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300-nm-thick AlInN films with InN molar fractions ranging from 0.114 to 0.197 were grown by metalorganic chemical vapor deposition on c-plane GaN on sapphire. It was confirmed that no lattice relaxation occurred for samples with InN molar fractions from 0.144 to 0.197, and the InAlN films with low InN molar fractions showed a relative smooth surface. However, it turned into a granular surface morphology resulting from a columnar polycrystalline structure when the InN molar fraction exceeded a compositional boundary of in-plane lattice matching. As for the smooth-surface AlInN single-layers, the optical constants as well as energy bandgaps were determined.

Multi-color emission from coaxial GaInN/GaN multiple-quantum-shell (MQS) nanowire-based light-emitting diodes (LEDs) was identified. In this study, MQS nanowire samples for LED processes were selectively grown on patterned commercial GaN/sapphire substrates using metal–organic chemical vapor deposition. Three electroluminescence (EL) emission peaks (440, 540, and 630 nm) were observed, which were primarily attributed to the nonpolar m-planes, semipolar r-planes, and the polar c-plane tips of nanowire arrays. A modified epitaxial growth sequence with improved crystalline quality for MQSs was used to effectively narrow the EL emission peaks. Specifically, nanowire-based LEDs manifested a clear redshift from 430 nm to 520 nm upon insertion of AlGaN spacers after the growth of each GaInN quantum well. This demonstrates the feasibility of lengthening the EL emission wavelength since an AlGaN spacer can suppress In decomposition of the GaInN quantum wells during ramping up the growth temperature for GaN barriers. EL spectra showed stable emission peaks as a function of the injection current, verifying the critical feature of the non-polarization of GaN/GaInN MQSs on nanowires. In addition, by comparing EL and photoluminescence spectra, the yellow-red emission linked to the In-fluctuation and point defects in the c-plane MQS was verified by varying the activation annealing time and lowering the growth temperature of the GaInN quantum wells. Therefore, optimization of MQS nanowire growth with a high quality of c-planes is considered critical for improving the luminous efficiency of nanowire-based micro-LEDs/white LEDs.

GaInN/GaN multiple quantum shells (MQS) nanowires and p-GaN shells were embedded with n-GaN layers through tunnel junction (TJ) shells using metalorganic chemical vapor deposition (MOCVD) method. The MQS nanowires were selectively grown on n-GaN/sapphire or GaN substrates. The fabrication process of laser structures with different resonators of 600500, 750, 1000 μm, and cavity widths of 7, 12, and 17 μm were investigated with insulating layer on the sidewalls of the ridge. The structures of the fabricated devices were characterized by scanning electron microscope (SEM) and current-voltage-light output characteristics were evaluated. Two different methods for mirror formation, etching and cleavage, were developed for the laser devices. During the investigation, a superior mirror formation suffered from the difference in etching rate between GaInN and GaN, generating concaves in the MQS region. Bluegreen light emission was observed from the entire ridge surface of the MQS index-guided laser structures. A maximum current density of emission at 17.9 kA/cm2 has been confirmed in the devices. The electroluminescence and cathodoluminescence measurements demonstrated that the r-plane and c-plane at the top of the MQS are dominant at low current densities, and the m-plane emission becomes stronger as the current density increases.

A tunnel junction and a n-GaN cap layer grown on the multi-quantum shells (MQS) /nanowires are introduced to decrease the resistivity and optical loss. The selective-area growth of the MQS/nanowire core-shell structures on the template was performed by metalorganic vapour phase epitaxy (MOVPE). Further, the MQS structure was covered with the tunnel junction and the n-GaN cap layer. Here, the growth conditions of the n-GaN cap layer were systemically investigated. The effect of p-GaN shape on the morphology of grown n-GaN cap layer was also assessed.

We will show our two recent results related to GaN-based VCSELs with AlInN/GaN DBRs. One is in-situ wafer curvature control of AlInN/GaN DBRs. Curvature data successfully provides us the strain condition in the AlInN layers, which leads to a high reproducibility for fabricating the AlInN/GaN DBRs. The other is laser operation of the VCSELs with large current apertures. While our previous VCSELs with only 8-15 µm apertures showed RT CW operations, our recent VCSELs now show RT CW operations with 5-30 µm apertures. such larger apertures will eventually provide much higher LOPs in the future.

