LEDs have been commercially available since the 1960's, but in recent years there have been remarkable improvements in performance. These technology developments have enabled the use of LEDs in a variety of colored and white lighting applications. Colored LEDs have already become the technology of choice for traffic signals, much of interior and exterior vehicle lighting, signage of various types often as a replacement for neon, and other areas. LEDs are expected to become the dominant technology for most colored lighting applications. LEDs are beginning to penetrate white lighting markets such as flashlights and localized task lighting. With further improvement LEDs have the potential to become an important technology for large area general illumination. White LED products already have performance of over 30 lumens/watt which is nearly 3x better than incandescents. White LEDs with outputs of more than 100 lumens are already available commercially, and higher power devices can be expected in the near future. LEDs can be used as point sources, or can be used with light guides of various types to provide distributed illumination. Developments that will need to occur for LEDs to be viable for large area general illumination are discussed.

The efficiency of a conventional light emitting diode (LED) is limited by coupling of light into guided modes in the structure. Several methods to increase the extraction efficiency of nitride based LEDs are studied from the perspective of the patterned structures in LEDs. The patterned structures are made in the interface between a semiconductor and a sapphire substrate and on the surface of a semiconductor or an indium tin oxide electrode. All of these approaches show an increased light output compared to that of reference samples, which means these kinds of scattering sources are inevitable to make a highly efficient light emitter in nitride-based semiconductor system.

Understanding of white light, the surrounding definitions that define it, and how these techniques are applied to traditional white light sources and solid-state light (SSL) sources, is challenging. As these definitions were developed for and are directly relational to the various forms of traditional incandescent and fluorescent lighting technologies, they can affect true understanding of newer SSL sources, such as LEDs. Additionally, this body of characterization techniques and their underlining calculus, that was defined and developed for traditional sources years ago, presents a challenging body of work to first understand and then be used to comprehend data obtained via these techniques from the testing of LEDs. As LEDs are used more and more in the replacement of traditional light sources, their definition from manufacturers, the very testing techniques used in their characterization, and the resulting data that is used to specify them, may be somewhat problematic in the accuracy and/or the understanding of the outcome. This paper looks at the underlining structure that currently defines lighting and then discusses and concludes that LEDs are a truly different kind of white light source that needs some differentiation in terms of the comprehension and standards used for their development, specification and use.

The critical semiconductor topics of forward voltage, lumen output, color and viewing angles will be discussed to realize commercially feasible LED lighting system solutions. An example of a LED fixture will be analyzed that optimizes the performance characteristics of color, electrical control, thermal management, and beam patterns using single chip white LEDs. System efficiencies including, robust manufacturing processes, efficient electrical design and other systems considerations will be discussed in the context of delivering an effective lumen/watt performance with respect to long life white LED luminaire.

Relatively intense deep-green/yellow photoluminescence emission at ~600 nm is observed for InGaN/GaN multi quantum well (MQW) structures grown on bulk AlN substrates, demonstrating the potential to extend commercial III-Nitride LED technology to longer wavelengths. Optical spectroscopy has been performed on InGaN MQWs with an estimated In concentration of greater than 50% grown by metalorganic chemical vapor phase epitaxy at 750^oC. Temperature- and power-dependence, time-resolved photoluminescence as well as spatially resolved cathodoluminescence measurements and transmission electron microscopy have been applied to understand and elucidate the nature of the mechanism responsible for radiative recombination at 600nm as well as higher energy emission band observed in the samples. A comparison between samples grown on bulk AlN and sapphire substrates indicate a lower degree of compositional and/or thickness fluctuation in the latter case. Our results indicate the presence of alloy compositional fluctuation in the active region despite the lower strain expected in the structure contrary to that of low In composition active regions deposited on bulk GaN substrates. Transient photoluminescence measurements signify a stretched exponential followed by a power decay to best fit the luminescence decay indicative of carrier hopping in the active region. Our results point to the fact that at such high In composition (>30%) InGaN compositional fluctuation is still a dominant effect despite lower strain at the substrate-epi interface.

