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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.

In this study, the improvement of reverse leakage current characteristics with a sputtered (SP) -AlN buffer layer in GaInN-based green light-emitting diodes (LEDs) has been presented for the first time. To understand the origin of the improvement, a detailed review and careful analysis of reverse leakage current characteristics were performed. The review and analysis identified that the improvement was primarily caused by the suppression of variable-range-hopping process obtained by replacing conventional low-temperature GaN buffer. Verification that threading dislocations and V- defects can enhance the variable-range-hopping process has been received. We believe that this study will contribute to the realization of green LEDs with advantages of high reliability, a long lifetime, and electrical robustness.

In this paper, a hybrid numerical simulation tool is introduced and performed for GaInN-based light-emitting diodes (LEDs) with metal-embedded nanostructure to theoretically predict external quantum efficiency (EQE), which composed of finite-difference time-domain, rigorous coupled wave analysis, and ray tracing. The advantage is that the proposed method provides results supported by sufficient physical background within a reasonable calculation time. From the simulation results, the EQE of LED with Ag-nanoparticles embedded nanostructure is expected to be enhanced by as high as ∼1.6 times the conventional LED device in theory.

As an attempt to elucidate the origin of the power-efficiency (PE) degradation at high current injection for GaInN-based light-emitting diodes, the injection current is quantitatively separated to radiative and nonradiative current components as a function of applied voltage. It is found that the conventional Shockley equation for the current-voltage curve of a Si pn diode is not adequate for the LED since the carrier transport and recombination processes are quite different from those of the Si pn diode. Hence, we propose a diode equation for an LED where the radiative and nonradiative currents are separately expressed as a function of applied voltage. By analyzing the proposed diode equation, it is concluded that the PE degradation at high injection currents is due to the increase of the junction voltage and the decrease of the internal quantum efficiency at the same time. The phenomena can be understood by the insufficient recombination rate in the active quantum wells.

Coaxial GaInN/GaN multiple-quantum-shells (MQSs) nanowires (NWs) were grown on an n-type GaN/sapphire template employing selective growth by metal-organic chemical vapour deposition (MOCVD). To improve the cathodoluminescence (CL) emission intensity, an AlGaN shell was grown underneath the MQS active structures. By controlling the growth temperature and duration, an impressive and up to 11-fold enhancement of CL intensity is achieved at the top area of the GaInN/GaN MQS NWs. The spatial distribution of Al composition in the AlGaN undershell was assessed as a function of position along the NW and analysed by energy-dispersive X-ray measurement and CL characterisation. By introducing an AlGaN shell underneath GaInN/GaN MQS, the diffusion of point defects from the n-core to MQS is effectively suppressed because of the lower formation energy of vacancies-complexes in AlGaN in comparison to GaN. Moreover, the spatial distribution of Al and In was attributed to the insufficient delivery of gas precursors to the bottom of the NWs and the anisotropy diffusion on the nonpolar m-planes. This investigation can shed light on the effect of the AlGaN undershell on improving the emission efficiency of NW-based white and micro-light-emitting diodes (LEDs).

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.

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.

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.

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.

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.

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.

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I&ndash, V curves, electroluminescence spectra, L&ndash, I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region.

Currently, the internal quantum efficiency (IQE) of GaInN-based green light-emitting diodes (LEDs) is still low. To overcome this problem, surface plasmon (SP)-enhanced LEDs have been intensively studied for the last 15 years. For an SP effect in green LEDs, Au and Ag are typically employed as the plasmonic materials. However, the resonance wavelength is determined by their material constants, which are theoretically fixed at ~537 nm for Au and ~437 nm for Ag. In this study, we aimed to tune the SP resonant wavelength using double-metallic nanoparticles (NPs) composed of Au and Ag to match the SP resonance wavelength to the LED emission wavelength to consequently improve the IQE of green LEDs. To form double-metallic NPs, Au/Ag multilayers were deposited on a GaN layer and then thermally annealed. We changed the thicknesses of the multilayers to control the Ag/Au ratio in the NPs. We show that the SP resonant wavelength could be tuned using our approach. We also demonstrate that the enhancement of the IQE in SP-enhanced LEDs was strongly dependent on the SP resonant wavelength. Finally, the highest IQE was achieved by matching the SP resonant wavelength to the LED emission wavelength.