As a potential material for white light source, high luminous efficiency of fluorescent SiC requires a concentration of higher than 1.0×1019 cm-3 for both the donor (Nd) and acceptor (Na) and a film thickness of about 120 μm. In this study, we focus on the epitaxial growth of fluorescent 4H-SiC by closed sublimation growth in terms of different growth temperatures to increase the doping concentration of boron and nitrogen. In addition, the effect of growth pressure on the surface morphology was investigated. The photoluminescence (PL) intensity of the sample grown at 1900 °C is higher than that grown at 1800 °C. This result indicates that a higher temperature of 1900 °C can increase the boron (B) and nitrogen (N) doping concentration. However, it was confirmed that the surface of the sample grown at 1900 °C was rough. The surface morphology was significantly improved with an increased pressure to 9000 Pa. Meanwhile, the growth time was extended to increase the film thickness, and the amount of BN, which is the doping source of B acceptor, was increased accordingly. As a result, strong PL emission intensity with high internal quantum efficiency (IQE) was successfully demonstrated in 4H-SiC sample, which is comparable to that in fluorescent 6H-SiC.

Ultraviolet (UV) semiconductor lasers are widely used in medical and industrial applications, and their commercialization is strongly desired. Recently, laser oscillation by current injection in the UV-C and UV-B regions has been reported . From now on, it is necessary to demonstrate CW operation, which is indispensable for practical applications. In order to realize this, it is important to understand the internal loss and optical gain of the current devices. In this presentation, we report the details of our UV-B lasers. Specifically, we would like to discuss the results of the variable stripe length analysis of the internal loss of the obtained device and the performance of the UV-B lasers. The results show that the internal losses of the optimized UV-B laser are relatively low and good values are obtained. We also discuss the details of polarization doping, which is very effective in the realization of these UV lasers.

The selective-monolithic growth of coaxial GaInN/GaN NWs was investigated by changing the TEG flow rate, barrier and well growth temperature during MQS growth. In incorporation increased with a higher TEG flow rate. However, In-rich flakes were formed the NWs resulting in the deterioration of crystal quality. Using a higher growth temperature of quantum barriers, abnormal growth at the top of NWs was eliminated. As a result, the CL emission intensity was enhanced. Furthermore, the occurrence of In desorption was suppressed by decreasing the growth temperature of quantum wells. Therefore, these results are promising for NW-based white LEDs.

The centimeter-scale laser lift-off (LLO) of a UVB laser diode structure on nano-patterned AlN was demonstrated by using a 257 nm pulsed laser. The mechanism of this LLO, which can be used for vertical light-emitting device fabrications, was analyzed in detail from the structural and optical properties. The large-area high-yield LLO without cracks was found to be enabled by taking advantage of the intentional in-plane periodic and nanometer-scale inhomogeneous distribution of the AlN molar fraction in the AlGaN layer introduced by growing AlGaN on nano-patterned AlN.

We demonstrated the structural optimization of AlGaN tunnel junction (TJ) deep-ultraviolet light-emitting diodes by changing the thickness and impurity concentrations of p+-type and n+-type AlGaN constituting the TJ. By decreasing the total thickness of the TJ to 23 nm, the operating voltage reached a minimum of 8.8 V at 63 A cm−2. Further decrease in TJ thickness markedly increases the operating voltage. This finding implies that the depletion layer width becomes greater than the TJ thickness if it is smaller than 12 nm. Therefore, we conclude that the TJ thickness must be greater than the depletion layer width.

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.

We have successfully fabricated vertical LEDs by separating a 1 × 1 cm2 wafer composed of deep-ultraviolet LED on a sapphire substrate from the substrate using a laser liftoff (LLO) method. Reproducible substrate separation was achieved by the LLO method using an Al0.68Ga0.32N underlayer film on an AlN template with periodic pillars. The fabricated vertical LED successfully demonstrated notable luminescence (peak wavelength: 298 nm) characteristics up to a current density of ∼43 kA cm−2 at room temperature and in pulsed drive, which is expected to be used in high-power LEDs and laser diodes.

This review paper describes the historical development, current issues, and future expectations of UV-B laser diodes, which are expected to be adopted in various applications such as in microfabrication and biotechnology in the near future. To achieve RT operation of UV-B laser diodes, several challenges need to be addressed, including the development of a crystal growth technique for high-crystalline-quality AlGaN films that enables laser oscillation with a low excitation carrier density, and the development of a semiconductor layer structure for simultaneous formation of a desirable optical cavity and injection of high-density carriers (operation of high current density), allowing laser oscillation in the active layer. These challenges and the corresponding technologies that have overcome them are reviewed. The current status of device characteristics and future challenges are also discussed.