This paper describes work leading to the development of a new packaging method for white LEDs, called scattered photon extraction (SPE). Previous work by our group showed that the traditional placement of the phosphor close to the die negatively affects the overall luminous efficacy and lumen maintenance of phosphor-converted white LEDs. The new SPE method enables higher luminous efficacy by placing the phosphor at a remote location from the die and by shaping the lens surrounding the die to extract a significant portion of the back-transferred light before it is absorbed by packaging components. Although the remote phosphor concept is not new, SPE is the first method to demonstrate efficient extraction of back-transferred light and show over 60 percent improvement in light output and efficacy compared to similar commercial white LEDs. At low currents, the prototype white LEDs based on the SPE technique showed over 80 lumens per watt. The SPE concept was tried on two types of commercial packages and both showed similar improvements.

The unique features of light emitting diodes (LEDs) such as intrinsic color generation and relative low temperature operation enable completely new lighting concepts. The ongoing increase in performance of LEDs reaching efficacy levels of more than 30 lm/W for illumination grade white light with the promise of reaching over 75 lm/W makes them also applicable for higher luminous applications such as spot and flood lighting, accent lighting and architectural lighting. For the new lighting feature of ambiance creation, which requires at least variation of the color temperature of the light and preferably also selection of more saturated colors, LEDs are ideally suited. In this paper we report on the overall system aspects to color variable LED spot lighting and on the performance of prototype spot modules. Mixing of the light is performed within the lighting module by a combination of dense packing of red/amber, green and blue emitting dice, and light collimation with facetted optics and small angle diffusion, resulting in a homogeneous appearance of the light source and a color point inhomogeneity Δu'v' in the beam (>90% of total flux) of less than 0.01. A color rendering index (R_a8) of over 80 can be obtained over a large temperature and color temperature range with the 3-color system for a specific combination of 5 nm wide wavelength bins. In the prototype spot modules, between 9 and 14 dice are mounted on a common substrate and integrated with the primary collimating optics that is based on total internal reflection. Nominal power of the spot module is 10W. The average thermal resistance between the die junctions and the housing is 2 K/W. The optical efficiency of the module is 70%. The maximum luminous flux in the beam, which has a full width at half maximum (FWHM) of 20-25°, is about 200 lm. The system has thermal and optical sensors that provide the signals for a closed control loop to compensate for run-up and differential ageing effects. The resulting color point accuracy in the u'v' color space is better than 0.01. This shows the feasibility of easy-to-use lighting modules that offer advanced lighting options with adjustable, reliable and accurate output.

The continuing research effort in high power LEDs will allow their use in high quality lighting systems in the (near) future. There are still a number of issues to tackle, for instance the LED's (strong) temperature dependence. This dependence will change the emitted flux and the spectral distribution of the LED. In addition, these parameters will also change as the LED ages. When creating white light by mixing red, green and blue LEDs, the temperature effects described above will already result in a visible color difference after a small rise in temperature. To overcome this issue, a number of LED color control loops have been developed. These loops can be based on: the heat sink temperature, flux measurements of each primary color, a combination of these last two and an integrated color point. For this purpose, an RGB test set up has been built, equipped with a temperature sensor and various photo-sensors. The appropriate color control loops have been implemented and tested in software. Some control loops use empirically determined LED parameters (dλ/dT or T₀), the value of these parameters has been determined for a different set of LEDs. In addition, initial optical LED (and sensor) calibration has been performed at a single temperature only. The color stability of the various color control loops has been measured for a temperature increase of about 50 degrees Centigrade. In this range, we find that, on short term, all color control loops show a significant improvement in the color error, except for the color control loop based on flux measurements of each primary color, which performs nearly as mediocre as open loop. However, the color control loop based on the heat sink temperature cannot offer color stability when the LED ages, which is expected to be significant. The color control loop based on an integrated color point seems the most expensive one.