Room-temperature pulsed oscillation with a laser wavelength of 290 nm and a threshold current density of 35 kA cm−2 was achieved by fabricating a UV-B laser diode on a thick AlGaN film formed on a 1 μm periodic concavo–convex pattern AlN (PCCP-AlN) on a sapphire substrate. The advantage of this method using PCCP-AlN is that it promotes the nucleation of AlGaN crystals. Planarization of this growth nucleus with AlGaN reduces the threading dislocation density at the top of the AlGaN growth layer while suppressing the formation of giant micrometer-sized hillocks and V-shaped pits that appear irregularly.

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.

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.

We report GaN-based vertical-cavity surface-emitting lasers (VCSELs) with buried GaInN tunnel junctions (TJs). Under room-temperature continuous-wave operation, the VCSEL with an 8 μm aperture exhibited a low operating voltage of 5.3 V at 10 kA cm−2 and a differential resistance of 110 Ω. In addition, the VCSEL with a 10 μm aperture showed a threshold current of 14.4 mA and a peak output power of 2.0 mW. An influence of an absorption loss at the GaInN TJ on laser performance was estimated using an analysis of a one-dimensional optical intensity profile based on a cross-sectional STEM image.

In this study, we aim to understand the thermodynamics inside the junctions of GaInN-based LEDs through optical operation of the device (the wavelength of the pumping laser is 405 nm for quasi-resonant optical excitation). First, to achieve this goal, the short-circuit current vs the open-circuit voltage curve and photoluminescence spectrum are carefully analyzed and compared with the current–voltage curve and electroluminescence spectrum. By this comparative study, we experimentally demonstrate the electrical–optical energy conversion efficiency (ECE) > 100% under optical excitation. Second, the intra-band phonon absorption and emission inside and/or outside the junction (PPeltier and PHC) are investigated through the power-loss analysis. In so doing, we ascertain that the carriers in the multiple quantum wells are energized by phonons, which introduces the ECE > 100%. Specifically, after the energization, the Boltzmann carrier distribution is no longer controlled by the ambient lattice temperature but by higher temperature, namely, that of the carriers themselves. Based on the analysis, we propose a thermally enhanced effective temperature for the Boltzmann distribution. Finally, we compare the junction efficiency by photoluminescence (PL), composed of the internal quantum efficiency by PL and the ECE by PL, of the sample at various temperatures to investigate the effect of PHC on the overall PL efficiency.

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).

Embedded growth of an n-GaN cap layer on multi-quantum shells (MQSs) and nanowires through a tunnel junction (TJ) was investigated for the improvement of current injection to the m-plane of the MQSs. Different growth conditions for an n-GaN cap layer were systematically studied to suppress Mg diffusion and void formation, which are serious problems in this structure. At a high temperature of 900 °C and above, the growth rate on a semi-polar r-plane decreased, and large voids were formed at the bottom part of the nanowires owing to the diffusion of Ga atoms from the r-plane to the m-plane. When the growth temperature decreased to 800 °C, the growth rate on both the m-plane and the r-plane increased, and the size of voids decreased. Simultaneously, Mg diffusion also disappeared because of the low growth temperature of 800 °C. It was found that the growth with an extremely low V/III ratio of 20 at low temperatures increases the lateral growth rate on the m-plane, and void formation is fully suppressed when the MQS height is 700 nm or less.

