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The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

Improving current injection into r- and m-planes of nanowires (NWs) is essential to realizing efficient GaInN/GaN multiple quantum shell (MQS) NW-based light-emitting diodes (LEDs). Here, we present the effects of different p-GaN shell growth conditions on the emission characteristics of MQS NW-LEDs. Firstly, a comparison between cathodoluminescence (CL) and electroluminescence (EL) spectra indicates that the emission in NW-LEDs originates from the top region of the NWs. By growing thick p-GaN shells, the variable emission peak at around 600 nm and degradation of the light output of the NW-LEDs are elaborated, which is attributable to the localization of current in the c-plane region with various In-rich clusters and deep-level defects. Utilizing a high growth rate of p-GaN shell, an increased r-plane and a reduced c-plane region promote the deposition of indium tin oxide layer over the entire NW. Therefore, the current is effectively injected into both the r- and m-planes of the NW structures. Consequently, the light output and EL peak intensity of the NW-LEDs are enhanced by factors of 4.3 and 13.8, respectively, under an injection current of 100 mA. Furthermore, scanning transmission electron microscope images demonstrate the suppression of dislocations, triangular defects, and stacking faults at the apex of the p-GaN shell with a high growth rate. Therefore, localization of current injection in nonradiative recombination centers near the c-plane was also inhibited. Our results emphasize the possibility of realizing high efficacy in NW-LEDs via optimal p-GaN shell growth conditions, which is quite promising for application in the long-wavelength region.

High crystalline quality coaxial GaInN/GaN multiple quantum shells (MQSs) grown on dislocation-free nanowires are highly in demand for efficient white-/micro-light-emitting diodes (LEDs). Here, we propose an effective approach to improve the MQS quality during the selective growth by metal-organic chemical vapor deposition. By increasing the growth temperature of GaN barriers, the cathodoluminescent intensity yielded enhancements of 0.7 and 3.9 times in the samples with GaN and AlGaN spacers, respectively. Using an AlGaN spacer before increasing the barrier temperature, the decomposition of GaInN quantum wells was suppressed on all planes, resulting in a high internal quantum efficiency up to 69%. As revealed by scanning transmission electron microscopy (STEM) characterization, the high barrier growth temperature allowed to achieve a clear interface between GaInN quantum wells and GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the correlation between the In incorporation and structure features in MQS was quantitatively assessed based on the STEM energy-dispersive X-ray spectroscopy mapping and line-scan profiles of In and Al fractions. Ultimately, it was demonstrated that the unintentional In incorporation in GaN barriers was induced by the evaporation of predeposited In-rich particles during low-temperature growth of GaInN wells. Such residual In contamination was sufficiently inhibited by inserting low Al fraction (∼6%) AlGaN spacers after each GaInN well. During the growth of AlGaN spacers, AlN polycrystalline particles were deposited on the surrounding dummy substrate, which suppressed the evaporation of the predeposited In-rich particles. Thus, the presence of AlGaN spacers certainly improved the uniformity of In fraction through five GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN MQS structures with the AlGaN spacer are supposed to improve the emission efficiency in white-/micro-LEDs.

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

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.

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.

In this study, first-principles calculations were carried out for the first time to systematically investigate the grain boundary segregation of transition metal alloying elements (Ti, V, Cr, Mn, Co, Ni, Cu, Nb, and Mo) in paramagnetic γFe and its dependence on the grain boundary character. The segregation energies of each element and site were comprehensively calculated for nine [0 0 1] symmetric tilt grain boundary models with Σ values from 5 to 41; the paramagnetic state was simulated by the antiferromagnetic double-layer (AFMD). By calculating the effective segregation energy for each grain boundary model from the obtained segregation energies, the grain boundary segregation behavior for each alloying element and its dependence on the grain boundary character were investigated. The segregation energy of transition metal elements is dominated by the Voronoi volume of Fe at the segregation site and arises from the elastic energy derived from the difference in atomic radii between the host and solute metals. Ti, Cu, Nb, and Mo have negative effective segregation energies (indicating a tendency toward segregation) at all investigated grain boundaries, and the absolute values of the effective segregation energy increase in the order of Cu

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.

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.

Tunable emission wavelengths from 415 nm to 600 nm were demonstrated in coaxial GaInN/GaN multiple quantum shells (MQS) grown on three-dimensional nanostructures. Uniform aperture arrays with different diameters and pitches were patterned on GaN/sapphire substrates by nanoimprint and e-beam lithography. Thereafter, the coaxial GaInN/GaN MQS structures were selectively grown on the nanostructures using metalorganic chemical vapor deposition (MOCVD). To achieve long wavelength emissions, a detailed investigation in terms of height, filling factor, diameter of apertures, and pitch was performed for clarifying the In incorporation in nanostructures. Scanning transmission electron microscopy (STEM) features demonstrate that the diffusion of precursors on SiO2 mask area plays an important role in the coaxially growth of MQS structures on naonopyramids. A growth model based on the diffusion length and pitch was proposed to explain the experimental results. It was clarified that the probability of In incorporation was promoted in nanopyramids with a small pitch (less than 3 times of the diffusion length), wherein direct impinged adatoms on nanopyramids also diffused and participated in the growth on the neighbor. With a decrease in the intervals of aperture patterns, emission longer than 600 nm can be achieved for white and micro light-emitting diodes.