Intentionally doped n-type bulk ZnO has been grown by patented melt technique at Cermet and was used as a substrate for homo-epitaxial growth of p-type ZnO films. The n-type ZnO has a carrier concentration on the order of 10¹⁸cm^-3 with a mobility of 113cm²/Vs, which is good for optical devices. Secondary ion mass spectroscopy (SIMS) profile shows a very uniform distribution of n-type dopant in the ZnO. Excellent transmission from the sharp absorption edge through the visible portion of the spectrum indicates that as grown n-type ZnO is perfect for any optical device applications. P-type ZnO thin films were successfully grown by MOCVD technique on n-type ZnO substrate to form ZnO based p-n junction structure. Cadmium and magnesium doped ZnO films were also grown by MOCVD and resulted in tunable bad gap energy of ZnO based alloy. Ohmic contact layer on n-type ZnO was formed by using Ti/Au and on p-type ZnO was formed by using Ni/Au. The current-voltage (I-V) characteristics of the ZnO based p-n junction exhibited rectification when reverse biased with a breakdown voltage of 10 V and turn-on voltage of 3.3 V. Post anneal of p-type ZnO films showed big improvement on the I-V characteristics. Electroluminescence (EL) spectra obtained from devices driven to 40mA are dominated by a peak at 384nm.

Although GaN is widely applied in UV light emitting diodes (LEDs) and laser diodes (LDs) applications, its intrinsic properties may limits its potential in the development of large scale consumer products. ZnO, on the other hand, is a known potent candidate for UV-LEDs and LDs. However, it is very difficult to fabricate p-type ZnO because of a strong self-compensation effect of intrinsic defects. In this study, we shall discuss the growth conditions that favor p-type ZnO based on first principles density functional theory (DFT) calculations. Selection of doping source and the corresponding thin film fabrication techniques and experimental results will be discussed also.

InGaN/GaN multiple-quantum-well light-emitting diodes have been fabricated on the (1010) m-plane of 4H-SiC substrates. The c-axes of the m-plane epitaxial films and the substrate are parallel. The surface of the epitaxial films has a ridged texture with the ridges aligned perpendicular to the c-axis. Xray diffraction shows superlattice features from the multiple-quantum well stack, and plan-view transmission electron microscopy shows a threading dislocation density of ~ 10¹⁰ cm^-2 and a basal plane stacking fault density of ~ 8 × 10⁶ cm^-1. The electroluminescence from the LED shows a strong polarization anisotropy with the majority of the light emitted perpendicular to the c-axis and a polarization ratio exceeding 0.8. The temperature dependence of the polarization ratio shows a 49 meV difference in energy gap between the valence band minima with different polarizations.

Enhancement of light extraction in GaN light-emitting diodes (LEDs) employing omnidirectional reflectors (ODRs) is presented. The ODR consists of GaN, ITO nanorod low-refractive-index layer, and an Ag layer. An array of ITO nanorods is deposited by oblique-angle deposition using e-beam evaporation. The refractive index of the ITO nanorods is 1.34 at 461 nm, significantly lower that that of dense ITO, which is n = 2.06 at 461 nm. It is experimentally shown that the GaN LED with GaN/ITO nanorods/Ag ODR show much better electrical properties and higher light-extraction efficiency than LEDs with Ag contact. This is attributed to enhanced reflectivity of the ODR by using an ITO low-refractive-index layer with high transparency, high conductivity, and low refractive index.

We observed a significant enhancement in light output from GaN-based light-emitting diodes (LEDs) in which two-dimensional photonic crystal (PC) patterns were integrated. We approached two types PC LEDs. One is top loaded PC LEDs. The PC patterns were generated on the top p-GaN layer. The other is bottom loaded PC LEDs. In this LEDs, PC patterns were integrated on the sapphire substrate. Two dimensional square-lattice air-hole array patterns, whose period was varied between 300 and 700nm, were generated by laser holography. Unlike the commonly utilized electron-beam lithographic technique, the holographic method can make patterns over a large area with high throughput. The resultant PC-LED devices with a pattern period of ~500nm had more than double the output power. The experimental observations are qualitatively consistent with three-dimensional finite-difference-time-domain simulation results.

