This PDF file contains the front matter associated with SPIE Proceedings Volume 10940, including the Title Page, Copyright information, Table of Contents, Introduction, Author and Conference Committee lists

We report a transparent 269 nm deep ultraviolet (UV) light-emitting diode (LED) with a thin Mg-doped AlN p-type ohmic contact layer. At 20 mA direct current, the forward voltage is 6.2 V and the optical output power is 11.8 mW, translating into wall-plug-efficiency (WPE) and external quantum efficiency (EQE) equal to 9.5% and 12.8%, respectively. The device maintains 70% of its original optical output power for more than 1000 hours (L70≥1000 hrs) at a current density (J) of 88.9 A/cm². Experimental data support that this device will have a significantly increased L70 for J ≤ 30 A/cm². We also demonstrate that for deep UV LEDs the EQE vs current-density (EQE-J) curve can be well fitted by the standard carrier recombination model (ABC model), and internal quantum efficiency (IQE) and light-extraction efficiency (LEE) can thus be extracted. Furthermore, we propose a method for quick assessment of LED’s lifetime, through fitting of EQE-J curves before and after short-term reliability test.

Two different structures of AlGaN/InGaN ultraviolet (UV) multiple quantum wells (MQWs) were grown in a metalorganic chemical vapor deposition (MOCVD) system, and their performance under optically pumped stimulated emission were experimentally investigated. During the MOCVD epitaxial growth of the AlGaN/InGaN MQWs, the growth rate of the AlGaN quantum barriers (QBs) was intentionally reduced to improve the surface morphology. Atomic-force microscopy (AFM) images show that the AlGaN QBs have a smooth surface with clear step flow patterns. The surface morphology of InGaN QWs was improved by thermal annealing effect when the growth temperature rose to the one of the AlGaN QBs. With optical confinement layers on both the n- and p-sides, the threshold pumping power density of optical stimulated emission for AlGaN/InGaN MQWs was determined to be 168 kW/cm². In order to reduce the negative effect of the interface between AlGaN QBs and InGaN QWs, another MQW structure with a larger quantum well thickness was designed and epitaxial grown. The optical investigation of sample B showed a threshold pumping power density of 124 kW/cm², which is 26% lower than sample A.

AlGaN-based deep ultraviolet light-emitting diodes (DUV-LEDs) have attracted much attention due to the potential applications in water purification, sterilization, and phototherapy etc[1]. However, the emission efficiency is still much lower than GaN-based blue LEDs because of the poor buffer quality, light extraction efficiency and carrier injection efficiency. In this study, sputtering technology was adopted to form the AlN nucleation layer and high-quality AlN buffer layer was grown by MOCVD. The full width at half maximum (FWHM) of X-ray diffraction (002) and (102) are <50 and <320 arcsec, respectively. Based on this technology, high-performance AlGaN DUV-LEDs with output power of ~32mW at operation current of 100mA was demonstrated.

UV-A LEDs have seen efficiencies reach almost 70% and a substantial reduction in cost over the last few years. These trends have enabled UV-LED-based curing to become a mainstream application with most LEDs being driven at 1-2 A/mm². At the same time novel applications, such as high-volume 3D Printing requiring higher power from UV LEDs have started to emerge. To address these applications, and to improve system speed in traditional applications, Luminus Devices has developed UV-A LEDs that can be operated at up to 4 A/mm² which is the highest current density of any commercially available LED on the market. These next generation LEDs are very reliable and at 4 A/mm² enable up to 74% higher power than LEDs driven at 2 A/mm2. This paper presents the performance characteristics of these nextgeneration LEDs, the novel and emerging applications they enable and provides a roadmap for future performance gains. UV-C LEDs are starting to become commercially viable but have low wall-plug efficiencies of 2%-4% and power levels reaching 100 mW. UV-C LEDs are not only replacing lamps but creating entirely new markets. The current status of UV-C LEDs is presented along with a discussion of the applications the LEDs will enable over the next 2-3 years.

The usage of UV LEDs is getting attractive for application such as phototherapy, plant growth, and disinfection due to the wavelength selective, narrow-band emission and a high potential for miniaturization of LED devices. Besides these benefits, the demands on optical power and long-term stability for these applications can often be well satisfied. For UVB LEDs most promising applications are in the field of medical skin therapy and novel concepts of horticulture and plant growth (irradiation of plants for the generation of phytamines or to reduce hormone-like mixtures). UVC applications focus on disinfection of air, surfaces and water at 265 nm or 280 nm. Each application field requires an individual UV dose, which is connected with the optical power output of the LED, and thus the number of LEDs and their long term stability. Typical doses for skin irradiation is 20 mJ/cm² at 310 nm and for water disinfection 20-60 mJ/cm² at 280 nm depending which target reduction factor log reduction of germicides is required. In this work a discussion on different factors influencing the reliability of LED modules, summarizing several years of research in this field will be given. Degradation effects are shown depending on LED design itself as well as the device assembly architecture including different mounting techniques. The most promising assembly technique was tested by a sample series of twice 400 single LED packages with a total yield of 87.7 % after mounting of UVB LEDs in single LED cases, cascading to an array on a main board by secondary soldering and burn-in of 48 hour at 50mA. In total 4% of the yield loss results by soldering issues of the LED on submount as well as another 8 % yield loss was measured after cascading of single LED packages on main board. Due to the burn-in process additional twenty UVB LEDs were lost. These reliability issues will be discussed using selected “state-of-the-art” LED device structures and examples of testing these LED devices in UV lighting lamp systems built at OSA opto Light will be given.

