Hideyuki Maki

Nitrogen Ion Behavior on Polar Surfaces of ZnO Single Crystals

Nitrogen radicals were irradiated on the (0001) and (0001) surfaces of the ZnO single crystals, and the stability and the states of N ions on the surfaces were investigated by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). On the (0001) surface, many N ions replaced the O ions of ZnO during annealing in oxygen gas or vacuum after nitrogen treatment. However, few N ions replaced the O ions of ZnO on the (0001) surface. These results suggest that the nitrogen doping on the (0001) surface is more effective than that on the (0001) surface.

Control of surface morphology of ZnO (0 0 0 1̄) by hydrochloric acid etching

Chemical etching mechanism of ZnO (0001) polar surface was investigated to control surface morphology. ZnO single crystal surfaces were etched with a HCl solution, and the morphology changes were observed with an atomic force microscope; hexagonal rings formed on the (0001) surface due to relatively rapid etching of its terrace faces in comparison with kinks or steps of etch pits. Square patterns of Pt thin films were periodically sputtered on ZnO surfaces using a mask; after etched in HCl solution for 1 day, many hillocks formed with keeping the original square shape. The pattern changed into pointed pyramidal shape after long-duration etching, suggesting that the shapes were influenced by the etching along not only the vertical direction, but also a little of the parallel direction to the surface.

The lattice relaxation of ZnO single crystal (0001) surface

Abstract Wurtzite-type ZnO is a polar crystal whose (0001) surface is terminated by Zn ions. The ZnO single crystals were etched for the surfaces to have regular step/terrace structures and analyzed by reflection high-energy electron diffraction (RHEED) and coaxial impact-collision ion scattering spectroscopy (CAICISS). RHEED patterns showed that the surface had a 1×1 structure. From the polar angle dependence of CAICISS time-of-flight spectra for Zn ions, a peak due to a focusing effect for fifth-layer Zn ions by the first-layer Zn ions was observed at 49.5°, which was 2° higher than the corresponding angle simulated for a non-relaxed surface. These results indicated that the uppermost Zn ions were surface-normally relaxed toward the outside of the surface by about 7% of the c -axis lattice constant.

An Electrically Driven, Ultrahigh-Speed, on-Chip Light Emitter Based on Carbon Nanotubes

The integration of high-speed light emitters on silicon chips is an important issue that must be resolved in order to realize on-chip or interchip optical interconnects. Here, we demonstrate the first electrically driven ultrafast carbon nanotube (CNT) light emitter based on blackbody radiation with a response speed (1-10 Gbps) that is more than 10(6) times higher than that of conventional incandescent emitters and is either higher than or comparable to that of light-emitting diodes or laser diodes. This high-speed response is explained by the extremely fast temperature response of the CNT film, which is dominated by the small heat capacity of the CNT film and its high heat dissipation to the substrate. Moreover, we experimentally demonstrate 140 ps width pulsed light generation and real-time optical communication. This CNT-based emitter with the advantages of ultrafast response speeds, a small footprint, and integration on silicon can enable novel architectures for optical interconnects, photonic, and optoelectronic integrated circuits.

Local Change of Carbon Nanotube-Metal Contacts by Current Flow through Electrodes

The new processes of current flow through electrodes at carbon nanotube (CNT)-electrode junctions were carried out to change the contact resistance of CNT conductors and the tunnel barriers of CNT quantum dots. When the current flow process was applied to CNT conductors with the Au/Ti electrodes deposited on multiwall CNTs (MWNTs), the contact resistance markedly decreased. This is caused by the formation of titanium carbide (TiC) at the electrode-nanotube junction due to the strong interaction between Ti and nanotubes. This process is useful for obtaining the CNT conductor with low contact resistance. Meanwhile, when the current flow process was applied to single-wall CNT (SWNT) quantum dots with Au-Ag alloy electrodes, the contact resistance hardly changed due to the weak nanotube-Au-Ag interaction. However, in the electrical measurement of these samples at low temperatures, a quantum dot with a strong confinement was obtained after the current flow process. Therefore, this process can be also used for the change of tunnel junctions of CNT quantum dots.