We investigated the effects of Si and Mg doping concentrations in the guide layers on the threshold power density of an AlGaN-based optically pumped ultraviolet-B band (UV-B) laser. For Si, a decrease in the threshold power density required for laser oscillation was confirmed at doping concentrations up to an order of 1017 cm−3. However, an increase was confirmed at high concentrations of approximately 1018 cm−3. For Mg doping, no decrease in the threshold power density of the optically pumped laser was confirmed even at low Mg concentrations, and it increased rapidly at 1018 cm−3. These results were characterized by weakly excited photoluminescence and variable stripe length, indicating that they were due to changes in optical gain with increasing or decreasing internal quantum efficiency and changes in internal loss. This suggests that Mg doping of the guide layer in UV-B lasers should be lower than 1018 cm−3 to realize low-threshold power density lasers. On the other hand, doping on an order of up to 1017 cm−3 for Si was effective at reducing the threshold power density. This indicates that the threshold power density increases if it is too high. These results could provide guidance for designing UV-B lasers in the future.

Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.

We investigated the dependence of the lasing threshold power density, optical gain, and internal loss on the dislocation density of an optically pumped AlGaN-based ultraviolet-B band laser. Reducing the dislocation density was found to not only increase the optical gain by reducing the non-radiative recombination centers but also reduce the internal loss. Furthermore, this reduction in internal loss was appropriately explained using an increased scattering model based on an increase in the refractive index fluctuations formed by the dislocations.

Structural characterizations of core-shell type nanowire-based materials are difficult because of their tiny 3D structure. In this paper, we prepared the GaInN/GaN multi quantum shell (MQS) grown on the sidewall of the n-GaN nanowire, and scanning transmission micro-scope (STEM) and 3D atom probe are employed for detailed structural characterization of such samples. Furthermore, cathodoluminescence (CL) measurement is carried out for characterization of optical properties. As a result of 3D atom probe measurement, an out-diffusion-like profile of In, where In atoms move from the wells to the outer barriers, was observed. This is considered that the incorporated In atoms in the outer barriers come from In–droplets caused on the top c-plane during the growth of GaInN.

An AlGaN ultraviolet-B laser diode at 298 nm was realized at room temperature using pulse operation. The laser diode has a lattice-relaxed Al0.6Ga0.4N layer from the underlying AlN/sapphire template and a composition-graded p-AlGaN cladding layer. The multimodal laser spectrum with proper polarization properties at 298 nm was obtained over the threshold current at 0.90 A corresponding to 67 kA cm–2. By broadening the width of the p-electrode to 11.5 μm, the threshold current density decreased to 41 kA cm–2.

We aimed to further improve performances of GaN-based vertical cavity surface emitting lasers (VCSELs) by applying a combination of conducting distributed Bragg reflectors (DBRs) and lateral optical confinement structures simultaneously, generally used in GaAs-based VCSELs. Si-doped conducting AlInN/GaN DBRs and buried SiO2 apertures were adopted in the GaN-based VCSELs. By comparing the VCSELs and micro LEDs to those with undoped non-conducting DBRs, we found that lower device resistances and more uniform lateral current distributions were obtained with the conducting DBRs. At the same time, the maximum light output power of 2.6 mW was observed from the VCSEL with the conducting DBR while 4.4 mW was obtained from the VCSEL with undoped DBR. Inferior characteristics of a GaInN quantum well active region was found on the Si-doped conducting DBR.

AlGaN-based deep-ultraviolet light-emitting diodes (LEDs) incorporating uneven multiple quantum wells (MQWs) with inclined and terrace zones, which were fabricated on an AlN template with dense macrosteps, have exhibited a high internal quantum efficiency (IQE). To investigate the microscopic structure of uneven MQWs, cathodoluminescence (CL) mapping characterization was carried out, and the maps of the CL intensity at 300 K relative to that at 38 K were obtained for uneven MQWs that targeted 265 and 285 nm LEDs. At an electron beam current of less than 1.0 nA, the signals from inclined and terrace zones of the uneven MQWs were confirmed to satisfy the nonsaturated excitation condition at 300 K. Nonradiative recombination (NR) was insufficiently frozen even at 38 K, specifically on the terraces in the 265 nm MQW, suggesting high concentrations of NR centers due to point defects (PDs). In contrast, NR in the 285 nm MQW at 38 K was closer to freeze-out. The concentration of PDs in the 285 nm MQW was likely to be lower than that in the 265 nm MQW. Finally, the ratios of the CL intensity at 300 K to those at 38 K were mapped, demonstrating an approach to creating an approximate map of IQE. The values in the CL intensity ratio maps are discussed by considering the analytical error factors. The results support the model of localized current injection through Ga-rich stripe zones in the n-AlGaN cladding layer.

