Shusaku Akiba

Buffer-enhanced room-temperature growth and characterization of epitaxial ZnO thin films

The room-temperature epitaxial growth of ZnO thin films on NiO buffered sapphire (0001) substrate was achieved by using the laser molecular-beam-epitaxy method. The obtained ZnO films had the ultrasmooth surface reflecting the nanostepped structure of the sapphire substrate. The crystal structure at the surface was investigated in situ by means of coaxial impact-collision ion scattering spectroscopy. It was proved that the buffer-enhanced epitaxial ZnO thin films grown at room temperature had +c polarity, while the polarity of high-temperature grown ZnO thin films on the sapphire was −c. Photoluminescence spectra at room temperature were measured for the epitaxial ZnO films, showing only the strong ultraviolet emission near 380nm.

Room-temperature epitaxial growth of indium tin oxide thin films on Si substrates with an epitaxial CeO2 ultrathin buffer

Abstract Room-temperature epitaxy of indium tin oxide (ITO) thin films was achieved on Si(111) substrates with an epitaxial CeO2 ultrathin buffer using a pulsed laser deposition technique. The epitaxial CeO2 buffer layer was also grown at room temperature. Reflection high-energy electron diffraction and pole figure X-ray diffraction analyses confirmed the formation of a double heteroepitaxial structure of ITO(111)/CeO2(111)/Si(111) with the epitaxial relationship of [−110]ITO//[−110]CeO2//[1–10]Si. The junction of [ITO: 100 nm thick/CeO2: 3 nm thick/p-Si(111)] fabricated at room temperature exhibited solar cell properties.

High-energy x-ray scattering in grazing incidence from nanometer-scale oxide wires

A method has been developed for analyzing the structure of crystalline nanowires deposited on a crystal surface. It combined in-air grazing-incidence surface scattering with high-energy synchrotron x-ray scattering. The technique has allowed a direct reciprocal-lattice space mapping of the x-ray intensities scattered from ultrathin nanowires. Using this method, the sheet-shape diffraction emanating from ultrathin NiO wires was observed. The average nanowire–nanowire distance of 46 nm and a crystallographic domain size of approximately 7.5 nm across the nanowire were evaluated.

Formation of a nanogroove-striped NiO surface using atomic steps

A nanogroove-striped pattern was formed on a NiO film surface. The periodic nanopattern was successfully obtained over the entire surface via high-temperature annealing of the epitaxial NiO thin film, which was grown on an atomically stepped sapphire substrate at low temperature. The depth, width and interval of straight nanogrooves were about 3xa0nm, 35xa0nm and about 100xa0nm, respectively. The periodicity of the stripe agrees well with that of the atomic steps of the substrate.

Structural analysis of NiO ultra-thin films epitaxially grown on ultra-smooth sapphire substrates by synchrotron X-ray diffraction measurements

Abstract Crystallographic structures of nickel oxide (NiO) ultra-thin films epitaxially grown on ultra-smooth sapphire (0xa00xa00xa01) substrates have been analyzed using synchrotron X-ray diffraction. Growth behaviors of a NiO crystal domain along both an in-plane and an out-of-plane directions were able to be explained at nano-scale resolution. They were drastically changed around 10–15-nm thick film range. Thermodynamic factors on the nucleation and growth was dominant in an ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range.

Nanoscale epitaxial growth control of oxide thin films by laser molecular beam epitaxy—towards oxide nanoelectronics

Abstract The nanoscale growth control of oxide thin films, such as ferroelectric and magnetic materials, were explored by a novel technique based on nanoscale substrate engineering as well as atomic layer control via laser molecular beam epitaxy (laser-MBE). Atomic-scale analysis of the terminating layer of perovskite oxide films was performed by in situ coaxial impact-collision ion scattering spectroscopy. The novel heteroepitaxies that could be attained were: (1) the termination-regulated molecular layer-by-layer epitaxy of BaTiO3 and La0.7Sr0.3MnO3 thin films and (2) the step-decoration epitaxy resulting in the nanowire or nanodot structures of magnetic oxides such as (Mn, Zn) ferrite on ultrasmooth sapphire substrates with straight atomic steps.

Room-temperature growth of ultrasmooth AlN epitaxial thin films on sapphire with NiO buffer layer

Room-temperature epitaxy of AlN thin films on sapphire (0001) substrates was achieved by pulsed laser deposition using an epitaxial NiO ultrathin buffer layer (approximately 6 nm thick). Four-circle x-ray diffraction analysis indicates a double heteroepitaxial structure of AlN (0001)/NiO(111)/sapphire (0001) with the epitaxial relationship of AlN [10-10] ‖ NiO [11-2] ‖ sapphire [11-20]. The surface morphology of room-temperature grown AlN thin films was found to be atomically smooth and nanostepped, reflecting the surface of the ultrasmooth sapphire substrate with 0.2-nm-high steps.

Crystallinity of Nio nanowires grown at step edges of sapphire substrate

NiO nanowires were formed along atomic step edges on the ultrasmooth sapphire (0001) substrates by laser molecular beam epitaxy. From atomic force microscopy, the nanowires were found to be ~20 nm in width and ~0.5 nm in height along the straight, 0.2 nm-high step edges of the substrate. The crystal structure of NiO nanowires was examined by in situ coaxial impact collision ion scattering spectroscopy (CAICISS). The CAICISS results on the azimuth dependences of Ni signal for the NiO nanowires as well as NiO (111) epitaxial thin films indicate that the NiO nanowires were epitaxially grown with (111) orientation under in-plane stress.

Anisotropic electric conduction derived from self-organized nanogroove array on Li-doped NiO epitaxial film

The self-assembly formation of straight and periodic nanogroove arrays was carried out on the surface of Li-doped NiO thin films by use of atomic steps on the substrate. The nanostructure was formed by annealing Li-doped NiO (111) epitaxial thin film prepared on an atomically stepped α-Al2O3 (0001) substrate via pulsed laser deposition at room temperature. V-shaped nanogrooves, with a depth of ∼20nm and an open-end width of ∼50nm, were observed over the entire substrate and characterized by cross-sectional transmission-electron microscopy. The separation of the aligned nanogrooves was from 80to100nm, comparable to the size of the atomic steps on the substrate. Anisotropic electric conduction was definitely attained for the Li-doped NiO thin film with the nanogroove array. A resistance ratio up to about 100 was obtained for both directions parallel and perpendicular to the nanogrooves. The resultant anisotropy was considered to be mainly caused by anisotropic grain growth due to self-organized reconstructi...

Fabrication of Micropatterns on Sapphire Substrates via Room-Temperature Selective Homoepitaxial Growth Induced by Electron Beam Irradiation

The surface microstructure of sapphire (α-Al2O3) substrates could be constructed at room temperature via selective sapphire homoepitaxial growth induced by electron beam irradiation in laser molecular beam epitaxy. The films grew homoepitaxially on sapphire (1012) substrates at room temperature only in the region electron-irradiated during film deposition, while amorphous aluminum oxide films grew in the non-irradiated area. The surface micropatterns on sapphire substrates were easily obtained by the selective H3PO4 wet etching of the modified sapphire surface coated with crystalline and amorphous film. The microwall and microgroove could be fabricated on the sapphire substrates at room temperature.