Deep ultraviolet (UV) light emitting diodes (LEDs) have a wide range of applications such as water treatment, medical diagnostics, medical device sterilization and gas sensing. The internal quantum efficiency of UVB and UVC LEDs is extremely low. Added to this is the high refractive index of the sapphire substrate. The electrical input power is converted to more than 95% to heat. Typically, ceramic packages of alumina with metal core or aluminum nitride are used. These promise a minimized thermal resistance. Comparative thermal simulations show that even Si with slightly lower thermal conductivity of 150 W / mK compared to aluminum nitride with 180 to 200 W / mK does not necessarily impair thermal management. From the thermal and optical calculations, basic information was extracted that forms the basis of the Si package layout. The advantage of the Si packing due to the possibility of integrating functional components has been worked out. An optimized Si package is presented that meets in particular the requirements of the assembly and packaging technology of UVB and UVC LEDs. The process technology was designed and implemented. The first samples with integrated protection diode, an optimized reflector and an optically adjusted single Fresnel lens are presented. The Si packages are designed for the flip-chip technology of UV LEDs with SnAg soldering, thermo-compression or thermosonic bonding and silver sintering. Furthermore, an outlook is given on the possibilities of an encapsulating technology to improve the light extraction.

Technology development of blue, green and red light micro LED display (uD) was developed in this study. Here, 64 x 32 pixels on the display, with size of 50 μm x 50 μm and pixel pitch of 100 μm were demonstrated. ITO as transparent conducting layer on the P-epilayer is used to be contact layer and contributes current spreading. Flip chip LED process as basic of process design is apply to this study, light is emitted from the back side of LED that it can prevent light emitting that is shaded from metal. Ti/Al/Ti/Au as electrode was deposited on the anode and cathode. Display was boned to IC by anisotropic conductive film, the process of display was finished eventually. Single pixel on matrix display can be driven by mix multi-electrodes addressable controlling circuit with passive IC, the image is demonstrated on the display successfully. The 430, 590 and 600 nit for blue, green and red uDs were obtained as the uDs were driven by PWM with voltage source at 5 V.

In this work, we study the optical emission from arrays of InGaN/GaN MQW nanofin and nanorod arrays with sizes ranging from a few micrometers down to sub-wavelength dimensions (i.e., nanometers). Such systems are of interest for developing arrays of single addressable nanoLEDs, which could be used to obtain a visible wavelength super-resolution microscope where the resolution is due to highly localized light spots with sub-wavelength LED-to-LED pitch. We have used commercial full-wave Maxwell solvers (COMSOL, CST) to calculate the optical field emitted from a single nanoLED in a periodic array for a wavelength of 450 nm. Simulations on 11×11 nanoLED arrays with pitches of 200 nm up to 800 nm and diameters of down to 50 nm have been conducted, in which the dependency of the emission pattern on different structural parameters is studied. In case of small nanoLED array with very narrow pitch, a large optical cross-talk between the activated LED and its neighboring pixels was found. Moreover, in presence of cross-talks, test objects smaller than the LED pitch placed on its surface with influence of near field could potentially be resolved by evaluating the varied emission patterns obtained by different pixel activations. Routes to achieve higher localized optical fields and reduce optical cross-talk have been also investigated by modifying the nanoLED array structures (e.g., by introducing filling material among the LED pixels).

This study is to produce three-dimensional structure by applying nano-mold process. The three-dimensional structure can be more suitably provided through a nano process using a patterning and etching process. The nano-mold process enables multiple wavelength light emitting diode. The nanorod structure through nano-mold technology is suggesting the new characteristics of nano structure by the way of realizing the light emitting diode of three-dimensional structure. The light emitting area will also be enlarged through changing three-dimensional from two-dimensional structure.

Nanotechnological approaches have been widely explored for improved light output in InGaN/GaN light emitting diodes (LEDs). In this talk, I will introduce the application of SiO2 nanoparticle (NP) embedded in GaN nanopillar template for high optical extraction, of Ag/SiO2 core/shell NP for a localized surface plasmon (LSP) LED, and of porous GaN templates made by combined wet chemical etchings. With the SiO2 NPs placed between the GaN nanopillars, subsequent overgrowth of GaN layer started only on the exposed tips of the nanopillars and rapidly switched to the lateral growth mode. This resulted in a high quality GaN layer sitting on the nanopillars and the layer of pores formed over the SiO2 nanoparticles. For LEDs grown on top of such template, ~3 fold increase in optical output was observed compared to reference samples. The effect is attributed mainly to the improved light extraction efficiency due to additional scattering in the nanopillars-SiO2-pores portion of the structure, also to the increased internal quantum efficiency caused by a decreased dislocation density and relaxed strain due to the GaN nanopillars. Practical approaches to making LSP-LEDs will be discussed. The stability problem of NPs could be solved by developing the technology of Ag/SiO2 core/shell NPs prepared by sol-gel method. The NPs places the LSP resonance peak near 450 nm matching it to the blue MQW LEDs. It is demonstrated that strong PL intensity enhancement can be achieved for MQW structures with Ag/SiO2 NPs. These studies seem to indicate that the extent of efficient spatial coupling covers the range of about 40 nm, in agreement with earlier calculations for Ag NPs. Measurements of the LSP-related PL enhancement stability over time due to Ag/SiO2 core/shell NPs is better than for Ag NPs. Free-standing GaN LED structure with high crystalline quality was fabricated by combining electrochemical and photoelectrochemical etching followed by regrowth of LED structure and subsequent mechanical detachment from a substrate. The structural quality and composition of the regrown LED film thus produced was similar to standard LED, but the PL and EL intensity of the LED structures on the etched template were several times higher than for standard LED. The performance enhancement was attributable to additional light scattering and improved crystalline quality as a result of the combined etching scheme.

