Shintaro Kobayashi

Highly (0001)-oriented Al-doped ZnO polycrystalline films on amorphous glass substrates

Very thin aluminum-doped zinc oxide (AZO) films with a well-defined (0001) orientation and a surface roughness of 0.357 nm were deposited on amorphous glass substrates at a temperature of 200 °C by radio frequency magnetron sputtering, which are promising, particularly in terms of orientation evolution, surface roughness, and carrier transport, as buffer layers for the subsequent deposition of highly (0001)-oriented AZO polycrystalline films of 490 nm thickness by direct current (DC) magnetron sputtering. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. DC magnetron sputtered AZO films on bare glass substrates showed a mixed (0001) and the others crystallographic orientation, and exhibited a high contribution of grain boundary scattering to carrier transport, resulting in reduced Hall mobility. Optimizing the thickness of the AZO buffer layers to 10 nm led to highly (0001)-oriented bulk AZO films with a marked reduction in the above contribution, resulting in AZO films with improved Hall...

Epitaxial thin films of p-type spinel ferrite grown by pulsed laser deposition

Epitaxial thin films of Ge-substituted Fe3O4 were grown on α-Al2O3 (001) using pulsed laser deposition, and their electrical and magnetic properties were investigated. A target with excess Ge compositions was employed for the film growth to compensate for the volatilization of germanium oxides during the high-temperature deposition. P-type conduction was achieved for the Ge-rich films. The Neel temperature of all the films was above 300 K. Furthermore, the films exhibited an anomalouos Hall effect at 300 K, suggesting that the carriers in the films are spin-polarized.

Double-layer fabrication of cubic-manganites/hexagonal-ZnO on various substrates by ion beam sputtering, and variable electrical property

Hetero double-layers of LaBaMnO3 (LBMO)/ZnO were fabricated by ion beam sputtering on substrates of MgO, sapphire (SP), LaAlO3 (LAO), and SrTiO3 (STO). All the surfaces of substrates, ZnO and LBMO have step-terrace morphology. The p-LBMO/n-ZnO/SP shows junction rectification at different temperatures. The junction resistance follows from colossal magnetoresistance (CMR) of LBMO based on DEC model. The different LBMO/ZnO junctions on the different substrates show different junction behaviors at room temperatures. LBMO/ZnO/STO has the largest rectification factor of 210. After running measurement currents, LBMO/ZnO/STO shows current?voltage (I?V) switchings. LBMO/ZnO/MgO shows very clear switching and large hysteresis between upward and downward voltage sweeps. These are interpreted by CMR and DEC model, and phase separation. The switching is caused by disconnection of percolation path consisting of ferromagnetic metallic grains. The higher resistant state cannot be quickly transformed back to the lower resistant state during the downward sweep.

Electrochromic properties of single-crystalline tungsten trioxide films grown by molecular beam epitaxy

C-axis oriented single-crystalline monoclinic WO3 films were grown by molecular beam epitaxy, and studied their properties by fabricating solution-gate electrochromic devices in H2SO4 aqueous solution. These devices exhibited reversible change in the following three states; a colored state with low electrical resistivity after biasing a positive gate voltage, a transparent state with low electrical resistivity after biasing a negative gate voltage, and another transparent state with high electrical resistivity after thermal annealing. At the former two states, the c-axis length of the monoclinic WO3 films was found to expand slightly compared with that of the latter state after annealing. These results were consistently explained in terms of the protonation and hydration of WO3.

Hetero-epitaxial Growth of Cubic La(Sr)MnO3 on Hexagonal ZnO, In-Plane Orientations of La(Sr)MnO3 (001), (110), and (111) Phases

Hexagonal ZnO was grown on a hexagonal (001) sapphire substrate. Then cubic La(Sr)MnO3 (LSMO) was grown on a ZnO underlayer by ion beam sputtering at various substrate temperatures to obtain double-layers of LSMO/ZnO and to investigate hetero-epitaxial orientation growth. Out-of-plane (001)-oriented ZnO was grown with an in-plane orientation of [100](001)ZnO ∥ [110](001)sapphire (B-mode). Three phases of LSMO with out-of-plane (001), (110), and (111) orientations were grown on (001) ZnO generally. However, each single phase of LSMO could be grown only by controlling the deposition conditions. We achieved the in-plane orientation growth of LSMO, [110](001)LSMO ∥ [100](001)ZnO (B-mode), [10](110)LSMO ∥ [100](001)ZnO (A-mode), and [10](111)LSMO ∥ [110](001)ZnO (B-mode). The A-mode is defined as the mode in which both in-plane fundamental axes are parallel. The B-mode entails no parallel axes. The LSMO(001)- and (110)-grains have three equivalent in-plane domains, and the LSMO(111)-grain has a single domain. Lattice matching calculation can be used to partially interpret the orientation growth.

Effects of the erosion zone of magnetron sputtering targets on the spatial distribution of structural and electrical properties of transparent conductive Al-doped ZnO polycrystalline films

We investigated the effects of the erosion zone of magnetron sputtering (MS) targets on the deposition rates of magnetron-sputtered Al-doped ZnO (AZO) polycrystalline films and on the structural and electrical properties of the resulting AZO films. We deposited AZO films on glass substrates at a substrate temperature of 200 °C, which were placed parallel to the target surface, by radio frequency (RF), direct current (DC) or RF-superimposed DC (RF/DC) MS. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. The structural and electrical properties were investigated by X-ray diffraction and Hall-effect measurements, respectively. For RF-magnetron-sputtered AZO films, the results of the electron probe microanalyzer and photo luminescence measurements show that the effects of the erosion zone of the targets on the properties were very small at any given substrate position, resulting in small differences in the structural and electrical properties among the AZO films grown at different substrate positions. In the case of DC-magnetron-sputtered AZO films, residual damage owing to recoiling Ar atoms was found, particularly for the films grown at substrate positions in the area opposite to the erosion zone of the targets. This gives rise to the formation of nonradiative recombination centers, low carrier concentration, and high contribution of grain boundary scattering to carrier transport owing to poor alignments between columnar grains, resulting in low Hall mobility. In this study, we prove that the use of the RF/DC MS technique is effective in reducing the amount of residual damage.

Protonation-induced change on optical, electrical, and structural properties of epitaxial WO 3 films

Protonation and deprotonation processes of c-axis oriented monoclinic WO3 epitaxial films were studied by fabricating a solution-gate device operated in H2SO4 aqueous solution. The protonation process by positive gate biasing brought about an optical transmittance decrease in visible (VIS) and near-infrared (NIR) regions. The deprotonation process by negative gate biasing recovered the decrease in VIS region while that in NIR one remained. The decrease in NIR region was stable at room temperature in air over several weeks. By analyzing the data on X-ray diffraction and Hall effect measurements, the decrease in VIS region was related to the formation of tetragonal HxWO3 tungsten bronze and that in NIR region was related to the formation of orthorhombic WO3·0.33H2O tungsten hydrate.