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Surface plasmons resonance is a promising way to improve the luminescence efficiency of light-emitting materials including InGaN/GaN-based quantum wells; however, it has rarely been used for GaN epilayer with metal nanoparticles (NPs). We demonstrated in enhancing the bandgap emission from GaN epilayer by localized surface plasmon resonance (LSPR) in a quadrupole oscillation mode using Ag NPs fabricated on the GaN. The shapes of the Ag NPs fabricated on the GaN substrates gradually changed over time and were eventually destroyed completely. This should be the reason why previous studies of enhancement of GaN-based materials by using Ag NPs have not performed as good results as expected. This problem was solved by employing oxide thin films to protect the Ag NPs from destruction or aggregation and obtain enhanced photoluminescence of the GaN epilayer. The localized and enhanced electric fields have been calculated by the finite difference time domain (FDTD) method to elucidate the enhancement mechanism. However, it has been still difficult to use this method to reproduce the enhanced emissions caused by the quantum interaction between the surface plasmons and excitons. We reproduced the highly efficient emissions attributed to the Purcell effect by evaluating the nanoantenna effect using the FDTD method. This technique confirmed that an oxide thin film between the GaN substrate and Ag NPs played an effective role in the local enhancement of the electromagnetic field and light extraction from the LSPR mode when several types of oxide thin films were considered.

In conventional phase-contrast imaging methods, an optimum distance that is essential for x-ray interference and wavefront separation must be maintained between the object and the x-ray detector. This distance determines the apparent focal size of the x-ray source and affects spatial resolution of the system. To improve the spatial resolution, we proposed a phase-contrast imaging method that eliminates the distance required for x-ray interference and wavefront separation by placing a scintillator in close contact with a Laue angle analyzer (LAA). We mathematically described the image formed by superposition of diffracted beams in two directions of LAA and introduced a method to reconstruct tomographic sections from multiple projections captured by this setup. We experimentally demonstrated the validity and the usefulness of the proposed method. Using a physical phantom, we showed that the proposed method provides higher spatial resolution than conventional x-ray dark-field imaging. The proposed method may be useful for studying and elucidating the fine 3D microanatomy and histopathology of various biological specimens.

Conventional neural structures tend to communicate through analog quantities, such as currents or voltages; however, as CMOS devices shrink and supply voltages decrease, the dynamic range of voltage/current-domain analog circuits becomes narrower, the available margin becomes smaller, and noise immunity decreases. More than that, the use of operational amplifiers (op-amps) and continuous-time or clocked comparators in conventional designs leads to high energy consumption and large chip area, which would be detrimental to building spiking neural networks. In view of this, we propose a neural structure for generating and transmitting time-domain signals, including a neuron module, a synapse module, and two weight modules. The proposed neural structure is driven by a leakage current of MOS transistors and uses an inverter-based comparator to realize a firing function, thus providing higher energy and area efficiency compared to conventional designs. The proposed neural structure is fabricated using a TSMC 65 nm CMOS technology. The proposed neuron and synapse occupy the area of 127 and 231 [Formula: see text], respectively, while achieving millisecond time constants. Actual chip measurements show that the proposed structure implements the temporal signal communication function with millisecond time constants, which is a critical step toward hardware reservoir computing for human–computer interaction. Simulation results of the spiking neural network for reservoir computing with the behavioral model of the proposed neural structure demonstrates the learning function.

Spin-torque oscillators (STOs) are unique spintronic devices in which spin current induces the precession of magnetization in the oscillation layer. In this study, we demonstrate coupled out-of-plane oscillation between two oscillation layers within an STO. The coupled oscillation state is observed in the magnetoresistance spectra as well as its degradation due to the mixing of the free oscillation state depending on the bias current. These behaviors are analytically described and numerically reproduced based on the Landau–Lifshitz–Gilbert equation. Since the STO is fabricated in the write head of a hard disk drive, the coupled oscillation state and the subsequent microwave magnetic field are applicable for microwave-assisted magnetic recording for high-density recording.