This paper describes the status and prospects of gallium nitride-based vertical-cavity surface-emitting lasers (VCSELs) with semiconductor-based distributed Bragg reflectors. These optoelectronic devices, which emit laser light from the violet to green region, are expected to be a superior light source for the next-generation of displays and illumination, such as retinal scanning displays and adaptive headlights. The development status and prospects are discussed in comparison with already commercialized gallium arsenide-based infrared VCSELs.

The formation process of porous SiC by anodic oxidation was investigated, aiming at the generation of pure white light with a high color rendering index (CRI) and high luminous efficiency. The efficiency of white light emission from porous SiC and its wavelength are strongly dependent on the porous structure such as the average pore size and porosity. In this study, we examined the structure and optical properties of porous SiC by adding potassium persulfate (K2S2O8) as an oxidant in HF solution to control the porosity of porous SiC formed by anodic oxidation. By increasing the amount of the oxidant, we enhanced the integrated light emission intensity of porous SiC to 81 times that of bulk SiC. Through the study of porous SiC we demonstrated that the peak wavelength of the porous SiC could be controlled from 370 to 500 nm. Porous SiC created by anodic oxidation was thus proven to have great potential for realizing high-CRI white light generation using LEDs.

We investigated the influence of polarization charges in nitride-based semiconductors. The influence due to polarization charges was calculated excluding the influences of the band offset. We found that the polarization charges (1 × 1013 cm−2) resulted in an energy spike of more than 100 meV at the location of the charges, which is a similar value to the band offset. We then proposed the concept of polarization dilution to suppress the energy spike for better hole transport by using a graded Mg-doped AlGaN layer in UV-LEDs. Device simulation results indicate lower operating voltage and higher injection efficiency by using the polarization dilution. So far, our actual 350 nm LED with the polarization dilution showed lower operating voltage. These results suggested such polarization-charge management is important in the design of the nitride semiconductors.

The UV semiconductor-based laser sources are important for a variety of fields, including medical, mechanical processing, chemical processing, biology, and photonics. However, the development of UV-B and UV-C laser diodes is strongly hampered because of the difficulties with current injection technology such as the realization of both a high hole concentration and low resistivity p-type AlGaN with a high AlN molar fraction. Because laser oscillation from AlGaN, with a high AlN molar fraction, can be obtained under optical pumping, UV lasers with controllable wavelengths should be realized if this problem can be solved. One promising technique for avoiding these problems is the use of electron beam excitation. Till date, nitride semiconductor-based lasers have been designed to achieve population inversion of the carrier and to oscillate due to current injection. However, as previously discussed, it is difficult to achieve wavelengths in UV-B and UV-C using this method. The conductivity control of nitride semiconductors is unnecessary using electron beam excitation. Therefore, it would be possible to expand the wavelength region for the laser action of nitride semiconductor-based lasers from deep UV to infrared if a nitride semiconductor-based laser could be oscillated via electron beam excitation. In this study, nitride-based lasers excited by electron beam were investigated, and laser emission was observed for the first time from a GaInN/GaN and GaN/AlGaN-based MQWs excited by an electron beam.

This study investigated electron beam laser excitation in the UV region using a GaN/AlGaN multiquantum well (MQW) active layer. Laser emission was observed when the GaN/AlGaN MQW was excited by an electron beam, with a wavelength of approximately 353 nm and a threshold power density of 230 kW/cm2. A comparison of optical pumping and electron beam pumping demonstrated that the rate of generation of electron-hole pairs when using electron beam excitation was approximately one quarter that of light excitation.

