Bare LED Research Articles

Study on Localized Surface Plasmon Coupling with Many Radiators.

Localized surface plasmon (LSP) coupling with many radiators are investigated. The LSP is generated by excitation of laser or electron beam on the random Ag nano particles (NPs) and arrayed ones embedded in the p-GaN of green LEDs. They couple with the excitons or radiative recombination in the quantum well (QW) and electron beam, which enhance or suppress the luminescence of the radiators. The photoluminescence (PL) intensity of periodic Ag NPs can get as much as 4.5 times higher than that of bare LED. In addition to the periodic structure, the morphology of Ag NPs also affects the localized SP (LSP) resonance intensity and light scattering efficiency. In the finite difference time domain (FDTD) simulation, five x-polarized dipoles are approximated to five quantum wells. Considering the interaction between the five dipoles and their feedback effect on LSP, the enhancement effect of SP dipole coupling with Ag NPs is amplified and the energy dissipation is reduced. The enhancement of cathodoluminescence (CL) was also found in green LEDs with Ag NPs. The three-body model composed of two orthogonal dipoles and an Ag NP is used for 3D FDTD simulation. The LSP-QWs coupling effect is separated from the electron beam (e-beam)-LSP-QW system by linear approximation. Under the excitation of electron beam, the introduction of z-dipole greatly reduces the energy dissipation. In the cross-sectional sample, z-polarized dipoles in QWs show more coupling strength to the dipole and quadrupole modes of LSP. The perturbation theory is used to separate the LSP coupling effects to x-dipole and z-dipole. At last, the resonator and the antenna effects are discussed for LSP coupling at different positions to the Ag NP.

Morphological effect of patterned sapphire substrate on efficiency of white-light phosphor LED package

Six-facets patterned sapphire substrates (A-PSS) are manufactured by wet-etching regular semispherical patterned sapphire substrates (r-PSS). The possible crystallographic planes of the etched planes on the semi-sphere pattern is the family of (1102) plane. Metalorganic Chemical Vapor Deposition (MOCVD) GaN epitaxial LED structure was grown on the c-plane sapphire wafer, r-PSS, and A-PSS, which were further processed as horizontal LED chips. Compared to LEDs on the flat c-plane sapphire, the light output power of LEDs on r-PSS and A-PSS is enhanced by 71% and 67%, respectively. The output power of the bare LEDs decreases after the white-light LED package. Remarkably, the output power of the white-light packaged LED on the A-PSS is larger than the white-light packaged LED on the r-PSS. The Monte Carlo ray tracing simulation shows that, before and after silicone-lens encapsulating, the LEDs on r-PSS and A-PSS emit more on the top surface. However, the sidewall emission power ratio of the LED on r-PSS decreases by 3.44% after silicone encapsulating. On the other hand, the sidewall emission power ratio of the LED on A-PSS increases by 1.06% after silicone encapsulating. Note that the emission light of LEDs out of the top surface travels a longer path in the silicone lens than the emission light out of the sidewalls. Hence, more re-extracted light out of the sidewall of LED on A-PSS would be the reason for the LED on A-PSS having a less package loss and the white-light package efficiency than the LED on r-PSS. In conclusion, the light radiation distribution pattern defined by the pattern morphology of A-PSS is the key for the better white-light package efficiency of the LED on A-PSS than that of the LED on r-PSS. • The patterned sapphire substrate morphology can alter the propagation direction of the light emission. • Monte Carlo ray tracing simulation shows the emission power of top surface and sidewall vary before and after lens package. • The emission light of LEDs out of the top surface travels a longer path in the phosphor layer than the sidewalls. • The light radiation distribution defined by the pattern morphology is the key for white-light package efficiency of LED.

Construction of a ternary channel efficient passive cooling composites with solar-reflective, thermoemissive, and thermoconductive properties

Current passive radiative cooling is a promising alternative to electrical cooling systems, but it is still difficult to be used in cooling devices with an internal heat source. Hence, we here present a ternary channel efficient passive cooling composites by embedding hexagonal boron nitride (h-BN) into high-density polyethylene (HDPE). Like the traditional passive radiative cooling materials, the h-BN/HDPE composite possesses a high solar reflectance (~87%) that minimizes solar heat gain, and a high thermal emittance (~0.71) that maximizes radiative heat loss, but the composites also have a high thermal conductivity (~1.47 W/(m·K)) that maximizes the internal heat conduction. Therefore, the combination of the optical and thermal properties allows for the LED's temperature drops of 5 °C, compared with the bare LED, under the average solar intensity of 1100 W/m2. The notably cooling effect provides the guidance for the future application of the ternary channel efficient passive cooling composites in cooling the devices with internal heat.