In this study, AlGaN-based ultraviolet-B band laser diodes with 150-mW peak output power in pulsed operation were demonstrated at room temperature. The oscillation wavelength, differential quantum efficiency, and slope efficiency of a laser diode were 300 nm, 3.6%, and 0.15 W/A, respectively. These results were obtained by increasing the injection efficiency and decreasing the positive fixed polarization charge formed at the interface between a p-side waveguide layer and an electron blocking layer when polarization doping is formed in a p-AlGaN cladding layer.

This study aims to improve the crystalline quality of 650-nm GaIn0.17N/GaIn0.38N red-emitting multiple quantum wells (MQWs) fabricated on a ScAlMgO4 (SCAM) substrate. When using the SCAM substrate, the diffusion and/or penetration of impurities, including Mg, Sc, O, and Al, from the SCAM substrate poses as a challenge. To address this issue, we introduced an Al0.74In0.26N layer between the SCAM substrate and MQWs, which was lattice-matched to the substrate. The Al0.74In0.26N layer effectively blocked the diffusion of impurities from the SCAM substrate into the adjacent layers during the metal-organic vapor epitaxy (MOVPE) growth. For further suppression, a thick AlN layer was deposited on the back of the SCAM substrate before the MOVPE growth, which effectively suppressed impurity penetration from the growth surface. The structure proposed in this study improved the crystallinity and the surface roughness of MQWs, resulting in the improvement of internal quantum efficiency by approximately three times compared to that of the conventional sample.

The development of flexible thermoelectric devices is gradually attracting increasing attention, particularly in the field of material design. In this study, we use first-principles calculations combined with Boltzmann equations to study the electronic and transport properties of Ag2S1− xSe x, a key material with many important properties and extraordinary ductility, as well as a wide range of thermoelectric applications. The effect of Se alloying on the electronic structure of Ag2S and defect formation is investigated, and the role of alloying in increasing the n-type carrier concentration is discussed. The electron–phonon coupling approximation is used to reproduce the experimentally observed transport properties reasonably well, which shows that this scattering model is suitable for predicting the transport properties of semiconductors in thermoelectric applications.

The structural and electronic properties of two-dimensional gallium nitrides (2D GaNs) on a van der Waals (vdW) epitaxial substrate are investigated using first-principles calculations. We show that the structures and electronic properties of mono- and bilayer GaN are hardly affected when deposited on the vdW substrate comprising a graphene sheet placed on the GaN(0001) bulk surface. A weak attractive interaction is found to work between the 2D GaNs and vdW substrate, which is still sufficient to maintain the 2D GaNs on the substrate and could also be exploited to control their stability. The present findings demonstrate that the vdW substrate is propitious to grow and hold the 2D GaNs without altering their intrinsic properties, in contrast to previously examined metal substrates.

Magnetic particles with a hollow structure have arisen as an important class of nanomagnets because of a large pore volume and higher surface-to-volume ratio compared with the same-sized solid particles. The hollow structure results in unique magnetic features such as enhanced surface exchange bias, spin freezing, and preferential stability of a magnetic vortex. Despite a recent growing understanding of sub-100 nm hollow spherical magnetic nanoparticles, magnetic properties of larger-sized hollow particles were not currently understood in detail. Here, we report results of observations of magnetic microstructures for 420 nm-sized hollow Fe3O4 spherical particles with an electron holography imaging technique, where a magnetic-vortex formation is inferred from bulk measurements. We directly observe a magnetic vortex in a remanence state with magnetization circularly oriented within the shell and the reduced stray field. Micromagnetic simulations demonstrate an increasing stability of a vortex for a hollow sphere and the formation of a field-induced curling double vortex with a pair of clockwise and counterclockwise vortices. This double vortex structure is not confirmed for the solid counterpart, and its stability enhances with decreasing the shell thickness. The present work provides useful knowledge in designing magnetic particles, where a hollow structure and a magnetic vortex are key factors for high-performance biomedical applications.