Inorganic-based micro light-emitting diodes (μLEDs) have been considered to substitute the conventional displays, because of their outstanding properties. The μLED applications are widely extending to various fields, such as augmented reality (AR), smart and biomedical devices. Furthermore, flexible μLEDs have been extensively investigated by several researchers for user-friendly optoelectronics. However, the μLEDs stability in harsh environments were not significantly investigated yet. Here, we realized wearable μLEDs using simple and rapid μLED fabrication process. Flexible μLEDs have excellent mechanical, thermal, humid, and photo stability in various environmental conditions. Finally, wearable μLED array was achieved on a general cloth.

Blue light emitting diodes (LEDs) currently exhibit wall plug efficiency exceeding 80%. This is the consequence of the extraordinary property of InGaN/GaN quantum wells (QWs) to emit light despite a high density of structural defects, i.e. threading dislocations. This has been ascribed to carrier localization in potential fluctuations induced by InGaN random alloy disorder. However, high efficiency LEDs commonly feature an InGaN layer, which is underneath the InGaN/GaN QW active region. This underlayer (UL) is known to dramatically increase the efficiency of blue LEDs. Several explanations have been proposed such as screening of threading dislocations due to V-pit formation, decrease of the internal electric field in the InGaN/GaN QWs due to band bending, or improved carrier injection. Another hypothesis is that defects are present at the GaN surface and lead to non-radiative recombination centers once incorporated in InGaN/GaN QWs. Thus, the role of the InGaN UL is to capture those defects resulting in a “clean” GaN surface before the growth of the InGaN/GaN QW active region. In this talk, we will present a comprehensive study of the effect of InGaN UL on the internal quantum efficiency of InGaN/GaN QWs. We will confirm that the role of this layer is to incorporate defects, which lie at the GaN surface. An origin of those defects will be proposed in view of secondary ion mass spectrometry analysis.

Within this paper, we summarize some of the degradation mechanism that still affect GaN-based optoelectronic devices. The most common source of the degradation is the creation of lattice defects, which lower the optical efficiency due to their role as non-radiative recombination centers, as proven in the case of UV-B LEDs. The local generation of defects is not the only possibility, with diffusion of impurities (possibly hydrogen from the p-side) being shown to be the limiting factor in the case of green laser diodes. Under extreme bias conditions, such as the EOS events, the robustness of the current carriers and spreading structures is critical, as shown by failure of bonding wires, metal lines and vias in white LEDs. In every optoelectronic device photons themselves possess an energy at least equal to the bandgap, and can be an additional source of degradation that cannot be eliminated.

We investigate the electroluminescence of blue LEDs in low bias (500 pA – 9 μA) at different temperature (15°C – 75°C). From 500 pA to 100 nA, the OP increases with bias current up to 10nA, and is stronger at higher temperature, as expected by radiative recombination through deep levels. The stronger contribution is at λ > 800 nm, i.e. at energies lower than the QW and GaN barrier midgap (720 nm). Above 100 nA the OP increases with current, and is compatible with QW emission. Its intensity decreases at higher temperature, as expected for non-radiative recombination. The experimental findings indicate that radiative recombination through deep levels can significantly influence the low current characteristics of the devices, even when those states are not at midgap.

Polarization-matched quantum wells (QWs) can lead to maximized electron-hole wave functions overlap and low efficiency droop at high current density. By using the modern theory of polarization with hexagonal reference, c-plane InAlN/InGaN QWs were explored and designed for polarization matching. The simulation results show that, even on c-plane, polarization-matched structures can be achieved by adjusting strain and material composition. The In composition of larger than 35% of InAlN was required to match the total polarization of InGaN at any given composition. Considering the bandgap’s bowing factors of III-nitride ternary alloys, In_0~0.1Ga_1.0-0.9N as quantum barrier (QB) provided enough potential barriers for In_0.35~0.45Al_0.65-0.55N to form a multiple QW (MQW) structure. The results indicated that improper resistance of MQW and the existing fixed charge between the interfaces of p-type region/MQW and n-type region/MQW could result in nonuniform carrier distributions and current leakage, respectively. Furthermore, we found that In_0.41Al_0.59N/In_0.1Ga_0.9N polarization-matched MQW had proper resistance; however, such structure produced a huge polarization fixed-charge between the junction interface. By studying the strain level of InAlN QW and GaN QB, which can be grown on AlN/GaN superlattice templates, the In_0.33Al_0.67N/GaN polarization-matched MQW structure has been specifically designed with small resistance and without inducing improper polarization fixed charge. By optimizing the number and thickness of QWs, the 425nm LED has relative IQE of 56% and efficiency droop of only 7% at high current density of 333 A/cm². This study provides guidance for development of In-rich InAlN materials.

Related publication Moon, Yoon-Jong; Moon, Dae-Young; Jang, Jeonghwan; Na, Jin-Young; Song, Jung-Hwan ; Seo, Min-Kyo; Kim, Sunghee; Bae, Dukkyu; Park, Eun Hyun; Park, Yongjo; Kim, Sun-Kyung; Yoon, Euijoon, "Microstructured Air Cavities as High-Index-Contrast Substrates with Strong Diffraction for Light-Emitting Diodes," in Nano Letters, Vol. 16, No. 3301, pp. 3308, Apr.-Apr., 2016. Jeonghwan Jang, Daeyoung Moon, Hyo-jeong Lee, Donghyun Lee, Daehan Choi, Dukkyu Bae, Hwankuk Yuh, Youngboo Moon, Yongjo Park, Euijoon Yoon, "Incorporation of air-cavity into sapphire substrate and its effect on GaN growth and optical properties," in Journal of crystal growth, Vol. 430, pp. 41, Nov.-Nov., 2015.