Approximately two billion people, one third of humanity still has no access to electricity, and thus relies on fuel-based lighting, a dangerous alternative of last resort that is unhealthy, expensive, and offers very poor levels of illumination. This lack of light makes it difficult to perform most evening activities including studies by children and adults alike and therefore represents a significant barrier to human development. Over the past five years The Light Up The World Foundation (LUTW) has pioneered the use of the white light emitting diode (WLED) as an alternative home lighting solution, bringing clean, affordable light to thousands of non-electrified homes around the world. The information presented herein is intended to increase awareness of the enormous potential possessed by this emergent technology, "Solid State Lighting" (SSL), to improve the quality of life of millions of people around the world. The feasibility of its implementation is demonstrated with results from comprehensive field experience and laboratory research work. The mutual economic, social and environmental benefits for both stakeholders and SSL suppliers are discussed. Strategies conducive to the dissemination of this technology throughout the developing world are also presented.

LED tapes are emerging as cost effective light sources for longitudinal applications, such as strip lighting and accent lighting on buildings, and shelf and cove lighting for interior lighting. Such applications can be sorted into direct viewing and illuminative. In the latter, uniform illumination is the most frequent desideratum, particularly on nearby surfaces spanning large solid angles from the light source. The LED tape by itself creates a hot line directly underneath it and relatively dim illumination at target's distant edge. Instead, extruded refractive lenses have been prototyped that spread out the light such as to provide uniform illumination over nearby target zones. Different lens profiles are available for different lighting configurations, via the redistribution of the light field of the LED tape. Each lens acts as a magnifier where candlepower is required for distant slanted surfaces and as a de-magnifier towards the closest surfaces.

In this paper, an experimental study is conducted to investigate three light uniformers based on liquid guide for their possible usages as color mixing elements. In this study, three types of light uniformers are systematically analyzed, which include a straight light guide, a tapered light guide, and a U-shaped light guide. This study consists of two parts: a computer-aided optical modeling using a commercial ray tracing software package; and an experimental study verifying the results obtained from the computer simulations. Beam uniformity, in terms of illuminance and color, is compared among different light uniformers. The experimentally observed results agree well with the simulated results. Light transmission efficiency is also calculated for each light uniformer based on the simulation results. It is found in this study that the U-shaped light uniformer has high light transmission efficiency and the best color mixing effect among the three investigated light guides and offers a compact, practical, and inexpensive solution for color mixing in many industries.

The growth of violet light emitting diodes (LEDs) was optimized using a statistical design of experiment (DOE) approach and several important interaction effects were found. The DOEs studied the effect of several variables on the well layer, the barrier layer, and the pAlGaN cladding layer. These variables included the gallium flow rate, the indium flow rate, the growth temperature, and the growth time for the well layer, the ammonia flow in the active region, the barrier growth time, and the Si doping of the barrier, as well as the growth time, growth temperature and Mg doping of the pAlGaN cladding layer. The LEDs were optimized based on combinations of several responses from photoluminescence and electroluminescence measurements. An overall process desirability was obtained, based on achieving the desired wavelength and maximizing the PL intensity and optical output power. Significant interactions between variables played a major role in the optimization of optical output power as well the emission wavelength. The understanding of these interactions led to the optimization of the LEDs both by improvements in the structure and improvements in the quality of the layers. Several of the interactions will be explained based on kinetic models of GaN growth by MOCVD.

Laser Diode Arrays continue to gain momentum as versatile, cost effective, reliable solution for a wide variety of existing and emerging illumination and pumping applications. In order to meet these growing demands, designers find themselves faced with three challenges: reducing system size, improving user serviceability, and managing cost. We developed a compact laser package platform that offers high output power, good reliability, and different beam collimation options. Both active cooling and passive cooling is possible with this new packaging concept. It has the footprint of the TO263 package and is based on packaging concepts that were developed for high power semiconductor devices and high volume opto semiconductor products like Light Emitting Diodes. High efficiency and high power laser bars are critical to various pumping and material processing applications. Wavelength multiplexing is an option to increase output power from laser systems. Typical wavelengths used are 808nm, 940nm and 980nm. We discuss the results of wavelength multiplexing of 880nm high power lasers.

