This PDF file contains the front matter associated with SPIE Proceedings Volume 8278, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Light-emitting diodes (LEDs) are penetrating into the huge market of general lighting because they are energy saving and environmentally friendly. The big advantage of LED light sources, compared to traditional incandescent lamps and fluorescent light tubes, is the flexible spectral design to make white light using different color mixing schemes. The spectral design flexibility of white LED light sources will promote them for novel applications to improve the life quality of human beings. As an initial exploration to make use of the spectral design flexibility, we present an example: 'no blue' white LED light source for sufferers of disease Porphyria. An LED light source prototype, made of high brightness commercial LEDs applying an optical filter, was tested by a patient suffering from Porphyria. Preliminary results have shown that the sufferer could withstand the light source for much longer time than the standard light source. At last future perspectives on spectral design flexibility of LED light sources improving human being's life will be discussed, with focus on the light and health. The good health is ensured by the spectrum optimized so that vital hormones (melatonin and serotonin) are produced during times when they support human daily rhythm.

Natural daylight is characterized by high proportions of blue light. By proof of a third type of photoreceptor in the human eye which is only sensitive in this spectral region and by subsequent studies it has become obvious that these blue proportions are essential for human health and well being. In various studies beneficial effects of indoor lighting with higher blue spectral proportions have been proven. On the other hand with increasing use of light sources having enhanced blue light for indoor illumination questions are arising about potential health risks attributed to blue light. Especially LED are showing distinct emission characteristics in the blue. Recently the French agency for food, environmental and occupational health & safety ANSES have raised the question on health issues related to LED light sources and have claimed to avoid use of LED for lighting in schools. In this paper parameters which are relevant for potential health risks will be shown and their contribution to risk factors will quantitatively be discussed. It will be shown how to differentiate between photometric parameters for assessment of beneficial as well as hazardous effects. Guidelines will be discussed how blue enhanced light sources can be used in applications to optimally support human health and well being and simultaneously avoid any risks attributed to blue light by a proper design of lighting parameters. In the conclusion it will be shown that no inherent health risks are related to LED lighting with a proper lighting design.

A skygazer is an astronomical visual observer who uses telescopes in a field. The lighting for them is important. Their requirements are solid-state, light weight, flicker-less, and dimming controls corresponding to photopic, mesopic and scotopic vision. Especially, the lighting never spoils a dark adaptation at moonless night. However, bright light is used to preparing the equipment at twilight. To satisfy their requirements, we clearly define the illuminance range and its wave length, and then we propose smart dimming. The main idea is the dimming method using PAM and the implementation to MCU sharing the DC-DC converter with LEDs. The next idea is efficient use of MCU built-in components.

LED has many advantages that will replace traditional light sources. In this article, we design a convergence lens and use a LED light source with ± 30° half-power angle. The lens can convergence light source and make it become parallel light. The goal to achieve high directivity can be applied to wireless sensors. According to the simulation results, compared to the LED with dome lens that has ± 0.6° half-power angle, the LED use our design's lens has ± 0.34° half-power angle. In the same transmission distance, the LED with our design's lens has more than 200 times received energy.

LED cluster is probably the most relevant among the emerging solid-state lighting techniques. Impressive scenarios of a wide range of color quality and luminous efficiency have been obtained, mostly at the condition of constant ambient temperature. This paper removes the constraint in ambient temperature. We present a methodology analogous to a general lens design rule to optimize step-by-step the spectral power distribution of a white-light LED cluster. The scheme enables the users to determine the optimal operation to meet requirements such as light efficiency, color quality, or other figures of merit over a wide range of color temperatures. All main factors influencing the spectral power distribution (SPD) are discussed, alongside the implementation of a pentachromatic R/G/B/A/CW platform suitable for clinic use. The result shows the multispectral cluster can be modulated within the color temperature from 2800K to 8000K in the range ambient temperature (10°C ~ 100°C) with high color quality scale (CQS > 85 points) and the possibly highest luminous efficiency.