In steel, P and S cause serious grain boundary (GB) embrittlement, which is associated with high segregation energies. To investigate the origins of such high segregation energies of P and S, we applied the combination of ab initio local energy analysis and crystal orbital Hamiltonian population (COHP) analysis for the GB segregation of Al, Si, P, and S in bcc-Fe, which can provide local energetic and bonding views of segregation behavior of each solute, associated with the replacement between solute–Fe and Fe–Fe bonding at GB and bulk sites. The local energy analysis revealed that GB segregation of such solutes is mainly caused by the difference between local energy changes of Fe atoms adjacent to a solute atom in the GB and bulk sites, and that the local energy change of each Fe atom depends on the solute–Fe interatomic distance with a unique functional form for each solute species. The COHP analysis showed that such distance dependency of the Fe-atom local energy change is caused by that of solute–Fe bonding interactions, relative to the Fe–Fe ones, governed by the valence atomic-orbital characters of each solute species. P and S have smaller extents of atomic orbitals and larger numbers of valence electrons; thus, they greatly lower the local energies of Fe atoms at interatomic distances shorter than the bulk first-neighbor one, and they greatly increase those of Fe atoms at longer interatomic distances around the bulk second-neighbor one. Thus, high segregation energies of P and S occur at GB sites with short first-neighbor distances and reduced coordination numbers within the bulk second-neighbor distance. The GB embrittlement by P and S was also discussed by this local-bonding viewpoint. The combination of local energy and COHP analyses can provide novel insights into the behavior of solute elements in various materials.

Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2&ndash, 3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 &deg, C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).

Developing steels with high strength and ductility is needed in order to improve the mechanical reliability and environmental performance of engineering products. The addition of Mn is a key technology for developing next-generation high-strength steels. However, the addition of Mn leads to a serious side effect, grain boundary (GB) embrittlement, which decreases the mechanical toughness of steels. Understanding the mechanism of GB embrittlement due to Mn is an essential process for improving the toughness of steels containing Mn. In this work, in order to reveal the fundamental mechanism of GB embrittlement by Mn, the effect of Mn on the cleavage fracture of bcc-Fe GBs, especially the influence of the difference in the magnetic coupling state between Mn and Fe, is investigated using uniaxial tensile simulations of the bcc-Fe $\mathrm{\ensuremath{\Sigma}}3(111)$ GB with and without Mn segregation using the first-principles density functional theory (DFT). The uniaxial tensile simulations demonstrate that Mn decreases the cleavage-fracture energy of the GB. In particular, the ferromagnetically coupled Mn substantially decreases the cleavage-fracture energy of the GB, promoting cleavage fracture. When ferromagnetically coupled Mn is present in the bcc-Fe GBs, the electrons contributing to the bonds between Mn and the surrounding Fe atoms easily localize to the Mn atom with increasing stress, and the bonding between Mn and the surrounding Fe atoms rapidly weakens, leading to a cleavage fracture of the GBs at a lower stress and strain. This unusual behavior is derived from the stability of the nonbonding Mn as a result of its half-filled d shell. These results show that the local magnetic state in GBs is one of the factors determining the macroscopic mechanical properties of steels containing Mn.

The presentation will report nonelectrical poling behaviors of guest-host polymers, consisting of Disperse Red1 (DR1) and poly (methyl methacrylate) and related second-order nonlinear optical susceptibilities. Our present experimental results found the emissions of the second harmonic generations from the polymer thin films on SiO 2 glass substrates after annealing the materials at the temperatures higher than the glass transition temperatures of the PMMA even in the absence of applying the external electric fields. The hydrogen bonds between the hydroxyl groups of the DR1 and the silanols of the substrate surely played essential roles for breaking the centrosymmetry in the alignments of the guests. The optimized conditions of the nonelectrical poling procedures were examined from the standpoints of the polymer film thickness and the concentrations of the guest chromophores.

Nonlinear optical (NLO) polymers and their applications have been studied for long decades. The fundamental structures of the NLO polymers consist of the host-guest structures; the guest chromophores play a role of nonlinear mixing of multiple waves, while the host polymers fix the positions of the guest. The symmetry of the materials is closely associated with the nonlinear optical susceptibilities. The second order nonlinearity requires the materials to break the centro-symmetry. In general, the NLO polymers exhibit the centro-symmetry in as-prepared conditions, because the guest chromophores are randomly distributed in the host. So-called poling procedure, the procedure applying the DC electric fields, is conducted so as to align the chromophores in the polar order and break the symmetry. In our previous study, we successfully obtain the second order nonlinear susceptibility by using the NLO polymers with the amorphous ferroelectric polymers as host without the conventional poling procedure [1]. Taking advantage of the polarization self-organization behaviors specific to the amorphous ferroelectric polymers, the noncentro-symmetry of the guest chromophores was induced just by annealing the materials at the temperatures higher than the glass transition points (Tg) of the polymers. In the present study, we will report that the nonelectrical poling is available even for the host-guest polymers with poly (methyl methacrylate) (PMMA), one of the most popular host materials for the NLO polymers.

In this manuscript, we describe the current manuscript describes the second-order nonlinear optical susceptibility of guest–host polymers possessing chromophores with strongly electron-accepting tricyanofuran (TCF). Chromophores substituted with different numbers of hydroxyl groups were prepared. Our experimental results demonstrated that the guest–host polymers exhibited nonlinear optical susceptibilities simply upon annealing at temperatures higher than the glass transition point of the host polymers even in the absence of applied external DC electric fields. Nonelectrical poling behaviors were only available for the materials possessing hydroxyl-group-functionalized chromophores. The results indicate that chemisorption of the hydroxyl groups on the substrate led to the orientation order of the guest chromophores. The orientation order of the chromophores was reproduced well by the model of poled polymers in previous studies.