Uniaxial ultra-high pressure dependent tuning of optical gain of W-shaped Type-II GaAsSb/InGaAs/InAlAs nano-heterostructure

In this article, the main emphasis has been given to understand the mechanism of tunability of the optical gain characteristics of W-shaped type-II GaAsSb/InGaAs/InAlAs nano-heterostructure under the variable applied pressure ranging from 1 to 20 GPa along (001), (100) and (110) directions. In pursuance of the calculations, it was examined that the application of inconstant pressure along (001) direction can improve the optical gain insistently with the red shift in wavelength; while the (100) directional pressure reduces the gain with the red shift in wavelength. But, the (110) directional pressure can reduce the gain up to a certain limit of the pressure (∼ 10 GPa), after that it begins to improve the gain characteristics. However, the tuning range of the gain characteristics was restricted in the wavelength limit of ∼ 1550–1900 nm. These consequences recommend that such tunable heterostructures can be employed in designing and fabricating the new high power IR-LEDs, bare LED chips, and modern tunable laser gain chips, which can have cantered wavelength at 1550 or 1900 nm.

Maximum Limits on External Quantum Efficiencies in Bare LEDs

Inevitable self-heating effects have been investigated for LEDs to date. In this paper, we present the thermally-induced penalty (TIP), and derive an upper external quantum efficiency (EQE) limit for a given bare LED by adding TIP to the real EQE. We find that the red-AlGaInP LED exhibits no efficiency droop under room temperature and the TIP is responsible for its real EQE droop. Among the TIPs of red, green, and blue LEDs, the red-AlGaInP LED suffers the most severe penalty. This TIP can serve as a criterion for LED cooling design, with its lower values leading to higher cooling rates.

Progress and expectation of quantum dots converted light emitting diode package

Quantum dots converted light emitting diode (QCLED) is a new-style lighting device with quantum dots (QDs) as the down conversion material. The QDs with tunable spectrum, remarkable saturation and high quantum yields result in the excellent performance of color rendering and saturation as well as the gamut of QCLED. As a consequence, the QCLED is attracting more and more attentions nowadays. Different from the traditional phosphor powders which are the down conversion material of white LED, the quantum dots are usually stored up in the fluid or solid matrix, and the most common package type of QDs is to be blended with the high polymer and then packaged into the bare LEDs. However, there are 4 crucial problems in the process of packaging: (1) The poor compatibility of QDs and high polymer matrix. Although techniques for incorporating QDs in high polymer films are well developed, the coexistence of QDs and bulk polymer matrix remains to be strengthened, or there will be some problems such as low yield of film, cluster of QDs, fluorescence quenching, and so on. (2) The poor thermal stability of QCLED. When the temperature rises, the organic ligands growing on the surface of QDs will fall off or become inactivated, then the surface defects are exposed to matrix, which will result in the reduction of fluorescence efficiency. (3) The poor stability of QDs against oxygen and moisture. The penetrative oxygen and moisture will corrode the surface ions and ligands on the surface of QDs, consequently resulting in defect trap states and furthermore the degeneration of optical performance of QCLED. (4) The optimization of optical spectrum. There are usually 3 or more spectrum distributions for QCLED light, meaning that 2 or more kinds of QDs are needed to combine with the light extracted from LED chip. In order to obtain the high performance QCLED, the collocation of the QDs and LED chip should be quite well designed. Aiming at these problems, the researchers have tried a good deal of solutions: (1) Ligand modification, surface passivation, QDs microspheres and etc. are employed to enhance the compatibility between the QDs and polymer matrix. (2) Design of QDs films composition structure and optimization of QCLED’s package structure are applied to improve the thermal stability. (3) Improvement of QDs chemical stability and protection of QDs films are conducted to improve the stability of QCLED against oxygen and moisture. (4) Kinds of optimization algorithms are employed to design the spectra distribution, and many packaging experiments are conducted to obtain the QCLED with high performance, and finally the advantages of the QCLED are highlighted at the researchers’ efforts. We summarize the recent developments of the QCLED in this review. The strengths of QCLED in illumination and display are overviewed, and the common packaging processes are introduced. What’s more, the problems mentioned above are explained at length, and the corresponding solutions are discussed one by one. In the last decades, the QCLED has achieved significant developments, and reviewing on these progresses will prompt the QCLED to tend to be mature.

Improvement of Light Extraction Efficiency in Flip-Chip Light Emitting Diodes on SiC Substrate via Transparent Haze Films with Morphology-Controlled Collapsed Alumina Nanorods.