Yb and Er-doped Y₂O₂S phosphor was synthesized by solid state flux fusion method and their upconversion spectral properties were studied as a function of different Yb concentrations. The solid state flux fusion results in well crystallized hexagonal shaped phosphor particles of average size 3.8 μm. The detailed optical characterizations such as absorption, emission, and fluorescence decay were performed to explore the emission processes in the UV-VIS-NIR as well as to quantitatively estimate the fluorescence quantum yield. Upconversion spectral studies show that for all the compositions, green emissions are stronger, particularly; the green emission intensity is 1.7 times stronger than the red one with composition of 8 mol% Yb and 1 mol% Er. Mechanisms of upconversion by two photon and energy transfer processes are interpreted and explained. The color coordinates are measured and the color tunability was analyzed as a function of the 980 nm excitation power. Results show that the Y₂O₂S:Yb,Er phosphor offers power dependent color tuning properties where the emission color can be tuned from 490 to 550 nm by simply changing the 980 nm excitation power from 10 to 50 mW.

The contribution of reduction of threading dislocation densities (TDDs) to optical properties is investigated for InGaN/GaN light-emitting diodes (LEDs) grown on sapphire substrate. The external quantum efficiency (EQE) curves depending on the TDDs are discussed both theoretically and experimentally. At the current density of <20 A/cm², the EQE increases with decreasing the edge-type TDD from 5 e8/cm² to 2 e8/cm². The current density at the maximum EQE shifts to lower value as the edge-type TDD decreases, whereas the EQE presents no remarkable difference in the highercurrent density range irrespective of the TDD. According to the rate equation (ABC) model, the peak shift reflects the Shockley-Read-Hall non-radiative process (A coefficient). Analysis of the photoluminescence (PL) decay and the dependence of integrated PL intensity on excitation power reveals that the threading dislocations act as non-radiative recombination centers in the multiple quantum well active region. The TDD of <2 e8/cm² is required for highly efficient blue LEDs operating at current density of around 15 A/cm², whereas the TDD of <5 e8/cm² in required for the LEDs operating at around 50 A/cm².

In this paper we review our progress in developing AlGaN-based deep UV LEDs with internal quantum efficiency (IQE) in excess of 50%. This is accomplished by growing the active region of the LEDs by plasma-assisted MBE under a growth mode which promotes the introduction of deep band structure potential fluctuations in the wells beyond the statistical ones due alloy disorder. AlGaN-based deep UV-LEDs emitting in the wavelength range from 320 nm to 265 were grown by this method and fabricated into devices. By combining high IQE AlGaN QWs in the active region with polarization field enhanced carrier injection layers, unpackaged deep UV-LEDs emitting at 295 nm and 273 nm were obtained with optical output power of 0.35 mW and 1.8 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency of these devices was calculated to be 0.4%, a result consistent with the low extraction efficiency of only 1-2%.

The bandgap engineering of ZnO nanowires by doping is of great importance for tunable light emitting diode (LED) applications. We present a combined experimental and computational study of ZnO doping with Cd or Cu atoms in the nanomaterial. Zn_1-xTM_xO (TM=Cu, Cd) nanowires have been epitaxially grown on magnesium-doped p-GaN by electrochemical deposition. The Zn_1-xTM_xO/p-GaN heterojunction was integrated in a LED structure. Nanowires act as the light emitters and waveguides. At room temperature, TM-doped ZnO based LEDs exhibit low-threshold emission voltage and electroluminescence emission shifted from ultraviolet to violet-blue spectral region compared to pure ZnO LEDs. The emission wavelength can be tuned by changing the transition metal (TM) content in the ZnO nanomaterial and the shift is discussed, including insights from DFT computational investigations.

Here we report the first realization of a current injection microcavity GaN exciton-polariton light emitting diode (LED) operating under room temperature (RT). The hybrid microcavity structure consists of InGaN/GaN quantum wells sandwiched between bottom epitaxial DBR and top dielectric DBR. The anti-crossing behavior of polariton LED denotes a clear signature of the strong interaction between excitons and cavity photons.