InGaN-based resonant-cavity light-emitting diode (RC-LED) structure with an embedded 1/4λ-stack nanopipe-GaN/undoped-GaN distributed Bragg reflectors (DBR) structure have been demonstrated. High refractive index Si-heavily doped GaN (n+-AlGaN:Si) epitaxial layers are transformed into low effective refractive index nanopipe AlGaN layers in the n+-AlGaN:Si/u-GaN stack structure through a doping-selective electrochemical and a lateral wet etching process. The anisotropic optical property of the nanopipe structure in the DBR structure provides an anisotropic and polarized high light reflectance spectra from ultra-violet to green light regions. The central wavelength blueshifted property with a high reflectivity stopband was observed in the angle-dependent reflectance spectra of the nanopipe DBR structure similar to conventional dielectric DBR structure. Ultra-short cavity length in the GaN-based resonant-cavity light-emitting diode had been realized by using the embedded and conductive nanopipe DBR with a narrow divergent angle property. Narrow linewidth, single cavity mode, and high linear polarized light are observed in the GaN-based RC-LED with an anisotropic nanopipe DBR structure which has the potential for the linear polarized vertical cavity surface emitting laser applications.

LED components today are complex systems with many optical, electrical, thermal, mechanical and transport interactions. These interactions imply to consider the LED component as a system, in order to identify potentials and remove deficits and improve the LED. Extensive research and development of last year’s allows to reach a high efficacy for LEDs – in the general lighting – of over 200 lm/W, and become commodity in the lighting market. In order to achieve higher efficacies, considering all mentored interactions is necessary. This talk gives an overview of system architecture of LED component today and shows projections into the future.

Adaptive Driving Beam (ADB) applications are proliferating from premium vehicles into the mainstream car models. Technology advances are creating the inflection point enabling cost effective digital headlighting to replace light steering by electro-mechanical swiveling shutters. Today’s LED matrix solutions support the mainstream ADB functionality with 5-25 independently addressable pixels. Meanwhile, the premium segment is evolving to a pixel count in the thousands to the one million range driven by additional functional value in beam content. We can predict the usual functionality migration from premium to mainstream segment and hence anticipate the adoption of increasing pixel count ADB solutions in the mainstream to support increased functionality, but at the appropriate cost point. Given the emerging range of high pixel count solutions today and an equally wide range of emerging technologies which support these, it is useful to shed light on which of these solutions have the potential to meet the mainstream requirements. This is the motivation for the work presented in this paper. We will look at all of the major competing technologies and offer an analysis on which technologies could support the requirements to enter the mainstream and based on this, predict the leading solutions with clear justifications. This will allow us also to forecast which new ABD functions are most likely to make the transition from premium to mainstream.

Laser-scanner headlamps, based on a blue laser spot steered by a MEMS mirror over a converter, can illuminate the road flexibly and efficiently. Some existing demonstrators suffer from insufficient performance of the converter (low contrast) or of the MEMS mirror (lack of flexibility). With a new high-contrast converter and highperformance MEMS mirrors, we have developed improved laser-scanner demonstrators optimized for different functions like full adaptive driving beam or visualization (symbols, guiding lines, and text). We explain the system design, converter technology, and high-bandwidth MEMS control, show the performance in video sequences, and discuss future applications of such systems.

Automotive headlight evolved from incandescent, to halogen, to xenon, to LED, and most recently, to laser phosphor lamps with increasing efficiencies and brightness. This paper presents the development of laser phosphor headlights using glass phosphor and single crystal phosphor for efficient and high power operations. Laser diodes are used for pumping the phosphors producing the white light to be projected to the roadway. In addition, various configurations of the laser diodes, which are individual addressable, are to be presented. Together with the used of DLP and LCD imagers, intelligent headlights are developed with the abilities selectively scanning the imagers illuminating the roadway with varying intensities. The design of the systems and the experimental results will be presented.

Increasingly, 3D sensing is becoming a ubiquitous technology – especially in consumer applications. Underlying this trajectory are diode lasers that emit light to measure, examine, and assess the world. There are two diode laser types: vertical-cavity surface-emitting laser (VCSEL), and edge emitters. Each laser with characteristics ideal for specific tasks. Reliability, consistency, and the ability to scale are critical for the demanding consumer markets. Even in the most severe environments – undersea -- diode lasers keep Internet traffic flowing. This presentation will examine diode laser history, recent gaming uses, and current-day gesture and facial recognition for smartphones. Also, promise for autonomous driving.

LEDs can be modulated at relatively high speeds to support wireless optical data communication (OWC). Yet, particularly LEDs optimized for illumination act as a non-linear low-pass communication channel. It has become clear in recent literature that their non-linearity and low-pass behavior cannot be seen as two separable, cascaded mechanisms. Although standard nonlinear equalizer schemes, e.g. based on Volterra Series, have been proposed and tested before, our recent research results show that a more dedicated approach in which we specifically analyze the hole-electron recombination mechanisms, yield a very effective and computationally-efficient compensation approach. In this manuscript, we will review the non-linear differential equations for photon emissions, its electrical equivalent circuit and a discrete-time variant with delays and non-linearities. This can be inverted, in the sense that we can actively eliminate or mitigate the non-linear dynamic LED distortion by adequate signal processing. We propose an aggressive simplification of the compensation circuit that allows us to use a relatively simple structure with only a couple of parameters.

With this work we report on the design, development and testing of near UV LED-based systems for oxygen gas sensing. The design and developed system is an optoelectronic setup based on 405 nm LEDs which excites and measures the photoluminescence emitted from a porphyrin based luminophor. By means of an accurate optical and optoelectronic setup, the system is able to operate without the need of avalanche photodiodes, thus resulting in a compact and low energy structure. The optical setup is specifically designed to maximize both the LED light exciting the luminophor and converted light acquired from the sensor.

