of ∼70 S cm−1 was obtained by the implantation Hall voltage measurements revealed that H+ implantation causes carrier generation in proportion to H+ fluence without reduction of electron mobility Following the post-annealing process resulted in further enhancement of the conductivity in each H+-implanted film, as conductivity and generation efficiency were found to increase up to ∼2×102 S cm−1 and ∼95% at the maximum, respectively This differs from the behavior of polycrystalline films in which conductivity decreased by post-annealing due to a decrease in the Hall mobility of electrons Thus it is concluded that crystal quality is crucial for heavy carrier doping by ion implantation, especially when utilizing post-annealing treatments to enhance the carrier generation efficiency without reduction of the Hall mobility

Reactive Solid-Phase Epitaxy: ∼ A novel growth method for single-crystalline thin films of complex oxides with superlattice structure ∼

Abstract: We have developed a novel growth method for single-crystalline film of natural superlattice oxides and named the method “Reactive Solid-Phase Epitaxy (R-SPE).” Single-crystalline thin films of homologous series In-GaO3(ZnO)m (m=integer) are fabricated by the R-SPE method and its growth mechanism, especially a role of ZnO epitaxial layer, is clarified. High-temperature annealing of bi-layer films consisting of an amorphous InGaO3(ZnO)5 layer deposited at room temperature and an epitaxial ZnO layer on YSZ substrate allows for the growth of single-crystalline film with a controlled chemical composition. The ZnO layer plays an essential role in determining the crystallographic orientation, while the thickness ratio between the two layers controls the film composition.