We report the results of crystal growth of thick AlGaN films on periodical 1 μm spacing AlN pillar concave-convex patterns and their impact on the performance of UV-B laser diodes. The formation of voids in the AlGaN film by increasing the AlN pillar height and the use of high-quality AlN templates were effective in improving the quality of AlGaN, and the dislocation density in the AlGaN film was reduced to approximately 3.4 × 108 cm−2. A UV-B laser diode was fabricated on the optimized AlGaN, demonstrating a threshold current density of ∼12 kA cm−2 and a high yield rate of over 90%.

This study aims to elucidate the carrier dynamics behind thermal droop in GaInN-based blue light-emitting diodes (LEDs) by separating multiple physical factors. To this end, first, we study the differential carrier lifetimes (DCLs) by measuring the impedance of a sample LED under given driving-current conditions over a very wide operating temperature range of 300 K–500 K. The measured DCLs are decoupled into radiative carrier lifetime (τR) and nonradiative carrier lifetime (τNR), via utilization of the experimental DCL data, and then very carefully investigated as a function of driving current over a wide range of operating temperatures. Next, to understand the measurement results of temperature-dependent τR and τNR characteristics, thermodynamic analysis is conducted, which enables to look deeply into the temperature-dependent behavior of the carriers. On the basis of the results, we reveal that thermal droop is originated by the complex dynamics of multiple closely interrelated physical factors instead of a single physical factor. In particular, we discuss the inherent cause of accelerated thermal droop with elevated temperature.

This study investigated the fabrication of porous fluorescent SiC using a constant voltage-controlled anodic oxidation process. The application of a high, constant voltage resulted in a spatial distinction between the porous structures formed inside the fluorescent SiC substrates, due to the different etching rates at the terrace and the large step bunches. Large, dendritic porous structures were formed as the etching process continued and the porous layer thickened. Under the conditions of low hydrofluoric acid (HF) concentration, the uniformity of the dendritic porous structures through the entire porous layer was considerably improved compared with the conditions of high HF concentration. The resulting large uniform structure offered a sizable surface area, and promoted the penetration of atomic layer-deposited (ALD) Al2O3 films (ALD&ndash, Al2O3). The emission intensity in the porous fluorescent SiC was confirmed via photoluminescence (PL) measurements to be significantly improved by a factor of 128 after ALD passivation. With surface passivation, there was a clear blueshift in the emission wavelength, owing to the effective suppression of the non-radiative recombination rate in the porous structures. Furthermore, the spatial uniformity of emitted light was examined via PL mapping using three different excitation lasers, which resulted in the observation of uniform and distinctive emissions in the fluorescent SiC bulk and porous areas.

Ultraviolet-B laser diode at 298 nm was realized by developing the technologies; growth method of lattice-relaxed Al 0.6 Ga 0.4 N layer on AlN/sapphire template and III-group composition-graded p-AlGaN cladding layer. The threshold current, current density and voltage were 5.0 A, 25 kA/cm2, and 34.4 V, respectively.

The three-dimensional core-shell structure consisting of GaN nanowire and GaInN/GaN multi-quantum shell (MQS) is thought to be promising for high-performance light-emitting devices, because of its advantages such as non-polar surface orientation, dislocation-free and tolerance of misfit strain due to small size crystal. However, their crystal growth mechanism and its crystalline quality have not yet fully understood, and they are still under the basic considerations. The key issues in the nanowire/MQS structure are its crystalline quality and shape control, which are greatly dependent on the growth condition. Thus, in this paper, selective growth of nanowire/MQS and its optical properties are described.

We analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode by phase-shifting electron holography. We clearly obtained a phase image reflecting the band alinement of the p–n homojunction and derived a depletion layer width of approximately 10 nm. In addition, the observed depletion layer width for the AlGaN TJ was in good agreement with the simulated one reflecting the diffusion profile of Mg and Si, thus enabling a discussion on the electrical conduction mechanism for an AlGaN p–n junction.

We investigated the effects of different growth facets of 6H-SiC and different voltage waveforms on the porous structure and luminescence properties. The structure formed on the surface after anodic etching significantly changed because of the difference in the growth plane, whereas dendritic and columnar pores were observed inside the Si- and C-face samples. These large porous structures were shown to promote the penetration depth of the atomic-layer-deposited Al2O3 films, and a recorded passivation depth of 30 μm layer was confirmed in C-face porous SiC. From the results using a fluorescence microscope and photoluminescence spectra measurement, it was concluded that the pulsed-voltage etching was preferable for fabricating uniform porous structures compared with the constant-voltage etching. However, the enhancement of the luminescence intensity needs to be further improved to realize high luminescent efficiency in porous fluorescent SiC.

