Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by ...assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs.
To elucidate the microscopic origin of the thermal droop, a blue-emitting indium gallium nitride (InGaN) quantum well grown on epitaxially laterally overgrown gallium nitride was investigated using ...temperature-dependent microphotoluminescence spectroscopy. Below 300 K, the sample exhibited a well-known dislocation-tolerant luminescence behavior. However, as temperature increases from 300 K to 500 K, the near band-edge emission at the wing region (with lower threading dislocation densities) was stronger than that at the seed region (with higher threading dislocation densities), indicating that threading dislocations are the microscopic origin of the thermal droop. Considering the carrier diffusion length, edge-type threading dislocations should play a major role in the thermal droop of heteroepitaxially grown InGaN-based LEDs.
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•A brief history and underlying mechanism of the surface-plasmon (SP)-enhanced light emissions were presented.•Enhancements of the spontaneous emission rates of the excited states ...were discussed by the terms of the coupling states between an exciton and SP.•Recent progress and current problems regarding device applications of plasmonic light-emitting diodes (LEDs) were reviewed.•Future possibilities of SP-enhanced light emissions were discussed to extend the wavelength regions from deep ultraviolet (UV) to infrared (IR).
Coupling between surface plasmons (SPs) and excitons can be used to enhance the emission efficiencies of light-emitting materials and devices. This approach had been theoretically predicted and, in 2004, was experimentally demonstrated by our group for enhancing the visible emission from InGaN/GaN quantum wells (QWs). Exciton–SP coupling increases the spontaneous emission rates of the excited states, causes a relative reduction in nonradiative relaxation, and ultimately increases the internal quantum efficiencies (IQEs) of such devices. Here, we present a brief history of the increases in emission efficiency that have been achieved and the underlying mechanism thereof. This method has the potential to enable the development of high-efficiency light-emitting diodes (LEDs), eventually leading to the replacement of fluorescent lights with solid-state light sources. After the initial discovery of this phenomenon, many device structures were proposed and reported; however, their emission efficiencies have thus far remained insufficient for practical application. Here, we also present recent progress on device applications and the current problems that must be solved. Finally, we explain the future possibilities regarding the extension of SP-enhanced light emission over a broader wavelength region, from the deep ultraviolet (UV) to the infrared (IR).
•Semipolar AlN films are grown on 15°-off (0001), (111¯02), and (112¯2) AlN substrates.•A high growth pressure realizes pit-free semipolar surfaces, unlike the (0001) growth.•The surface pits are not ...due to threading dislocations in the substrates.•The semipolar surfaces after pit-elimination are atomically smooth.•Strong and narrow near band edge emissions dominate the semipolar AlN emission.
Semipolar AlN homoepitaxial films, which are expected to act as underlying layers of highly efficient light emitters, are fabricated on 15°-off (0001), (11¯02), and (112¯2) AlN substrates using the metalorganic vapor phase epitaxy method. In conventional (0001) AlN growth, low reactor pressures are preferred to enhance the migration of Al adatoms and to suppress parasitic reactions between trimethylaluminum and ammonia. In contrast, low-pressure growth generates numerous pits on the surface of semipolar AlN grown homoepitaxially, which are derived from defects formed in the initial growth stage. Herein we experimentally demonstrate that higher-pressure growth can drastically decrease the pit density. A higher-pressure growth realizes atomically smooth surfaces, strong near-band-edge emissions with narrow line widths (∼1–2 meV), and well-suppressed deep level emissions. The optimal reactor pressure to eliminate pits is 500 Torr in terms of the growth rate and nucleation density.
GaN layers are grown on Al-pretreated ScAlMgO4 (0001) substrates by metal-organic vapor-phase epitaxy (MOVPE) without using low-temperature (LT) buffer layers. The Al pretreatment is performed under ...a H2 and N2 gas mixture and results in the nucleation of AlN on the ScAlMgO4. High-resolution transmission electron microscopy reveals that the ScAlMgO4 (0001) surface is terminated with the rocksalt phase ScO (111). Because of the chemical nature of the ScO layer, the AlN nuclei are N-polar. However, subsequent GaN MOVPE inverts the polarity to metal-polar in the vicinity of the boundary between the interfacial AlN nucleation layer and GaN layer, which is attributed to the GaN surface during MOVPE being more stable when it is metal-polar. The grown GaN layers exhibit better crystalline quality compared with GaN grown on ScAlMgO4 or sapphire (0001) substrates using conventional LT-buffer technology, thereby demonstrating the promise of the method to produce high-quality GaN epilayers on ScAlMgO4.
GaN/AlN ultrathin quantum wells (QWs) emitting in the deep UV spectral range are fabricated by metalorganic vapor phase epitaxy. The GaN thickness is automatically limited to the monolayer (ML) scale ...due to the balance between crystallization and evaporation of Ga adatoms. This growth characteristic facilitates the fabrication of highly reproducible GaN ML QWs. The strong quantum confinement within the GaN ML QWs achieves emissions below 250 nm. The photoluminescence intensity at room temperature with respect to that at low temperature, which is closely related to the emission internal quantum efficiency, is drastically improved from 0.1% for a conventional Al0.8Ga0.2N/AlN QWs to 5% for a 1 ML GaN/AlN (0001) QW and 50% for a 2 ML GaN/AlN (11¯02) QW under weak excitation conditions. These higher emission efficiencies are attributed to the suppressed nonradiative recombination in the GaN QWs and the enhanced radiative recombination in the (11¯02) QW.
Ultrathin GaN/AlN quantum wells (QWs) on the monolayer (ML) scale drastically improve the emission internal quantum efficiency in the deep ultraviolet spectral range, compared with conventional AlxGa1−xN‐based QWs. This achievement is attributed to the suppressed nonradiative recombination due to the point‐defect reduction and enhanced radiative recombination in GaN‐based ML QWs.
Broadband ultraviolet (UV) emission is achieved using AlGaN microstructure 2D arrays. 2D arrays of trenches are initially formed on AlN templates on sapphire (0001) substrates. AlGaN‐based quantum ...wells (QWs) are subsequently regrown on top of the patterned templates. Bunched steps are formed within the trench, inducing variations in the Al composition and AlGaN thickness in the QWs. Regions with and without bunched steps coexist and cause emission wavelength variations. The formation mechanism of bunched steps is attributed to the position‐dependent AlN growth rate due to variations of the source–precursor flow within narrow trenches.
Semiconductor emitters inherently show single‐peak emissions determined by the bandgap, whereas current major ultraviolet (UV) light sources of Hg lamps show multiple peaks. Herein, multiple‐component UV emission is achieved from semiconducting AlGaN using 2D microstructure arrays. This achievement may promote the use of environmental‐friendly AlGaN‐based UV emitters as alternatives to Hg lamps.
Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum ...efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and temperature dependences of the product (IQE × CIE) are derived. The temperature dependence of the EQE cannot be simply explained by the Auger recombination processes. This observation enables the CIE and IQE to be separately extracted by rate equation analysis. The room-temperature EQE of the AlGaN-based DUV LEDs is limited by the CIE and not the IQE. We propose that the relatively low CIE may originate from the nonradiative recombination process outside quantum-well layers.
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