We demonstrate GaN-based flip-chip light emitting diodes (FC-LEDs) on SiC substrate achieving high extraction efficiency by simply attaching the optically transparent haze films consisting of collapsed alumina nanorods. Through controlled etching time of alumina nanorods, we obtain four types of films that have different morphologies with different optical transmittance and haze properties. We show that the light output power of the FC-LEDs with film, which has 95.6% transmittance and 62.7% haze, increases by 20.4% in comparison to the bare LEDs. The angular radiation pattern of the LEDs also follows the Lambertian emission pattern without deteriorating the electrical properties of the device. The improvement of light extraction is mainly attributed to the reduced total internal reflection (TIR) via efficient out-coupling of guided light from SiC substrate to air by collapsed alumina nanorod structures in the film. The high transparency of film and reduced Fresnel reflection via graded refractive index transition between the film and SiC substrate also contribute to the extraction enhancement of the device. We systematically investigate the influence of haze film's geometrical or optical properties on the extraction efficiency of FC-LEDs, and this study will provide a novel approach to enhance the performance of various optoelectronic devices.

Analysis of wide color gamut of green/red bilayered freestanding phosphor film-capped white LEDs for LCD backlight.

In this study, we propose green/red bilayered freestanding phosphor film-capped white light-emitting diodes (W-LEDs) using InGaN blue LEDs and narrowband red and green phosphors to realize a wide color gamut in a liquid crystal display (LCD) backlight system. The narrowband K2SiF6:Mn4+ (KSF) red and SrGa2S4:Eu2+ (SGS) green phosphors are synthesized using a facile etching synthetic process and flux-aided solid state reaction under a H2S atmosphere, respectively, and the freestanding phosphor films are fabricated using a delamination method with water-soluble polymer, polystyrene sulfonic acid, PEDOT/PSS, and interlayered phosphor film. Various phosphor concentrations of green/red bilayered freestanding phosphor film-capped W-LEDs exhibit a correlated color temperature (CCT) and luminous efficacy range of 11,390 K ~6,540 K and 99 lm/W ~124 lm/W, respectively, with an applied current of 60 mA. The W-LED with green (12.5 wt%)/red (40 wt%) bilayered phosphor film, which exhibited luminous efficacy of 105 lm/W at the CCT of 8,330 K, is selected and the color gamut of the bare LED and phosphor RG and the filtered RGB triangle is calculated to be more than ~95% and ~86.4%, respectively, relative to the NTSC in the 1931 CIE color coordinates space.

Study of MID Technologies for Automotive Lighting and Light Signaling Devices

The production of Molded Interconnect Devices (MIDs) may be achieved through different processes. In this work Centro Ricerche Plast-optica (CRP) has chosen to evaluate, implement and assess Laser Direct Structuring (LDS) and In Mold Labeling (IML) technologies for the production of MIDs. Both alternative methods have been analyzed starting with a 2D component, mainly used for implementation and optimization of the process, and finally a more complex 3D component, that has been designed and produced. The first phase of the activity regards the production of several conductor patterns on planar substrates dedicated to evaluate properties as conductors resistance, adhesion, SMD components solderability, wire bondability of bare LED dice etc. In the case of IML, a flexible circuit has been over-molded during the production of the component by injection molding process: planar parts have been tested also in terms of adhesion of flexible circuit to the injected polymer. The second phase of the activity concerns the production of 3D circuits. Both technologies have been implemented in order to select to materials, process conditions and parameters, design rules and to verify the reliability in the automotive severe conditions. Exploitation of technologies have been performed on an automotive rear lamp. A prototype has been produced in LDS technology and allowed to define the conditions that make economically affordable this solution. Differently, the IML technology allowed to produce a completely new device by embedding a planar electronic circuits into the plastic material.

Large area flexible lighting foils using distributed bare LED dies on polyester substrates

Integration of LEDs on flexible foil substrates is of interest for flexible lighting applications and for backlights for flexible displays. Such a large area lighting device can be made by integrating a matrix of closely spaced LEDs on a flexible foil substrate. Preferably, these LEDs are integrated unpackaged, i.e. as bare dies, as this reduces footprint, thickness and cost. As substrates, low cost materials like polyethylene terephthalate (PET) should preferably be used. However, the use of these materials also imposes limitations. Especially, their low thermal stability limits the maximum temperatures during the processing and the thermal dissipation of the LED during operation will pose constraints on the thermal design. This paper describes the results of research on possibilities for integrating bare die LEDs with such low cost flexible PET foils. Bonding of LED dies on PET substrates with copper circuitry using conductive adhesives was performed. Both anisotropic conducting adhesives and isotropic conducting adhesives were investigated. An experimental comparison is made between the different techniques based on temperature/humidity reliability and flexural stability of the bonded LEDs. Additionally, finite element (FE) thermal modeling results of adhesively bonded LED-on-foil configurations are presented. The role of the different materials and the effect of their geometries on the temperature distribution in the simulated devices are discussed. The results are compared to experimentally observed temperature distributions using infrared thermal imaging in LED on PET foil reference devices. Finally a demonstrator device of 64 LEDs on flexible copper–PET substrate is presented.

