Table of contents

Vertical GaN Schottky barrier diodes (SBDs) were fabricated on freestanding GaN substrates with low dislocation density. High quality n-GaN drift-layer with an electron mobility of 930 cm² V^-1 s^-1 was obtained by optimizing the growth conditions by reducing the intensity of yellow luminescence using conventional photoluminescence measurements. The specific on-resistance (R_onA) and the breakdown voltage (V_B) of the SBDs were 0.71 mΩ cm² and over 1100 V, respectively. The figure of merit (V_B²/R_onA) was 1.7 GW/cm², which is the highest value among previously reported SBDs for both GaN and SiC.

A dopant cluster consisting of two antimony atoms facing each other in a six-member ring was found in the silicon crystals of a dopant concentration of 5×10²⁰ cm^-3, slightly above the critical value for generating electrically deactivated clusters. This cluster was detected with a 50-pm-resolution aberration corrected scanning transmission electron microscope. A large angle convergent beam, 30 mrad in semi-angle, yields section images with an improved depth of field of about 2 nm. The dopant clusters were discriminated from isolated antimony atoms by statistical analysis of image intensity. These clusters can cause electrical deactivation at critical dopant concentration.

The growth of InGaN/AlGaN multiple quantum wells (MQWs) structures is highly effective for realizing high quality semipolar (2021) active regions for green light emitting diodes (LEDs) and laser diodes (LDs). The use of AlGaN barriers significantly improved internal quantum efficiencies and the uniformity of the emission compared to InGaN or GaN barriers. 516 nm lasing wavelength was demonstrated on semipolar (2021) GaN substrates by introducing three periods InGaN/AlGaN MQWs and the AlGaN-cladding-free optical waveguide consisting of GaN cladding and InGaN guiding layers.

We analyse the effect of a magnetic field parallel to the growth axis of a biased semiconductor superlattice on the amplitude of the terahertz (THz) emission that follows a fast interband optical excitation. We predict a significant increase of the THz emission in the presence of a high field, associated with the freeze out of the in-plane motion of photoexcited carriers.

Nitride laser diodes are pivotal for optical data storage and laser projection but their performance is limited by large internal absorption losses. Here we quantify the internal loss due to free-carrier absorption using first-principles calculations and show that indirect free-carrier absorption is an important loss mechanism in nitride lasers. The dominant loss process is the phonon-assisted absorption by holes in the p-type layers. Parameters for the absorption coefficients have been extracted for use in device modeling. This work constitutes an important step towards the understanding of the efficiency problems in lasers and may assist the design of future devices.

The authors report on the quantum efficiency (QE) of UV-excited photoluminescence measured in SnO–ZnO–P₂O₅ glass developed as rare earth (RE)-free material for light emitting diode (LED) applications; we report what is, to the best of our knowledge, the highest value of QE ever reported. It is notable that the QE value of the present RE-free glass (∼90%) is comparable to that of RE-doped glass. For future LED applications, we have emphasized that the low-melting glass will be one of the most industrially favorable inorganic materials to replace organic sealants that suffer degradation by strong LED irradiation.

We report an extremely widely tuneable monochromatic Cherenkov phase-matched terahertz (THz) wave generator using a bulk lithium niobate crystal. This was achieved by optimizing the incident conditions of the pump beams, and the surface-emitting condition was produced by total internal reflection of the pump beam at the THz wave output surface of the nonlinear optical crystal. Effective generation at higher frequencies was realized by reducing the absorption inside the crystal, and as a result, we achieved a tuning range from 0.2 to 6.5 THz with a very simple configuration.

Organic p–i–n solar cells comprising metal-free phthalocyanine and fullerene, which was covered with an aluminum doped zinc oxide protection layer, were successfully operated for 42 days under white light (100 mW cm^-2) irradiation. Decreases in the short-circuit photocurrent density and conversion efficiency during this period were just 3 and 5%, respectively. These results suggest that organic solar cells possess sufficient potential for practical long-term durability.

A white light continuum was generated by focusing the terawatt 800 nm femtosecond laser pulse in Kr gas. The white light spectrum spanned in a broad wavelength range from 300 to more than 2200 nm. The white light continuum was used for the first time to perform direct absorption spectroscopy of CO₂ at around 2000 nm in a laboratory absorption cell. The present system can be used for measuring the CO₂ concentrations with an accuracy of 1–2 ppm (ca. 0.5%) in atmosphere for around 5.5 km propagation lengths through the air.

We fabricate a buckling structure by thermally depositing aluminum on a polymer surface and cooling it to room temperature, and utilize the structure as a microlens aggregate for organic light emitting diodes. The aspect ratio of the lens is increased by a multiple deposition process. We demonstrate that the characteristics of the outcoupled emission of this device are improved from various perspectives such as efficiency, emission angle, and reduced spectral change. These improved characteristics originate from randomized double-curvatured lens structure with full surface coverage.

The performance of discrete multilayer and continuously graded antireflection coatings for omnidirectional broadband applications are compared. It is shown that in practical cases where refractive index choices are constrained, discrete antireflection coatings can surpass the performance of continuously graded coatings by taking advantage of interference effects, which continuously graded coatings are expressly designed to avoid. A four-layer antireflection coating designed using a genetic algorithm is fabricated, and is experimentally shown to have reflectivity lower than what is achievable for continuously graded designs.

The electric control of the motion of multiple domain walls (DWs) is demonstrated by using the Pt/Co/Pt nanotracks with perpendicular magnetic anisotropy. Because of the weak microstructural disorders with a small DW propagation field (approximately a few mT), a purely current-driven DW motion is achieved in the creep regime at current densities less than 10⁷ A/cm² at room temperature. It is confirmed by using a scanning magneto-optical Kerr effect microscope that several DWs are simultaneously and identically displaced by the same distance in the same direction. Utilizing such DW motion, we succeeded in realizing the writing and transferring of random bits in a device prototype of 4-bit shift registers.