AlInN alloys are promising as cladding layers in GaN-based visible laser diodes (LDs) because they show a large index contrast to GaN or InGaN in whole visible wavelengths at an alloy composition lattice-matched to GaN. To consider the application to cladding layers in LDs, a sufficiently-thick film with a smooth surface is necessary. In this study, therefore, we grew 300-nm-thick AlInN films with various alloy compositions on a c-plane GaN/sapphire template or a free-standing (FS) GaN substrate by metalorganic chemical vapor deposition (MOCVD). The results showed that no lattice relaxation occurred for samples with InN molar fractions from 0.144 to 0.197, and the InAlN films with low InN molar fractions showed a relative smooth surface. However, it turned into a granular surface morphology resulting from a columnar polycrystalline structure when the InN molar fraction exceeded a compositional boundary of in-plane lattice matching. Eventually, it was confirmed that epitaxial AlInN films with a good crystal quality and smooth surface roughness were grown at alloy compositions almost perfectly lattice-matched to GaN/sapphire and FS-GaN. As for the smooth-surface AlInN single-layers, the optical constants as well as energy bandgaps were determined. On the day in the conference, we would like to present new proposals for applications of the high-quality AlInN films to components in devices other than cladding layers in LDs.

The 2D transition metal dichalcogenides MoS2 and WS2 have attracted great interest due to their unique (opto)electronic properties. For their fabrication on an industrial scale, high-productivity MOCVD systems are most suitable because of precisely controlled precursor fluxes, advanced temperature control and superior homogeneity. Here, we report on the development of an MOCVD process for 2D WS2 and 2D MoS2 on sapphire (0001) substrates in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Molybdenum hexacarbonyl (MCO), tungsten hexacarbonyl (WCO) and di-tert-butyl sulfide (DTBS) are used as sources, respectively. A one-step process was developed to control nucleation and lateral growth of both 2D materials at 30 hPa total pressure in an N2 atmosphere. It was found that a fine-tuned S-to-metal ratio can inhibit the parasitic deposition of carbon contaminations. Investigations of the influence of deposition temperature on lateral growth of WS2 confirm previous findings for MoS2. The optimum growth temperature for MoS2 and WS2 is 845 °C. WS2 deposition experiments show that in order to achieve a fully coalesced 2D film, it is necessary to supply a sufficient amount of WCO, the limiting species during WS2 growth. Increasing the WCO flow from 1 nmol/min to 20 nmol/min raises the total substrate coverage from 2.5 % to >50 % in 10 h processes. Extending gradually the growth time to 20 h results in deposition of fully coalesced WS2 samples without carbon contaminations and only sparse bilayer nucleation. Moreover, the fully coalesced WS2 samples exhibit strong PL signals.

One of the big challenges of micro LED displays is to reduce cost/increase yield and establish excellent manufacturability. Galliumnitride on silicon (GaN-on-Si) LED epiwafers offer fundamental cost advantages to the entire process flow for micro LEDs compared with conventional GaN-on-sapphire LED epiwafers. However, due to the difficulties of epitaxial growth of GaN-on-Si, demonstration of such cost advantages in micro LED application is not wide-spread yet. In this presentation, we have demonstrated excellent emission uniformity with well-controlled strain by precise strain-engineering. This opens the way to use the advantages of GaN-on-Si LED epiwafers in the entire supply chain of micro LED making and thus reduce cost significantly and enable high yield manufacturing.

Today, Quantum Dots are perfecting LCD displays, enabling a new generation of brighter, more efficient televisions with lifelike colors. This has given LCD technology an important edge as it battles new display entrants such as WOLED. What’s next for this novel nanotechnology? This paper looks at how quantum dots are the technology platform for displays including their evolving use in LCD displays, as well as how they enhance and are being used in OLEDs and micro-LED displays, and how they are being developed as emitter materials for future printable electroluminescent displays.

The most widely used light sources for projection system and spotlights are discharge lamps. With tremendous advancements over the last decade in blue laser developments, laser excited phosphor systems have been developed for various applications including projectors and spotlights. One major challenge remains in the very high power applications where multi-kilowatt xenon lamps are still being used. In this paper, an advance material, namely, single crystal phosphor has been developed with high optical efficiency, high power handling capability, and a melting point of 1,950°C. To enable such single crystal phosphor to be used to its full capacity, a major effort was placed on the heat sinking of the crystal phosphor pumped at high power, over 70 W of blue laser power from a 4 by 6 array of laser diodes. The nominal dimension of the crystal phosphor of one of the system measures 2 mm by 2 mm by 4 mm and is end-pumped from one end with a set of focusing lenses directing the output from 24 lasers onto the surface of the crystal phosphor. The 4 sides of the crystal phosphor is specially coated and attached to the heat sink for efficient dissipation of heat, keeping the temperature of the crystal low enough for efficient emission. The output from the crystal phosphor is extracted using a CPC reducing the total internal reflection effect inside the crystal phosphor. To accommodate the high power laser at the input face of the crystal phosphor, various methods are used to prevent the local burning of the input face, including the use of diffusers, light pipes, and light tunnels. The computer simulation and experimental results will be presented.

Due to the weak thermal and chemical stability of organic resins which are used for conventional white LEDs to embed phosphors, inorganic color converters such as phosphor ceramics and phosphor-in-glasses are currently being used to replace conventional color converters based on organic materials, especially for high power and high brightness applications. In this paper we report on the study of sintered glass ceramics based on low melting glass in which commercial YAG:Ce3+ phosphors are embedded. A low Tg is necessary to avoid high temperature sintering which can damage the optical properties of the embedded phosphors. Two different types of glass have been studied: borosilicate and tellurite. The compositions have been optimized in terms of stability, sintering efficiency and thermal conductivity. Selected samples were optical charterized using a GaN high power multimode 450 nm Laser Diode, with a maximum output power of 1.6 W at 1.5 A. We investigated both the effect of high irradiation density and high operating temperature, as well as their color-rendering index. The sintered glass ceramic based on borosilicate glass showed better high power stability because of its higher thermal conductivity.

