Nanocrystal-based multi-quantum shell (MQS) structure is promising for next-generation high-performance light-emitting devices. Red MQS-LEDs are thought to be able to achieve highly efficient light emission by growing quantum shells on the semipolar surfaces in the n-GaInN pyramids. Regarding blue MQS-lasers, laser oscillation has been confirmed in a p-GaN embedded structure, and it seems that the potential as a high-efficiency laser is quite high. In n-GaN buried structures, lowering the resistance of the tunnel junction and the adjacent p-GaN is the key to realizing laser oscillation. These new quantum shell light-emitting devices are essential for the development of application fields such as micro-LED displays, laser processing, and optical wireless power transfer, and their future progress is strongly desired.

  Using InGaN/GaN-ordered nanocolumn (NC) arrays, two-dimensionally arranged multicolor (blue, green, yellow, and red) NC micro-LED (μLED) pixels with a luminous area of 5×5 μm² were demonstrated, exhibiting attractive prospects for application in μLED displays. The red μLED pixel exhibited low light intensity with a broad emission spectrum. To improve the red luminous property, semipolar InGaN/AlGaN multiple-quantum-wells were integrated into NC LED crystals, thereby realizing a φ2.2 μm red NC μLED with a high on-wafer external quantum efficiency of 2.1%. Additionally, honeycomb-lattice NC plasmonic crystals were investigated, which demonstrated a 4.8-fold enhancement of red light emission. Photonic crystal (PhC) NC LEDs exhibited directional light beam radiation and wavelength stability against current changes, with red wavelength stability at 615 nm. The monolithic integration of multicolor (green to orange) PhC NC LEDs was demonstrated. These results are promising for the development of three primary colors (RGB) integrated NC LEDs as next-generation full-color LEDs.

  Arbitrary visible spectra synthesis and switching are important for a variety of fields, including advanced lighting, high-performance displays, visible light communications, and chemical and biological analyses. However, conventional light-emitting diodes (LEDs) with planar structures emit light at a particular wavelength determined by the semiconductor material’s bandgap. This limitation restricts the spectral designability of InGaN-based LEDs. As a result, the assembly of multiple LEDs with different colors becomes a complex and time-consuming process, posing a challenge for the development of full-color micro-LED displays, which are a key device for the aforementioned applications. To overcome this issue, we have proposed InGaN quantum wells on novel three-dimensional (3D) structures with polychromatic emission properties. In this report, we present the current status and future prospects of polychromatic InGaN LEDs using tailored 3D structures for micro-LED displays.

  Here, we report the crystal growth of the InGaN-based red light-emitting diodes (LEDs) and their device properties for the next generation micro LED display applications. InGaN material is possible to obtain the primary colors: red, green, and blue which are suitable for the displays. However, the efficiency of the InGaN-based red LEDs is the bottleneck for the RGB micro-LEDs because of the fundamental issues in the high-in-content InGaN growth. From point of the view of the high crystalline quality InGaN growth, we focus on the key growth technologies such as higher growth temperature and strain engineering. The red LEDs perform high external quantum efficiency with a narrow line width. Our works demonstrate the prospect of the development of InGaN-based red LEDs. In this review, we describe the outcome of the InGaN-based red LEDs by several growth technologies.

  Micro-light-emitting-diodes (μ-LEDs) have recently attracted much attentions as novel light sources for next-generating ultrahigh-resolution display. To realize such high resolution, monolithic integration of three primary color LEDs based on the same material system using growth technique is indispensable. The development of efficient red LEDs based on GaN-platform is key technology to realize μ-LED displays. In red LEDs using Eu-doped GaN（GaN:Eu), the emission wavelength is ultra-stable with respect to ambient temperature and current injection levels. This is because the red emission originates from the intra-4f shell transitions of ⁵D₀-⁷F₂ in Eu³⁺ ions. The output power of the red LED has already proceeded to 1 mW due to the intrinsic and extrinsic control of local structures around Eu³⁺ ions. The effect of carrier sidewall-related non-radiative recombination on photoluminescence quantum efficiency is currently negligible due to the limited carrier diffusion length of GaN:Eu. We also demonstrate a monolithic vertical stack full-color LED consisting of GaN:Eu and InGaN quantum wells.

  One of the fundamental issues in the fabrication of μ-LED display using bottom-up crystal growth process is the low luminous efficiency of red LED, and further development of GaInN crystal growth technology is required. We have studied on the possibility of the structure in which GaInN/GaInN multiple quantum wells (MQWs) are inserted in pn-GaInN matrix, instead of the structure with GaInN/GaN MQWs in pn-GaN. This structure is expected to suppress the generation of crystal defects in the MQWs due to large lattice mismatch. This is also expected to increase the probability of electron-hole recombination and the large wavelength shift due to the quantum confined Stark effect (QCSE). In this article, we introduce our studies on the radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) growth of GaInN, especially on (i) a novel growth method of DERI (Droplet Elimination by Radical Beam Epitaxy) for the growth of film structure, (ii) in-situ monitoring and control in the initial heteroepitaxial growth using synchrotron X-ray diffraction (XRD) and (iii) nanocolumns and its device application.

  ScAlMgO₄ (SAM) has attracted attention as a substrate for fabricating bulk GaN crystals and InGaN template substrates. Radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) is preferred for the growth of the first (In)GaN template layers to avoid the contamination of the epilayers by the constituent element atoms of the SAM substrate. We have grown GaN and InGaN films directly on SAM substrates without any buffer layers using RF-MBE. The (In)GaN/SAM interface was atomically abrupt and no significant contamination in the epilayer was observed. Nearly lattice-matched InGaN was obtained by the optimal growth temperature and source fluxes. However, the extraordinarily large step height close to three bilayers of wurtzite (WZ-) GaN and low temperature growth in MBE results in the mixing of the metastable zincblende (ZB) phase in the GaN films. High-temperature growth on a SAM substrate with wider terraces and increasing the film thickness suppressed the ZB mixing and a pure WZ-GaN layer was obtained.