Wakana Hara

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.

Atomically Stepped Glass Surface Formed by Nanoimprint

We investigated atomic-scale surface modifications of silicate glass by nanoimprint using an atomically stepped sapphire (α-Al2O3 single crystal) plate as nanopattern mold. The sapphire mold had regularly arranged straight atomic steps, with uniform height and terrace width of about 0.2 and 80 nm, respectively. During pressing, vertical positions of the sapphire mold and glass plate significantly affected the morphology of the imprinted glass surface. The nanopattern was transferred to the glass surface when the mold was set on the glass plate, while the nanowave pattern was formed on the glass surface when the glass plate was set on the mold.

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.

RECIPROCAL-LATTICE SPACE IMAGING OF X-RAY INTENSITIES DIFFRACTED FROM NANOWIRES

Yoshimoto Res. Group, Tokyo Institute of Technology [3]. We used two samples that were similar except for the nanowires’ orientations; in addition, both substrates had a miscut of approximately 0.1 . Sample A had nanowires almost perpendicular to the sapphire [1 0 1 0] direction; and sample B had nanowires nearly parallel to the [1 0 1 0] direction. The coordination is expressed using a hexagonal symmetry. The samples were grown using a pulsed laser ablation method. The surface morphology of sample A was observed by atomic force microscopy (AFM) in air (Fig. 1). The nanowires were found to be of the order of mm in length, which was surprisingly long, by scrutinizing hundreds of successive AFM images (scan size 10 μm × 10 μm, not shown here). The images indicate that the nanowires were grown along step edges and were approximately 20 nm in width and 0.5 nm (two atomic planes) in height. The density was a nanowire per 50 nm. H ere, the nanowires that we are interested in are very thin, narrow, and long. If the wires are 1D crystalline, the scattering pattern or diffraction domain (the Fourier transform) in the reciprocal-lattice space shows sheets (which are perpendicular to the wires and whose diameters are inversely proportional to the width s of the wires). Let us assume that the sheets elongate along the sample surface normal from bulkcrystal Bragg points due to a surface truncation effect. The Ewald sphere can simultaneously intersect some of the sheets accordingly (Fig. 2). We used possibly high-energy X-rays to excite many diffracted X-rays. Measurements were performed at the undulator

Nanoscale Smoothing in Crystallization of Amorphous Indium Tin Oxide Thin Films Induced by Glass Ultrathin Overlayer

Nanoscale smoothing in the crystallization of amorphous indium tin oxide (ITO) thin films was achieved using an ultrathin silicate glass overlayer (about 3 nm thick). An amorphous ITO thin film with an ultrathin glass overlayer were deposited by pulsed laser ablation using the ITO ceramics and silicate glass targets, respectively. The amorphous ITO film covered by the ultrathin glass layer was crystallized by annealing at 700 °C. When the ultrathin glass layer was removed by wet etching in aqueous H3PO4, an ultrasmooth crystallized ITO film surface appeared. The surface roughness of the crystallized thin film (about 0.3 nm rms) was one order of magnitude smaller than that of the ITO films crystallized with no glass overlayer (3.1 nm rms).

Fabrication of Optically Active Er3+-Doped Bi2O3–SiO2 Glass Thin Films by Pulsed Laser Deposition

We examined the fabrication conditions of optically active Er3+-doped Bi2O3–SiO2 glass thin films prepared by pulsed laser deposition using a glass target with a nominal composition (mol %) of 41Bi2O357SiO22Er2O3. The transparent glass films were obtained at 200 °C under 5 ×10-2 Torr O2. The glass films were found to possess a composition and a refractive index close to those of the bulk glass used as the target. The films exhibited a broad emission of Er3+ ions at around 1.5–1.6 µm as well as Er3+ upconversion-pumped fluorescence in the 500–700 nm range.

Nanoscale growth control of functional thin films on atomically surface-controlled substrates by laser MBE

The nanoscale growth control of functional ceramic thin films was examined by our originally developed technique, which was based on the nanoscale substrate engineering as well as atomic layer control via laser molecular beam epitaxy (laser-MBE). The atomically controlled surface of the substrate such as the ultrasmooth sapphire (single-crystal Al2O3) substrate with atomic steps and atomically smooth terraces was found to enhance atomically layer-by-layer growth as well as self-assembled nucleation along the atomic steps. The novel epitaxial growths could be attained on the physically or chemically controlled substrates, that is, (1) termination-regulated molecular layer-by-layer epitaxy, (2) step-decoration epitaxy resulting in the nanowire or nanodots, (3) room-temperature epitaxial growth of AlN, and (4) self-organized formation of the nanogroove-striped pattern on the film surface.