Resonant absorption and scattering suppression of localized surface plasmons in Ag particles on green LED

The metallic-structure dependent localized surface plasmons (LSPs) coupling behaviors with InGaN QWs in a green LED epitaxial wafer are investigated by optical transmission, scanning electron microscopy (SEM) and photoluminescence (PL) measurements. Ag nanoparticles (NPs) are formed by thermal annealing Ag layer on the green LED wafer. SEM images show that for higher annealing temperature and/or thicker deposited Ag layer, larger Ag NPs can be produced, leading to the redshift of absorption peaks in the transmission spectra. Time resolved PL (TRPL) measurements indicate when LSP-MQW coupling occurs, PL decay rate is greatly enhanced especially at the resonant wavelength 560 nm. However, the PL intensity is suppressed by 3.5 folds compared to the bare LED. The resonant absorption and PL suppression are simulated by three dimension finite-difference-time-domain (FDTD), which suggests that Ag particle with smaller size and lower height lead to the larger dissipation of LSP.

High Density Attachment of HB-LED Die to Heat Sink Surfaces Using Aerosol Jet Printing

High LED junction temperature leads to reduced LED output and color shift, and are a factor in most LED failure mechanisms. Every interface and layer of packaging between the junction and the heat sink increases the device operating temperature necessitating more aggressive heat sink design. Packaging also increases the footprint of the device, limiting packing density. The ideal design would have the bare LED die attached directly to the heat sink with a micron-scale bond of thermally conductive adhesive. This paper discusses the use of an Aerosol Jet ® system to create micron-scale bonds of adhesives for both top and bottom-side (flip-chip style) connections. The system is also used to write topside connections, replacing wire bonding. In order to limit the temperatures of LED junctions with power densities of 250 W/cm2 to below 100 °C, it is necessary to limit the thermal resistance of a direct die-heat sink bond for a 0.4 W, 0.16 mm2 die to approximately 100 C/W. Assuming 50% coverage, this implies a thickness of approximately 1.5 um for an unfilled adhesive (K=0.2) or 40 um for a filled adhesive (K=5). For a 0.16 mm2 die this requires an adhesive volume of 65 pl and 1600 pl respectively, far below the 17,000 pl in the 8 mil hemisphere from a syringe dispense. Aerosol Jet systems are able to produce patterned deposits with total volumes in the 100 pl range. This paper will describe the results of work with top and bottom-side attachment of 450 nm InGaN LED die to thermally conductive substrates.

Study on photoluminescence of GaN-based UV-LEDs with refractive index gradient polymeric nanopatterns

For the GaN-based UV-light emitting diodes (LEDs), a lot of studies have been performed to improve the internal quantum efficiency; however, low external extraction has been another confronting problem. Here, we constructed nanopatterned polymer layers on UV-LEDs. Employing curable polymer (P4MS), nanopatterned polymer layers were successfully prepared on UV-LEDs by curing the pre-deposited first layer and imprinting the second uncured layer with the nanopatterned soft mold, followed by UV curing. We considered the nanostructured P4MS layers as multiple layers from bottom to top, which is expected to improve external light extraction of UV-LEDs with small difference of refractive indices. As compared to the bare LED, PL intensity showed about 25% increment for the surface-treated UV-LEDs with nanopatterned polymer in blue light region in thickness of continuous layer from 21 to 120 nm.

Modeling Light‐Extraction Characteristics of Packaged Light‐Emitting Diodes

We employ a Monte Carlo ray‐tracing technique to model light‐extraction characteristics of light‐emitting diodes. By relaxing restrictive assumptions on photon traversal history, our method improves upon available analytical models for estimating light‐extraction efficiencies from bare LED chips, and enhances modeling capabilities by realistically treating the various processes which photons can encounter in a packaged LED. Our method is not only capable of calculating extraction efficiencies, but can also provide extensive statistical information on photon extraction processes, and predict LED spatial emission characteristics.