This study analyzes optical confinement factor and light emitting mode order for three different GaN LEDs: a conventional LED, thin Film LED, and thin Film LED with a photonic crystal (PhC) grating. For the first structure, we increase the thickness of AlxGa1-xN from 0 to 600nm, alter the x composition in AlxGa1-xN from 0.05 to 0.2 in steps of 0.05, and adjust the p-GaN and n-GaN thicknesses each from 0 to 200nm. For the second structure, we alter the n-GaN substrate thickness from 300-1000nm in steps of 100nm and 1000- 4000nm in steps of 1000nm. These simulations show that increasing the substrate thickness causes the light emitting mode order to increase. The higher the mode, the more current is needed to make the device emit light. Higher current leads to shorter device lifetime. The last structure contains a photonic crystal grating with a period T = 100nm, 230nm, 460nm, 690nm, 920nm, 1500nm, 2000nm, 3000nm and 50% duty cycle. For each grating period, we display the effects on optical confinement factor and optical field intensity. The results show that changing the grating period does not affect the mode order, but does affect the optical field intensity. A larger grating period corresponds to lower optical field intensity. Maximizing optical field intensity increases the brightness of the device. The simulation method above can be used to improve the efficiency, brightness, and lifetime of GaN LEDs by reducing the effects of transverse mode coupling and maximizing the optical field intensity.

In this study, we propose to enhance an output power for 380 nm UV-LED with a hexagonal pyramid structures (HPS) on the interface of sidewall between AlGaN and AlN layers. The HPS are formed by inserting a 50 nm AlN as a sacrificial layer in n-AlGaN than using a selective wet etching process in KOH solution at 90 °C for 60 min. From the scanning electron microscope (SEM) image, the HPS can be clearly seen on the interface of AlGaN, the facet angles and the average of structure height of pyramid are 58° and 0.5-μm, respectively. According to the electroluminescent (EL) results, 12% enhancement of the light extraction efficiency can be expected in the UV-LED with HPS. Furthermore, we measured the output power at 20 mA between the UV-LED with and without HPS are 2.69 mW and 3.01 mW, respectively. As a result, the light extraction efficiency can be improved by this approach because of changing the routes of light reflection around the sidewall.

With this work we propose an innovative method for the analysis of the reliability of LED and Laser devices and systems. The basic idea of the proposed method is the separation of the different degradation forces that lead to the decrease of LED performances during ageing. By using a specific reliability analysis procedure it is possible to separately evaluate the effects of the single accelerating factors: temperature, current intensity, applied signal waveform, voltage overstress, optical and mechanical solicitation. To individually determine the degradation kinetics it is fundamental to separate the effects of temperature and current. For these reasons we carried out iso-currents reliability tests, where several devices have been stressed with the same current at different junction temperatures, and iso-thermal stresses, where junction temperature is instead constant for different applied currents. The result of the analysis will be a multivariable law that relates the several degradation parameters in the form of degradation kinetics. This will allow the estimation of the devices lifetime for a very wide operating conditions region. During degradation an extensive set of measurements have been carried out at fixed steps in the form of photometric, optical, electrical, capacitive, mechanical and thermal characterization. The combination of these results allows the understanding of what degradation mechanisms are taking place and therefore it is a fundamental tool to improve system reliability. Degradation has also been studied by analyzing catastrophic damages by means of failure analysis; the failure investigation is useful for the catastrophic damage: melting of bonding wire, contacts evaporation, facet melting (for laser diodes).

GaN epilayer can be grown on sapphire substrate with a Ga₂O₃ sacrificial layer. It was employed for the epilayer transferring from sapphire substrate to Cu substrate using the chemical lift-off process application. The (-201) oriented β-Ga₂O₃ thin film was first deposited on the c-plane sapphire substrate, followed by the GaN growth via metalorganic vapor phase epitaxy under N₂ and H₂ environment in sequence. The crystal quality of GaN epilayer can be improved dramatically with the regrowth in a H₂ ambient. A GaN epilayer with an electroplated copper substrate was demonstrated using a chemical lift-off process where the Ga₂O₃ sacrificial layer can be laterally etched out with a hydrofluoric solution. It is worthy to mention that the separated sapphire substrate can be cleaned and reused for LED epitaxial growth next time. It is benefiting the cost down for the LED fabrication and Green Photonics Development.