In this study, the growth mechanisms of n-type gallium nitride (GaN) cap layers for embedding nanowire-based multi-quantum-shell (NW-MQS) with a tunnel junction were investigated using the metal–organic vapor-phase epitaxy method. Herein, instead of a p-type GaN layer, n-type GaN cap layers were applied as a contact layer to the anode electrode via a tunnel junction. NW-MQSs were prepared in a rectangular grid arrangement aligning with the m- and a-axis of GaN. The growth mode of the cap layers was controlled in three stages using different growth pressures and temperatures. The first cap layer was a faceted growth mode primarily comprising the r-plane, the second was a lateral growth mode that expanded the area of the c-plane, and the third was an acceleration of lateral growth. Finally, cap layers with a flat surface were realized. Void formation in the cap layers occurred only between the NW-MQSs along the m-axis of GaN. This observation can be attributed to the difference in the coalescence of the cap layers in the a- and m-axis directions. We believe that the NW-MQS structure with the cap layer that was optimized in this study can be used as a highly efficient optical device.

A pressure-induced valence transition has attracted much attention in Eu-compounds. Among them, EuRh2Si2, EuNi2Ge2, and EuCo2Ge2 reveal the valence transition around 1, 2, and 3 GPa, respectively. We have succeeded in growing single crystals of EuT2X2 (T: transition metal, X: Si, Ge) and studied electronic properties under pressure. EuRh2Si2 indicates a first-order valence transition between 1 and 2 GPa, with a large and prominent hysteresis in the electrical resistivity. At higher pressures, the first-order valence transition changes to a cross-over regime with an intermediate valence state. Tuning of the valence state with pressure is reflected in a drastic change of the temperature dependence of the electrical resistivity in EuRh2Si2 single crystals. Effect of pressure on the valence states on EuRh2Si2, EuIr2Si2, EuNi2Ge2, and EuCo2Ge2, as well as an isostructural related compound EuGa4, are reviewed.

To reduce the number of threading dislocations (TDs) in nonpolar a -plane GaN ( a -GaN) epi-layers grown on flat r -plane sapphire substrates ( r -FSS), we investigated the effects on the crystalline quality of the a -GaN epi-layers of high-density patterned r -plane sapphire substrates ( r -HPSS), the patterns of which were placed at intervals of several hundred nanometers. Two types of r -HPSS, the patterns of which had diameters and heights on the order of several hundred nanometers ( r -NHPSS) or several micrometers ( r -MHPSS), were prepared with conventional r -FSS. The effect of these r -HPSS on the a -GaN epi-layers was demonstrated by evaluating the surface morphology and the crystalline quality of the epi-layers. The surfaces of the a -GaN epi-layer grown on r -FSS and r -NHPSS were pit-free and mirror-like, whereas the surface of the a -GaN epi-layer grown on r -MHPSS was very rough due to the large, irregular GaN islands that grew on the patterns, mainly at the initial growth stage. The crystalline quality of the a -GaN epi-layer grown on r -NHPSS was better than that of the a -GaN epi-layer grown on r -FSS. We confirmed that there were fewer TDs in the a -GaN epi-layer grown on r -NHPSS than there were in the a -GaN epi-layer grown on r -FSS. The TDs propagating to the surface in a -GaN epi-layer grown on r -NHPSS were mainly generated on the flat sapphire regions between the patterns. Interestingly, it was also found that the TDs that propagated to the surface concentrated with a periodic pitch along the c -axis direction. The TD densities of a -GaN epi-layers grown on r -FSS and r -NHPSS were estimated to be approximately 5.0 × 10 10 and 1.5 × 10 9 cm −2 , respectively. This knowledge will contribute to the further development of a -GaN epi-layers for high-performance devices.

This study aimed to investigate and analyze the impurity doping characteristics in tunnel junctions (TJs) grown on core–shell structures, comprising GaInN/GaN multiple-quantum-shells (MQSs) and GaN nanowires. To this end, the impurity, structural, and electrical properties of the samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, atom probe tomography (APT), nanoscale secondary ion mass spectrometry (NanoSIMS), and electroluminescence of the device which was fabricated for a prototype laser device to demonstrate an electrical operation of the MQSs layer. From the experimental results of NanoSIMS and APT, we demonstrated that the Mg-related problems in the TJ, such as the diffusion to the n++-GaN layer from the p+-GaN layer and formation of clusters in p+-GaN, are critical. Consequently, they cause a high operating voltage and dot-like spot emission of the light-emitting device. Based on the analysis, we suggested remedies and strategies to further improve the TJs that work well.