There is an increasing need for high power density light sources, e.g. for the next generation of car headlights, diode laser pumped white light sources and projection devices. However, saturation and droop at high excitation densities limit the light output in high power devices. Excited state absorption and long excited state lifetimes play a role, but the relation between light output and excitation power is a poorly understood and is complex interplay of quenching processes including reabsorption and (transient) color center formation. The development of superior materials is crucial and relies on a better understanding of droop processes and the relation with the nature and processing condition of light conversion materials. In this presentation a basic (and hopefully insightful) overview of known luminescence quenching processes will be followed by a discussion on how we can increase our understanding of luminescence quenching with a focus on high power applications. A variety of quenching mechanisms will be evaluated and illustrated for known and new luminescent materials. New experimental and theoretical capabilities will be discussed that may help to acquire new insights in what limits the light output in current and future light sources.

Although LEDs have penetrated successfully in many lighting domains, high brightness light source applications are still suffering from their limited luminance. High power LEDs are generally limited to less than 100 Mnit (10⁸ lm/m²sr), while dedicated devices for projection may achieve pulsed peak luminance values up to 200 Mnit for phosphorconverted green. For high luminous flux applications with limited etendue, like in stage or architecture spot lighting or in front projection, in the beam only very modest luminance values can be achieved with LEDs compared to systems based on discharge lamps. In this paper we evaluate light engine concepts based on static luminescent converters pumped by blue laser diodes, and concepts based on luminescent concentrators pumped by blue LEDs. Both concepts break through the flux and brightness requirements for these applications by enabling luminance values that are a factor five to ten higher than what can be achieved with LEDs. With continuous wave irradiation of a 10 mm² static converter by multiple laser diodes, 47 klm yellow-green emission was achieved at 1.5 Gnit source luminance, or 40 klm @1.2 Gnit in a collimated beam. With yellow-green light concentrator modules, 16 klm yellow-green emission was achieved at 1.2 Gnit collimated beam luminance. Thermal conditions are much more relaxed in luminescent concentrator modules than for static laser diode (LD) pumped converter systems. The High Lumen Density (HLD) LED-based luminescent concentrator, with its advantage of scalability in both flux and luminance, enables breakthrough performance in projection systems and in a wide variety of other applications. Laser-pumped converters, on the other hand, easily scale in flux proportional to their source size at constant luminance. They show very high flux capability and comparable brightness, enabling scope extension with extremely high flux solid state light sources.

Lighting applications that require luminance levels above several 100 cd/mm² cannot be addressed with LEDs today. Laser pumped phosphor (LPP) light sources are an emerging technology that helps to address this field with solid state lighting (SSL). Digital projectors with a LPP light source are already commercially available. They make use of phosphor wheels that are irradiated with blue laser light to generate light via photo-luminescence. The power limit of the phosphor wheel is determined by the peak temperature of the phosphor at the laser spot. We present experimental and numerical studies on the temperature contribution of both, the wheel geometry and the phosphor material, and show that ceramic phosphor rings are the best choice for high power and high luminance applications. Results for power levels close to 700W and an irradiance above 180 W/mm² are presented.

The low growth temperature technology Remote Plasma Chemical Vapour Deposition (RPCVD) is currently being developed by BluGlass Ltd. for use in high-brightness LED applications. The unique growth conditions of RPCVD are demonstrated to produce Activated As-Grown (AAG) buried p-GaN for achieving GaN-based tunnel junctions (TJ) for use in current spreading and potential use in cascade LED and LD applications. Hybrid RPCVD/MOCVD TJs were grown on commercial full blue LEDs, and all-RPCVD TJs were grown on commercial partially completed blue LEDs and the devices were processed into 1.1 mm x 1.1mm chips. The LEDs with hybrid TJ displayed a 4.4% increase in light output power (LOP) and an increase in forward voltage (Vf) of 0.68 V compared to LEDs using indium-tin oxide (ITO) at a current density of 26 A/cm² . The LEDs with all-RPCVD TJs displayed a 3.6% increase in LOP and an increase in Vf of 0.88 V at 26 A/cm² .

During the last decade a number of both theoretical and experimental studies have shown the importance and the possible effects of random alloy fluctuations in InGaN. Interesting results have been obtained in particular with atomistic simulation models. Based on experimental evidence, most theoretical studies so far concentrated on a uniform random alloy, i.e. where the probability of finding an indium instead of a gallium atom is spatially constant. In this work, we calculated the density of states, the spontaneous emission spectrum and the radiative coefficient for InGaN/GaN single quantum wells and for bulk InGaN in presence of alloy non-uniformity, using an empirical tight binding approach. We considered an indium concentration of 20%, and 10 nm large supercells. The non-uniform indium distribution has been obtained by distributing a certain percentage of all indium atoms with uniform probability, and the rest with a probability that depends on the number of indium atoms already present locally. This allows to produce structures ranging from random alloy up to strong clustering. We find that non-uniformity reduces the band gap and the peak energy of the optical emission spectrum. Moreover, increasing degree of clustering decreases the average value of the ground state transition matrix element, which can be explained by the carriers’ spatial localization, combined with quantum confined Stark effect in quantum wells. The radiative coefficient on the other hand is not substantially influenced by light non-uniformity, while it increases for stronger degree of clustering, compatible with a transition to a quantum dot system.