For high-quality general lighting, a white light source is required to exhibit good photometric and colorimetric performance along with a high level of electrical efficiency. For example, a warm white shade is desirable for indoors, corresponding to correlated color temperatures ≥4000 K, together with color rendering indices ≥90. Additionally, the luminous efficacy of optical radiation (LER) should be high, preferably ≥380 lm/W_opt. Conventional white LEDs cannot currently satisfy these requirements simultaneously. On the other hand, color-conversion white LEDs (WLEDs) integrated with quantum dots (QDs) can simultaneously reach such high levels of photometric and colorimetric performance. However, their electrical efficiency performance and limits have been unknown. To understand their potential of luminous efficiency (lm/Welect), we modeled and studied different QD-WLED architectures based on layered QD films and QD blends, all integrated on blue LED chips. The architecture of red, yellow and green emitting QD films (in this order from the chip outwards) is demonstrated to outperform the rest. In this case, for photometrically efficient spectra, the maximum achievable LE is predicted to be 327 lm/W_elect. Using a state-of-the-art blue LED reported with a power conversion efficiency (PCE) of 81.3%, the overall WLED PCE is shown to be 69%. To achieve LEs of 100, 150 and 200 lm/Welect, the required minimum quantum efficiencies of the color-converting QDs are found to be 39, 58 and 79%, respectively.

An electrically driven nanopyramid green light emitting diode (LED) was demonstrated. The nanopyramid arrays were fabricated from a GaN substrate by patterned nanopillar etch, pillar side wall passivation, and epitaxial regrowth. Multiple quantum wells were selectively grown on the facets of the nanopyramids. The fabricated LED emits green wavelength under electrical injection. The emission exhibits a less carrier density dependent wavelength shift and higher internal quantum efficiency as compared with a reference c-plane sample at the same wavelength. It shows a promising potential for using nanopyramid in high In content LED applications.

Although planar heterostructures dominate current optoelectronic architectures, 1D nanowires and nanorods have distinct and advantageous properties that may enable higher efficiency, longer wavelength, and cheaper devices. We have developed a top-down approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter. This approach avoids many of the limitations of bottom-up synthesis methods. In addition to GaN nanorods, the fabrication and characterization of both axial and radial-type GaN/InGaN nanorod LEDs have been achieved. The precise control over nanorod geometry achiveable by this technique also enables single-mode single nanowire lasing with linewidths of less than 0.1 nm and low lasing thresholds of ~250kW/cm².

The paper discusses various factors affecting internal quantum efficiency (IQE) of state-of-the-art III-nitride lightemitting diodes (LEDs). A general figure of merit for LED heterostructures, namely the quality factor, is proposed on the basis of a simple recombination model, which enables comparison of overall performances of the structures either emitting light at different wavelengths or having substantially different designs. The relationships between the quality factor, maximum value of IQE, and IQE droop with the current density are revealed. Some ways for IQE improvement and reducing its droop are considered. Among them, the use of short-period superlattice (SPSL) active regions is found to be quite promising. The operation of such structures and their properties are examined in detail by simulations accounting for quantum corrections to electron and hole transport within the quantum-potential concept.

We report the efficiency droop behaviors of InGaN/GaN blue LEDs with different thickness of GaN quantum barriers (QBs). The droop percentage from efficiency peak to 70 A/cm² is only about 10% as reducing the thickness of GaN QBs from 104 Å to 33 Å. A less carrier localization has been observed from wavelength dependent time resoled photoluminescence measurement as reducing the thickness of GaN QBs. The alleviation of droop percentage may due to more uniform distribution of electron and hole carrier in the active region, which resulted from super-lattice (SL) like active structure. The crystalline quality does not become worse from the results of v-pits density even thickness of GaN QBs is as low as 33 Å. The SL like active structure could be a potential structure to alleviate the efficiency droop for the application of solid state general lighting.