We investigated the relationship between the optical constants and localized states near the band-edge in high-quality crystalline Al1−x In x N alloys, with an indium content x ranging from 0.12 to 0.22, grown on a c-plane freestanding GaN substrate. Optical constants were obtained by spectroscopic ellipsometry. The tanΨ and cosΔ spectra were fitted by the Adachi’s critical-point (AC) model. The effects of the near-band-edge localized states on the optical constants were characterized by the spectral broadening factor γ, which was obtained by the AC model. The γ increased with increasing x and this tendency also confirmed by the γ obtained by the photoluminescence excitation (PLE). The bandgap energies obtained by the AC model agree well with those obtained by the PLE. It is suggested that the indium-related near-band-edge localized states cause the spectral broadening of the refractive index and extinction coefficient spectra in Al1−x In x N alloys.

Inspired by the ideas of many authors, we provide insight on state-of-the-art potential technologies for the prevention and prediction of infectious diseases before they spread. This review also surveys virus sterilization with ultraviolet light and low temperature plasma technologies. Researchers in the various fields of medicine, materials, electronics, and plasma sciences have addressed increasingly challenging demands and the discussion encompasses the major challenges in societies that are faced with the threat of infectious diseases. In addition, technologies that use nanomaterials are evaluated for infection prevention and hygiene purposes. Advances in biomedical diagnostics for health care in terms of complementary metal-oxide-semiconductor transistors-based devices and telemetry for health monitoring are also reviewed.

Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a -plane GaN ( a -GaN) grown on an r -plane sapphire substrate. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, and 180 nm were used. We found that the surface morphological transitions with increasing thickness were different between the sp-AlNs and the ep-AlNs, and that the sp-AlNs had poorer crystallographic orientations than did the ep-AlNs. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of the sp-AlNs; however, the positive effect of annealing was limited because the in-plane crystallographic orientation degraded with increasing layer thickness. The optimal buffer layer was found to be the 30-nm-thick annealed sp-AlN, which was composed of uniformly arranged oval grains with better crystallographic orientation than the other sp-AlNs and annealed sp-AlNs. The crystalline quality of the a -GaN epilayer grown on 30-nm-thick annealed sp-AlN had a narrower X-ray rocking curve–full width at half maximum for both the on- and off-axis planes compared with that grown on any other AlN buffer layers.

In this study, we discuss the influence of SiH4 flow rates on the structural and optical properties of GaN nanowires (NWs) with multiple-quantum-shells (MQSs). To this end, we prepared two n-GaN core NW samples with different SiH4 flow rates. Subsequently, MQS active layers of the same structure were grown on each n-GaN core NW under identical growth conditions. The samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray, and cathodoluminescence (CL) mapping. From the experimental results, we ascertained that a Si-rich layer was created between the sidewall of the NWs and MQSs, in which the number of Si atoms was mainly determined by the SiH4 flow rate. These Si atoms diffused into the MQSs during the growth, and significantly impacted the structural and optical properties, such as the shape and crystalline quality of the MQSs and NWs, and the CL intensity of the MQSs. On the basis of experimental results, we conclude that the SiH4 flow rate during the NW growth plays a critical role in the performance of an MQS-based optoelectronic semiconductor device.

In this study, we attempted to elucidate the carrier dynamics behind the abnormal characteristics of photogenerated current and voltage ( I Ph and V Ph ) at cryogenic temperature in III-nitride optoelectronic semiconductors by employing space-charge theory. To this end, we carefully investigated and analyzed excitation-power-dependent I – V (PDIV) curves operated by quasiresonant excitation of an AlGaInN-based p − i − n junction semiconductor at 300 K and 15 K. At 300 K, the curves exhibited typical characteristics and were well described by the conventional theory. However, the PDIV curves at 15 K could no longer be described by the conventional theory. To elucidate the mechanism behind this phenomenon, we proposed a model in which the space-charge effect (SCE) plays a key role. Based on this model, we proposed the modified Shockley diode equation, which can explain the PDIV characteristic at 15 K, including the SCE. We also discussed the SCE on the efficiency of devices.

We have successfully synthesized a uranium intermetallic compound U2Pt6Ga15 via the gallium flux method. The crystal structure, magnetization, specific heat, and resistivity are investigated. U2Pt6...