The dependence of threshold current on applied magnetic field for domain wall depinning from a pinning potential in Gd doped permalloy wires is measured using pulsed current measurements. By increasing the Gd concentration we find a marked reduction in threshold currents. This is shown to arise due to the enhanced non-adiabatic spin-transfer torque: we calculate the non-adiabaticity parameter β to be around 0.11 for 10% Gd concentration. On the other hand we show that the adiabatic spin-transfer torque is largely unaffected by Gd concentrations up to 10%.

We proposed a vertical InGaAs channel metal–insulator–semiconductor field effect transistor (MISFET) with an ultranarrow mesa structure, an undoped channel, and a heterostructure launcher. With the aim of obtaining a narrow mesa structure, we proposed the concept of performing selective undercut etching after dry etching. We fabricated the proposed device with a 60-nm-long and 15-nm-wide channel mesa structure. In the fabricated device, the observed drain current density was 1.1 A/mm. Because the channel mesa width was 15 nm, the drain current density per unit area was 7 MA/cm². Thus, a high current density was achieved for an ultranarrow mesa structure.

We studied a new method for large area imaging by Fourier transform holography in both soft and hard X-ray regions. In soft X-ray region, magnetic domain image of the perpendicular magnetized film was observed in the 8 µm area at an X-ray energy of 778 eV. In hard X-ray region, the method was applied to observe both the artificially patterned sample in 7 µm area and the cross-section of Cu-interconnect-line, at an X-ray energy of 5500 eV. The spatial resolution, estimated from the 10 to 90% intensity change, was 42 and 75 nm at 778 and 5500 eV, respectively.

We fabricated a device with a two-dimensional Si-nanodisk array (2D ND array) with spiking neurons. The 2D ND array was prepared using a 2D array of iron-oxide cores as a uniform mask and a defect-free chlorine neutral beam as an etcher. The transformation from a pulse input signal (voltage) to a decayed analog output (current) was clearly observed, which may have resulted from the random hopping of electrons in the 2D ND array. Additionally, these analog outputs could be integrated in this 2D array by applying consecutive pulse inputs.

To achieve three-dimensional super-resolution microscopy (3D-SRM) using fluorescence depletion, a two-color annular phase plate has been applied to a commercial laser-scanning microscope (LSM). When two-color beams passing through the plate are focused, only one of the beams is modulated, so that a minute 3D-dark-hole is generated at the focal point. Using these two beams for illumination lights for 3D-SRM, both the lateral and the axial resolutions overcame the diffraction limit. Inserting the plate into the optical pass for illumination in an LSM, the setup of 3D-SRM works as a very simple and compact analysis tool in life and material science.

Black metal absorbing light can be used for the light absorber material for a solar thermophotovoltaic system, which is expected to be a photovoltaic system in the next generation. Here we show that the fiberform nanostructured tungsten formed by helium irradiation absorbs the light from all angles of interest and is virtually black for a solar spectrum, from visible to near infrared wavelength; the absorptivity of the total solar power could be 98%. It is revealed that the nanostructure is formed by a novel process, self-growth of the helium nano-bubbles.

We present a simple and economical method to produce a potential open microfluidic polymeric device. Biomimetic superhydrophobic surfaces were prepared on polystyrene using a phase separation methodology. Patterned two-dimensional channels were imprinted on the superhydrophobic substrates by exposing the surface to plasma or UV–ozone radiation. The wettability of the channels could be precisely controlled between the superhydrophobic and superhydrophilic states by changing the exposure time. The ability of superhydrophilic paths to drive liquid flows in a horizontal position was found to be significantly higher than for the case of hydrophilic paths patterned onto smooth surfaces.

A long-wavelength infrared thermal imaging camera (IR camera) was applied to visually evaluate the thermal influence of defects in semi-insulating 6H-SiC substrates. Defects in substrates were rapidly and effectively detected by IR camera observation, and the dependence of the temperature on the defect size could be observed precisely. We have applied an IR camera to show clearly, for the first time, the change in heat propagation in areas of defects by observation of temperature distribution images in real time. Consequently, the IR camera can be considered as an effective technique for evaluating the thermal influence of defects.

An outstanding timing resolution of 75 ps at full width at half maximum (FWHM) has been achieved in coincidence measurement of positron annihilation radiation. Pure CsBr crystals characterized by an ultrafast response of less than 70 ps and a considerably low light output of 20 photons/MeV are utilized as ultrafast scintillators by coupling them with microchannel-plate photomultiplier tubes (MCP-PMTs) and a fast digitizer. The achieved timing resolution corresponds to a time-of-flight (TOF) localization of 11 mm in positron emission tomography (PET). The results can serve as a new standard in the development of scintillator materials to achieve a timing resolution better than 100 ps.

Three dimensional (3D) stress distributions in Si, surrounded by copper (Cu) filled through silicon vias (TSVs) with various dimensions and pitches, are non-destructively characterized and stress contour maps generated at different depths using a long focal length, polychromator-based, multi-wavelength micro-Raman spectroscopy system. It was found that stress and crystallinity in Si (in both planar and depth directions) was strongly influenced by the proximity to a TSV, as well as, the dimensions of the TSV. In addition to characterizing semiconductor materials, Multi-wavelength micro-Raman spectroscopy was extremely effective for characterizing process-induced variations in crystalline stress and quality where 3D interconnects and packaging technology is introduced.