The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation result of high resolution x-ray diffraction (HRXRD) ω-2θ curve by using dynamical diffraction theory, the Al composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are 43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 × 10¹⁷ #/cm³. It could be explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer. Therefore, the light output power of n-GaN UV-LED is slightly higher below the forward current of 250 mA. Nevertheless, the output power was enhanced about 22% as the injection current was increased to 600 mA. Furthermore, the external quantum efficiency (EQE) of n-AlGaN UV-LED was nearly retained at the 600 mA (only 20% droop), whereas the UV-LED with n-GaN exhibits as high as 33%. We attributed this improvement to the less self-absoption by replacing n-GaN underlayer with n-AlGaN.

The authors report the formation of air-voids at GaN/cone-shaped-patterned-sapphire-substrate interface by laser scribing and lateral etching with one-step growth. With 5 and 20 min lateral etching, it was found that pyramid-like airvoid was formed with an average height of 0.98 and 1.9 μm, respectively, on top of each corn of the substrate. It was also found that we can enhance output power of GaN-based light-emitting diodes by 6.6 and 11.5%, respectively, by immersing the wafer in a mixture of H₃PO₄ and H₂SO₄ solution at 220°C for 5 and 20 min, respectively.

Praseodymium-doped fluorogermanate 60PbGeO₃-20PbF₂-20CdF₂ nano-structured phosphors were synthesized by thermal treatment of precursor glasses. Luminescence features of praseodymium ions incorporated into low-phononenergy PbF₂ nanocrystals dispersed in the fluorogermanate glass matrix was evaluated. The luminescence spectra exhibited visible emission signals peaked at 490, 525, 613, 643 nm. White-light emission was observed in praeodymium single-doped phosphor excited using a LED at 460 nm. The dependence of the luminescence emisson intensity upon annealing temperature, and rare-earth concentration was also evaluated. The results indicated the existence of optimum annealing temperature and activator ion concentration to obtain intense emission light with CIE 1931 chromaticity coordinates within the white-light boundary region. Results suggest that the novel nanocomposite glass material herein reported is a promissing phosphor for white-light LED applications

In this study, several combinations of multi-package white light-emitting diodes (LEDs), which combine an InGaN blue LED with green, amber, and red phosphor-converted LEDs (pc-LEDs), were characterized by changing the peak wavelength of green pc-LEDs between 515nm and 560nm (515, 521, 530, 540, 550, 560nm) in color temperature of 6,500K and 3,500K. Various green monochromatic pc-LEDs were fabricated by capping a long-wave pass-filter (LWPF) on top of pc-LEDs to improve luminous efficacy and color purity. LWPF-capped green monochromatic pc-LED can address the drawback of green semiconductor-type III-V LED, such as low luminous efficacy in the region of green gap wavelength. Luminous efficacy and color rendering index (CRI) of multi-package white LEDs are compared with changing the driving current of individual LED in various multi-package white LEDs. This study provides a best combination of four-color multi-package white LEDs which has high luminous efficacy and good CRI.

This paper compares the optical effects of a two-dimensional (2D) SiO₂ and SiN_x photonic crystal layer (PCL) on Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) yellow ceramic plate phosphor (CPP) to achieve enhanced extraction efficiency of YAG:Ce CPP on top of a blue InGaN light-emitting diode (LED) cup. The low external efficiency of YAG:Ce CPP is due to low light extraction efficiency by the total internal reflection (TIR) and waveguide effect occurring in film-type phosphor. To reduce the TIR and the waveguide effect on YAG:Ce CPP, 2D triangular-lattice air-hole nanoarrays of SiO₂ and SiN_x with various lattice constant (330, 420, 580, and 720nm) as PCL were fabricated on the YAG:Ce CPP by nanosphere lithography (NSL). The optimum properties on light extraction efficiency of YAG:Ce CPP are realized by adding the 2D SiN_x PCL with a 580nm lattice constant. By adding the 2D SiN_x PCL with a 580 nm lattice constant, integrated yellow emission was improved by a factor of 1.72 compared to that of a conventional YAG:Ce CPP capped on a blue LED cup.