One of the main loss mechanisms in photovoltaic solar cells is the thermalization of photogenerated hot carriers via phonon-mediated relaxation. By inhibiting these relaxation mechanisms and reducing thermalization losses, it may be possible to improve the power conversion efficiency of solar cells beyond the single gap limit. Here, type-II InAs/AlAsSb multi-quantum well (MQW) structures are investigated to study the impact of the phononic properties of the AlAsSb barrier material in hot carrier thermalization. Experimental and theoretical results show that by increasing the barrier thickness (increasing the relative contribution of AlAsSb content in the superlattices), the relaxation of hot carriers is reduced as observed in power-dependent photoluminescence and thermalization analysis. This is attributed to an increase in the phononic bandgap of the MQW with increasing AlAsSb composition reducing the efficiency of the dominant Klemens mechanism as the phononic properties shift toward a more AlSb-like behavior.

This Letter reports the polarization induced hole conduction in composition-graded AlInN epitaxial layers grown by metalorganic chemical vapor deposition. First, the composition-graded AlInN layer with an InN mole fraction from 0.12 to 0.20 was formed on c-plane GaN on sapphire, and they were confirmed to show the p-type hole conduction with a less temperature dependence, which is a feature of polarization-induced carriers. Then, blue light-emitting diodes (LEDs) with the composition-graded AlInN layers inserted in the p-type side were fabricated and their vertical current injection was investigated. The electroluminescence (EL) spectra confirmed that the fabricated LEDs exhibited a single-peak blue-light emission with the help of the impurity Mg doping. The LED simulation indicated that the impurity and polarization co-doping effectively compensated and overcame the residual oxygen donors in the AlInN layer and promoted the carrier recombination at the light-emitting layers. Finally, LEDs with the Mg-doped and composition-graded AlInN insertion layer exhibited good current–voltage characteristics with a low forward voltage drop of approximately 3 V in addition to the good EL spectra.

To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode.

In this study, we attempt to identify the presence of surface defects (SDs) at an n-type GaN surface after high-temperature growth and gain insight into their intrinsic features. To this end, first, we carefully investigate n-type GaN samples with different surface etching depths. Low-temperature photoluminescence (PL) spectra reveal that SDs are most likely nitrogen vacancies (VN) and/or VN-related point defects intensively distributed within ∼100 nm from the n-type GaN surface after a high-temperature growth. We investigate the effect of SDs on the internal quantum efficiency (IQE) of green light-emitting diodes (LEDs) by preparing GaInN-based green LEDs employing a surface-etched n-type GaN, which exhibits a prominent enhancement of the PL efficiency with an increase in the etching depth. This effect is attributable to the reduced non-radiative recombination centers in multiple-quantum-well active regions because the SDs near the n-type GaN surface are removed by etching. We discuss strategies of in situ engineering on SDs to further improve the IQE in GaInN-based green LEDs on the basis of the results presented in this study.

In this study, the electronic structures of A-site layer-ordered double perovskite YBaCo2O6 and YBaCo2O5.3 epitaxial thin films were investigated by x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. The valence band PES spectrum of the YBaCo2O6 thin film exhibited a finite density of states (DOS) at the Fermi energy (EF), while that of the YBaCo2O5.3 thin film exhibited no DOS at EF, corresponding well with the metallic and insulating transport properties, respectively. The Co L-edge XAS measurements revealed that the Co ions in YBaCo2O6 are in a mixed valence state of high-spin (HS) Co3+ (50%) and intermediate spin (IS) Co4+ (50%), while HS Co3+ (80%) and HS Co2+ (20%) coexist in YBaCo2O5.3. The XMCD measurements led to the conclusion that both HS Co3+ and IS Co4+ contribute to the ferromagnetism of YBaCo2O6.

Rutile-related (tetragonal rutile and monoclinic distorted rutile) ReO2 epitaxial thin films were grown on rutile TiO2 single crystalline substrates by reactive pulsed laser deposition of the Re metal target under a N2O atmosphere. The ReO2 film underwent structural phase transition from the monoclinic to tetragonal phase with the decreasing film thickness, driven by compressive lattice strain from the substrate. X-ray photoelectron spectroscopy and electrical transport measurements revealed that both phases had metallic properties above 25 K. The distinct difference in valence band spectra between the two phases might be due to reconstruction of the band structure dominated by Re 5d t2g.