In this study, we have aimed to improve the light extraction efficiency by using p- and n- high-reflectivity indium tin oxide (ITO)/Al electrodes with a reflectivity of ∼77% at 350 nm, laser lift-off (LLO) method, and silicone sealing. By a combination of these three methods, the light output power and operating voltage of UV LEDs were improved to 23.8 mW and 6.6 V at 100 mA. Furthermore, the external quantum efficiency and light extraction efficiency were improved to 6.6 and 25.3% at 100 mA. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We propose moth-eye patterned sapphire substrate (MPSS) as a solution to improve the light extraction efficiency and to reduce the production cost of LEDs. The MPSS samples' surfaces consisted of a triangular grid of about 375 nm high truncated cones with different pitches of 460 nm, 500 nm, 600 nm, 700 nm and 800 nm. A commercially available patterned sapphire substrate (PSS) and a flat sapphire substrate (FSS) was also used in the experiments for comparison. According to the cathode luminescence observation, the GaN template on MPSS with a thickness of 3 µm showed a threading dislocation density (TDD) of around 1.9 × 108 cm–2. Transmission electron microscope observation revealed that many of the dislocations were bent and disappeared as loops formed in the vicinity of MPSS cones. On the other hand, PSS required a GaN template thicker than 5µm to achieve a level of TDD equal to MPSS. The LED on MPSS with a pitch of 600 nm showed the highest light output power among the evaluated LEDs as it was 1.89 times higher than that on FSS and 1.05 times higher than that on PSS. The pitch dependence of the light output improvement agrees with the pitch dependence of the simulated transmissivity at the GaN/sapphire interface. As a result of our comparison, we concluded that MPSS provides the most cost effective solution for high performance LEDs.

We found out the reduction of contact resistance in V-based electrode for high AlN molar fraction n-type AlGaN by using thin SiNx intermediate layer. The contact resistivity for n -type Al0.70Ga0.30N with the V/Al/Ni/Au electrode using SiNx intermediated layer reached 1.13 × 10-6 Ω cm2. Moreover, contact resistivity using SiNx intermediated layer had been reduced more than one order of magnitude in all AlN molar fractions from 0.62 to 0.87. Using this electrode, we also demonstrated UV light-emitting diodes (LEDs) on n -type Al0.70Ga0.30N underlying layer with an emission wavelength of approximately 283 nm. An operating LED voltage using a V/Al/Ni/Au electrode with SiNx intermediated layer was 3.3 V lower at 100 mA current injection than that without SiNx intermediated layer. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We fabricated a GaInN/GaInP/GaInAs/Ge four-junction solar cell by wafer bonding a GaInN solar cell and a GaInP/GaInAs/Ge three-junction solar cell. We performed our wafer bonding at high pressure (500 N) and temperature (450 °C) by using p-type GaN and n-type GaAs. The open-circuit voltage (V OC), J SC, and fill factor of our four-junction solar cell under the condition of AM 1.5 G, i.e., 1 sun are 2.85 V, 0.219 mA cm−2, and 0.74, respectively. The improved V OC of our four-junction solar cells was confirmed by a series connection. The J SC is almost comparable to the theoretical value (itself based on the assumption of an ideal series junction), but the V OC is approximately 1 V less than that predicted by theory. Our research will improve the solar cell efficiency and help meet future energy needs.

AlGaN multiple quantum wells (MQWs) targeting 265 and 285 nm deep-ultraviolet LEDs grown on AlN templates with macrosteps were compared. In the cathodoluminescence images of the 285 nm MQW, high-intensity zones were observed along the edgelines of the macrosteps with wavelengths of 290–296 nm and on the terraces with wavelengths of 286–288 nm. Dark spots related to threading dislocations were seen in the entire 285 nm MQW. For 265 nm, high-intensity zones were limited along the edgelines and dark spots showed a low-contrast, which is likely to be caused by nonradiative recombination centers due to point defects.