Advanced multi-channel illuminants consist of four or more strategically differentiated non-white LED illuminants. When simultaneously combined they can exhibit exceptional color fidelity, stability, efficacy, die area utilization, and more in contrast to systems of PC-white LEDs. They represent a fundamental forward-looking path in illumination – capable of fully addressing the limits of efficacy and spectral dynamics of human factors (temporal/circadian stimulus, color, intensity), imaging/sensors, and horticulture. The underlying properties of an optimized class of 5-channel illuminants are examined in contrast to systems of state-of-art phosphor-converted white illuminants. Factors including: color fidelity limits (CRI, R9, IES TM30-18 Rf for human eye, and CAM02-UCS deltaE's for camera sensor case) relative to white reference illuminants over broad CCT range (1100K - 10000K), die utilization, efficacy, die binning sensitivity, circadian stimulus minimization and maximization over CCT at specific lux levels, and short and long-term color stability are compared. (results summary forthcoming).

Traditional illumination systems uses various lamps selected based on certain requirements of the applications. One common issue is the trade-off between output brightness and lamp lifetime. LEDs with long lifetimes have been used in many applications. This paper describes a multi-colored LED illumination system with individually controlled red, green, and blue outputs combined together with the etendue of a single LED, having enhanced green and red output brightness with supplementary excitation of the phosphor-based green and red LEDs from additional blue LEDs, increasing the overall output of the system.

We reported on the design, demonstration, and analysis of white lighting systems based on GaN laser diodes. Compared to light-emitting-diodes (LEDs), lasers have been proposed for the development of high-power light sources for many potential advantages, including circumventing efficiency droop, reduced light emitting surface, directional beam characteristics. Laser-based white light sources are also attractive for visible light communication (VLC) applications that enabling lighting and communication dual functionalities. In this work, we detailed the color-rendering index (CRI), correlated color temperature (CCT), and luminous flux analysis of laser white light sources by using the GaN laser diode exciting color converters at various driving conditions. By using a blue-emitting laser exciting a yellow YAG phosphor crystal, a luminous flux greater than 600 lm has been achieved with a moderate CRI of 67.2. By constructing a white lighting system using phosphor crystal array based on a reflection configuration, an improved CRI of 74.4 and a luminous flux of ~400 lm with a CCT of 6425 K was obtained at 3A. Using a novel ceramic phosphor plate as color converter, the CRI for the white light source has been further improved to ~ 84.1 with a CCT of ~ 4981 K, which suggests that the laser-based white light source is capable of high-quality illumination applications. The CCT of the white laser sources can be engineered from 5000 K to 6500 K and a potential approach to use laser array for high power white lighting is discussed.

Electronic illumination systems are now available with multiple independently controllable LEDs of different spectral power distribution (SPD). These independent SPD can be combined in the proper proportions to replicate a target SPD. Generally at least four channels and sometimes eight or more channels are needed for a sufficiently broad tuning range with sufficiently high quality replication. As the number of independent channels rises above three, there are an infinite number of SPD that can achieve the same chromaticity. Algorithms employing a merit function are used to quickly sort to a good solution. In addition to spectrum, time is often an important quality of illumination such as with daylight and fire. A system that combines spectral tuning and time synchronization is referred to here as a light player. There are multiple optimization goals that may be of interest in determining the synthesized SPD. Examples include efficacy, color rendering, color saturation, high/low melanopic suppression, and smallest SPD error. One application of a light player is to replicate the time-varying SPD of daylight towards a healthier and more enjoyable indoor illuminated environment. Other applications include, retail appearance enhancement, medical diagnosis, horticulture, and standard illuminant synthesis such as D65. This talk will explore the general architecture of light players, the algorithms for synthesizing an SPD, summary information of daylight recordings, and a brief demonstration of a light player.

Methods for evaluating light source color rendition have recently undergone substantial changes. This article explores the ability of color rendition specification criteria to capture preferences for lighting color quality. For a compilation of five recent psychophysical studies on perceptions of colors, recently proposed specification criteria using ANSI/IES TM-30-18 substantially outperformed all currently used specification criteria in identifying preferred lighting conditions. To understand the consequences of changing color rendition specification criteria, the performance of a set of 484 commercially-available SPDs was evaluated.

LED lighting systems using Power-over-Ethernet (PoE) technology have been introduced to the lighting market in recent years as a network-connected lighting solution. PoE technology can provide low-voltage direct current (dc) power and control information to LED lighting over a standard Ethernet cable. One of the commonly claimed benefits of the PoEbased lighting system is higher system efficiency compared to traditional line voltage alternating current (ac) systems. This is due to the fact that in the case of PoE systems, the ac-dc power conversion losses are minimized because the acdc power conversion takes place at the PoE switch rather than at all the LED drivers within the lighting fixtures. However, it is well known that power losses can occur as a result of increased voltage drop along the low-voltage cables. The objective of this study was to characterize a PoE lighting system and identify the power losses at the different parts of the system. Based on the findings, we developed a methodology for characterizing the electrical efficiency of a PoEbased LED lighting system and then used this methodology to characterize commercially available PoE-based LED lighting systems and compare their performance. The electrical efficiency characterization included both the system as a whole and each individual component in the systems, such as the power sourcing equipment, powered device, Ethernet cables, and LED driver. The study results also investigated the discrepancy between the measured and reported energy use of the system components.

In this work, we show a spectrally tunable LED lighting system that uses the SPDs of the different channels of the light engine to optimize for different parameters, i.e. flux, illuminance, efficacy, CRI-Ri, TM30-15-Ri, Damage Potential, melanopic flux, PPF/YPF, CLA/CS, etc. Our results show how this tool and their associated methods can be used to design complex lighting applications in a meaningful and objective manner.

LED A-lamps are used in many types of lighting fixtures; however, these lamps can experience different thermal environments and use patterns (on-off switching), resulting in system life that varies in different applications. A recent study showed that on-off switching negatively affects LED system lifetime, and solder joint failure was the main reason. The goal of this study was to investigate and identify a theoretical model that can be used to predict LED A-lamp failure, when the failure is mainly due to solder joint failure. Although several models for solder joint fatigue failure exist in the electronics industry, the Engelmaier model is the most commonly used in industry standards. The study presented here showed that the Engelmaier model with modified fatigue ductility exponents provided a better fit to the experimental lifetime data for LED A-lamps. This paper describes the Engelmaier model prediction method for LED A-lamp failure.