Short-Wavelength Electroluminescence from Single-Walled Carbon Nanotubes with High Bias Voltage

Short-wavelength electroluminescence (EL) emission is observed from unipolar and ambipolar carbon nanotube field-effect transistors (CNFETs) under high bias voltage. EL measurements were carried out with an unsuspended single-walled carbon nanotube (SWNT) in high vacuum to prevent the oxidation damage induced by current heating. Short-wavelength emission under high bias voltage is obtained because of the Schottky barrier reduction and the electric field increase in a SWNT. The simultaneous measurements of transport and EL spectra revealed the excitation mechanism of impact excitation or electron and hole injection dependent on the conduction type of unipolar or ambipolar characteristics. In addition to the EL emission, blackbody radiation was also observed in a p-type CNFET. Taking into account the device temperature estimated from blackbody radiation, the contribution of impact excitation and thermal effect to the exciton production rate was evaluated.

Photon antibunching in single-walled carbon nanotubes at telecommunication wavelengths and room temperature

We investigated the photoluminescence of individual air-suspended single-walled carbon nanotubes (SWNTs) from 6 to 300 K. Time-resolved and antibunching measurements over the telecommunication wavelength range were performed using a superconducting single-photon detector. We detected moderate temperature independent antibunching behavior over the whole temperature range studied. To investigate the exciton dynamics, which is responsible for the antibunching behavior, we measured excitation-power and temperature dependence of the photoluminescence spectra and lifetime decay curves. These measurements suggested an exciton confinement effect that is likely caused by high-dielectric amorphous carbon surrounding the SWNTs. These results indicate that SWNTs are good candidates for light sources in quantum communication technologies operating in the telecommunication wavelength range and at room temperature.

Electrically driven, narrow-linewidth blackbody emission from carbon nanotube microcavity devices

We report electrically driven narrow-linewidth blackbody emission from carbon-nanotube with Fabry-Perot microcavities. We fabricated two types of devices with microcavities consisting of either gold mirrors or distributed Bragg reflectors (DBR). Gold-mirror microcavity devices exhibit blackbody emission with narrowed full-width at half-maximum of ∼50 nm in contrast to the broad normal blackbody emission spectrum. The spectra from these devices can be explained by theoretical calculations accounting for the temperature-dependent intrinsic blackbody spectra and transmittance spectrum of the microcavity. Moreover, the DBR microcavity devices show a narrower resonant peak inside the photonic bandgap, compared with the gold-mirror microcavity device.

Electrochemically switchable photoluminescence of an anionic dye in a cationic metallo-supramolecular polymer

A fluorescent dye anion (sulforhodamine B, SRB) was successfully introduced into a Fe(II)-based metallo-supramolecular polymer (polyFe) by exchanging the counter anion. The exchange was confirmed by elemental analysis, UV-vis absorption spectroscopy, 1H-NMR spectroscopy, and FTIR spectroscopy. A solid-state device using the obtained polymer (polyFe–SRB) shows electrochromic properties based on the electrochemical redox of Fe(II) ions by applying ±2.8 V. The device shows electrochemical switching of photoluminescence at 584 nm by applying ±2.8 V. This switching was caused by the appearance and disappearance of the metal-to-ligand charge transfer (MLCT) absorption of polyFe–SRB. It is considered that the MLCT absorbs the excitation light at 560 nm or the emission from SRB.

Manipulation of Faraday rotation in Bi-substituted yttrium-iron garnet film using electromagnetic interaction between Au nanoparticles in two-dimensional array

The Faraday rotation in Bi-substituted yttrium-iron garnet thin films, in which an artificially fabricated Au nanoparticle array is embedded, is studied as a function of lattice spacing compared with the extinction spectra. With decreasing lattice spacing in the Au array, the wavelengths corresponding to the enhanced Faraday rotation and the extinction peak showed blueshifts in the same manner. This indicates that Faraday rotation can be manipulated by means of the wavelength shift of localized surface plasmon resonance that originates from the change in electromagnetic interaction between Au nanoparticles.