Museum lighting present challenges due to the demand for a high color rendering index (CRI), color uniformity and the damaging effects of both visible and invisible radiation. Golden objects are furthermore normally displayed with illumination which has a correlated color temperature (CCT) of 2200 K, a CCT that is not commercially available from single LEDs. An LED system that conforms with these requirements has been developed and implemented at The Royal Danish Collection at Rosenborg Castle. Color mixing of commercial LEDs (red, cyan, and white) was employed to achieve the spectral power distribution needed for the CCT and a CRI above 90, for all CRI test color samples. Replacing the traditional low voltage incandescent lighting has shown energy saving above 70 %. Harmful IR radiation was reduced by 99 %. Temperature fluctuations in the display cases were reduced from several degrees Celsius to below one, despite the fact that the lighting units were placed within the display case. Spatial color uniformity of the illumination and uniformly colored shadows was achieved by use of a highly diffusing reflector dish which avoids direct illumination from the LEDs.

Given the problem of metamerisms inherent in color mixing in light-emitting diode (LED) systems with more than three distinct colors, a method for optimizing the spectral output of multicolor LED system with regards to standardized light quality parameters has been developed. The composite spectral power distribution from the LEDs are simulated using spectral radiometric measurements of single commercially available LEDs for varying input power, to account for the efficiency droop and other non-linear effects in electrical power vs. light output. The method uses electrical input powers as input parameters in a randomized steepest decent optimization. The resulting spectral power distributions are evaluated with regard to the light quality using the standard characteristics: CIE color rendering index, correlated color temperature and chromaticity distance. The results indicate Pareto optimal boundaries for each system, mapping the capabilities of the simulated lighting systems with regard to the light quality characteristics.

In recent times, the transportation industry has generated a number of developments involving new technology in signaling. Important developments have involved the production of light by means of light emitting diode (LED). The optical proprieties of LEDs depended on junction temperature. Since the heat from the junction must be dissipated into the ambient somehow, changing the ambient temperature affects the junction temperature and hence the emitted light. When the LEDs have been used in the railway or traffic signals, the optical proprieties of these have to maintain more rigorous color specifications. Besides, the limits imposed must be respected in ample range of variation of the ambient temperature (typ. 233-347K). The peak wavelength of LED shifts proportionally to changes in junction temperature. Therefore, to respect color specifications with signals using LED as light source it is not simple. In this paper, we describe problems of the temperature dependent changes of colorimetrical parameters of LEDs. Besides we will introduce a method through which to build a correct signal with feedback that, according to the measured temperature, it reacts to correct the optic characteristics of signal.

The mechanism responsible for the efficiency droop in AlGaInP based vertically-structured red light-emitting diodes (LEDs) is investigated using dynamic measurement techniques. Short electrical pulses (~100ps) are pumped into this device and the output optical pulses probed using high-speed photo-receiver circuits. From this, the internal carrier dynamic inside the device can be investigated by use of the measured electrical-to-optical (E-O) impulse responses. Results show that the E-O responses measured under different bias currents are all invariant from room temperature to ~100°. This is contrary to most results reported for AlGaInP based red LEDs, which usually exhibit a shortening in the response time and degradation in output power with the increase of ambient temperature. According to these measurement results and the extracted fall-time constants of the E-O impulse responses, the origin of the efficiency droop in our vertical LED structure, which has good heat-sinking, is not due to thermally induced carrier leakage, but rather should be attributed to defect recombination and the saturation of spontaneous recombination processes.

Staggered quantum wells (QWs) structures are numerically studied to reduce the influence of the efficiency-droop effect on the InGaN-based green light-emitting diode (LED). The location of high In-content InGaN layer in staggered QWs considerably affects the distribution of the electrostatic-field of an LED. When the high In-content InGaN layer is suitably located in the staggered QWs, the localized electrostatic-field with high intensity increases the transport efficiency of injected holes across the active region, improving the overall radiative efficiency of the LED. Most importantly, as accumulation of injected holes in the last QW is relieved, the Auger recombination process is quenched, suppressing the efficiency-droop in the LED. Theoretically, the incorporation of the staggered InGaN QWs in the green LED (λ = 530nm) can ensure an extremely low efficiency droop of 11.3%.