High-resolution vehicle headlamps represent a future-oriented technology that can be used to increase traffic safety and driving comfort. Typically, selective absorbing of light using a spatial modulator like DMD, LCD or LCoS creates the light distribution of such headlamp systems. A similar effect can be generated by using LED arrays. Its additive principle generates light only in specific segments if necessary. In general, these arrays can be distinguished between conventional LEDs arranged in an array and micro pixel LEDs. Conventional LED arrays characterize by the design (THT or SMD) with typically a few millimeters edge length. In contrast, a micro-pixel LED uses COB technology, in which individual LED dies are packed in a single housing directly next to each other at a distance of a few microns. By increasing the array resolution, the challenges in designing an optical system for high-resolution headlamps rise. High efficiencies and contrasts call for small, accurate lens geometries and negligibly scattered light effects. Due to limited installation space and manufacturing tolerances, compromises have to be made. Ideally, the optics have to be accurate enough to image each pixel of the micro LED with high contrasts and high efficiency and still be too blurry to project the gaps between each pixel. This results in small distances between LED and optics and therefore in diffcult to manufacture radii of curvature. In this paper we specify the challenges to implement micro pixel LEDs in headlamp systems, as well as present the controllability of scattered light effects of these systems.

Higher optical power and higher UV doses come along with a higher operation temperature of UV LED based light sources. Silver sintered pastes offer a robust lead-free alternative to solder pastes increasing the lifetime of the device and enabling higher heat dissipation. Due to the design of UV LEDs, they have to be connected to a heat spreading submount by flip chip joining. Well established processes are flip chip soldering and thermal compression bonding. However, both methods do not achieve optimal heat dissipation in practice. Solder joining material offers a thermal conductivity in the range of 50-60 W/mK which can be further reduced by voids or uncovered areas. Gold contacts for thermal compression bonding offer excellent thermal conductivity of 320 W/mK, but show a maximum coverage of 50- 70%. Silver sinter paste adapted to the UV LED contact system, is a promising alternative flip chip joining material. In order to evaluate different joining methods, UV LED devices were assembled by thermal compression bonding, diffusion bonding, soldering and sintering and compared according to, thermal resistivity, optical-electrical and mechanical behavior and reliability issues. In addition, different silver sinter pastes were tested and their thermal resistivity was adjusted via processing parameters (pressurization, sintering temperature and time). For pressureless approaches the thermal conductivity and layer thickness are in the range of solder material or below. Using pressure for sintering, several advantages will be introduced. The interconnection thickness can be adjusted to be as thin as possible (below 5 micron), which enhanced the heat dissipation. A thin sintered layer of a few microns shows a lower shrinkage and a better adhesion to the joining partners. The thermal conductivity can be enhanced as well. After sintering, the silver interconnection layer is thermally stable up to 800 °C. These facts speak for sintering pastes as a real alternative for UV LED assembly.

The abundance of commercial LED lighting fixtures in the marketplace has resulted in price erosion, forcing manufacturers to look for ways to lower manufacturing costs. 3D printing holds promise for providing new solutions that not only can increase the value of lighting but can potentially reduce costs. During the past few years, 3D printing has been successfully adopted in industries such as aerospace, automotive, consumer products, and medical for manufacturing components. For the lighting industry to adopt 3D printing for fabricating light fixtures, it has to show that different subcomponents of an LED light fixture, including thermal, electrical, and optical components, can be successfully made. Typically, optical components are either transmissive or reflective type. In both cases, the component’s optical properties affect fixture efficiency and beam quality. Therefore, the objective of this study was to understand how short-term and long-term optical properties are affected when using 3D printed optical components. In the case of transmissive optics, several optical elements were printed and aged at higher than ambient temperatures and their corresponding spectral transmissions were measured over time. Similarly, several reflective optical elements were printed and characterized for spectral reflectivity as a function of print parameters, including print layer height, print orientation, and the number of print layers before and after aging the parts at higher ambient temperatures. These results are useful for optical component manufacturers to understand the possibilities of using 3D printing to make high-quality optics for lighting fixture applications and for 3D printing material and printer hardware manufacturers to understand the requirements of optics for the illumination applications.

Avalanche generation is a physical mechanism responsible for the breakdown at extremely high field, such as in the reverse bias conditions typical of ESD discharges. In this work, for the first time we provide experimental evidence that avalanche generation can take place in state-of-the-art InGaN-based blue LEDs. We measured the current-voltage and electroluminescence curves of the devices while pulsing them with increasing reverse voltages. We investigated a wide span of temperatures (from cryogenic to room temperature) in order to verify that the increase in leakage current detected below -80 V is related to avalanche generation (positive temperature-coefficient). Numerical simulations show that in this bias condition the band-to-band tunneling barrier thickness is low, leading to the possible injection of highly-energetic electrons from the p-side to the n-side that can start the avalanche process. The spectral shape shows a broad emission, covering the spectral range between 1.25 and 3.5 eV; the low energy side slowly decreases below 2.2 eV, and two sharp edges are seen at the high-energy side. Since an avalanche generation process is present, we can interpret the spectrum as follows: (i) hole and electron pairs generated by the avalanche process recombine, emitting photons; (ii) high-energy side: reabsorption of the emitted photons in the In-containing layers and nGaN side, confirmed by the red-shift at higher temperature; (iii) low-energy side: internal photoluminescence of the defects in the n-GaN layer, confirmed by PL measurements with external excitation. A theoretical computation based on this model is able to reproduce the experimental data.