High-performance, blue micro-size InGaN light emitting diodes (LEDs) with diameters of 3 to 20 μm have been fabricated. An ion implantation technique and a 12 micron electro-ridge were used to simplify fabrication processes. The 3 to 20μm LEDs that exhibited a large emission photon blue shift (87.5meV ~52.9meV) were observed in electro-luminescence (EL) spectra. Under an increased injection current, the quantum wells become populated with charge carriers, which screened the internal piezoelectric field and caused the energy blue shift of EL eventually. A high injection current caused a high junction temperature that narrowed the band gap (red shift). The size dependent energy shift is largely owing to the competition between the blue and the red shifts. At a bias voltage of 8.96V (which is 140% of the turn on voltage, 6.4V), the 10 μm device exhibited an injection current of 7.9mA. This value exceeds that in literature, i.e., 4mA at a bias voltage of 14V (which is 140% of the turn on voltage, 10V). This phenomenon may be owing to that the ion implantation and electro-ridge designs herein involved a lower series resistance. The external quantum efficiencies (E.Q.E.) of the micro size LEDs herein were all 0.4%~3.3%, which is better than the values reported in literature, which were ranged between 0.004% and 1.29% for an individual LED and an array LED, respectively. The E.Q.E. of the 15μm device at maximum injection current had the optimum value yet obtained for micro-size LEDs. The dependence of the blue shift and the E.Q.E. on the size warrants further study.

Phosphor-converted Light Emitting Diodes (pc-LEDs) to generate white light from blue or UV emitting diodes can be made using Eu²⁺ doped nitridosilicates - M2Si5N8- and/or oxo nitridosilicates - MSi2O2N2 with M=alkaline earth. Luminescence properties of some out of this new class of color converters have been investigated. As expected from strong absorption the decay times of the internally excited Eu²⁺ is short (around 1 microsec) and depends on the cation sharing the unit cell. Time resolved spectroscopy illustrates the behavior even more clearly. Laboratory pcLEDs using 2 of the nitride phosphors show excellent drive and temperature stability of all color properties as expected.

Current technology of lighting is Solid state lighting using LED's (SSL-LED). The aim of the present study is to find the critical concentration of Eu²⁺ for high emission intensity and also the role of Ce³⁺ co-doping on the absorption and emission properties in the host BaMgSiO₄. Photoluminescence emission of Eu²⁺ in BaMgSiO₄ when excited with 370 nm shows a broad band in the region 450 to 550 nm with a maximum at 502 nm and a shoulder at ~480 nm and one more band at ~ 400nm. The three emissions are due to Eu²⁺ in three different Ba sites in the lattice. Studies on Ba_{1 x}Eu_xMgSiO₄ [x = 0.0025 - 0.1 in steps of 0.0025] show that the emission intensity is maximum for x = 0.075 and a decrease in emission intensity is observed for higher x values. Ce³⁺ luminescence is studied for the first time in BaMgSiO₄. Ce³+ emission occurs as a broad band with maximum at 430 nm when excited with 356 nm. The Eu²⁺ excitation that occurs in the region 250 - 420 nm covers both the Ce³⁺ absorption and emission. Hence Ce³⁺ to Eu²⁺ energy transfer is possible in BaMgSiO₄. In the case of Ba_{0.99 x}Eu_0.01Ce_xMgSiO₄ [ x = 0.0025 - 0.1 ], it is observed that the emission intensity of Eu²⁺ increases with increasing Ce³⁺ content up to 0.01. This result proves the energy transfer from Ce³⁺ to Eu²⁺. Thus, the co-doping of Ce³⁺ also enhances the absorption of Eu²⁺ in the near UV to blue region where the LED emission occurs. BaMgSiO₄:Eu²⁺, Ce³⁺ with bright green emission can find potential application as a green phosphor for SSL-LED technology.

We have developed a Ce:YAG (Y₃Al₅O₁₂) glass-ceramic phosphor for the white LED. The glass-ceramic phosphor was obtained by a heat treatment of a Ce-doped SiO₂-Al₂O₃-Y₂O₃ mother glass between 1200°C and 1500°C for the prescribed time of period. We confirmed that, by XRD measurements, only YAG crystal precipitated in the mother glass after the heat treatment. It was shown from SEM observation that the YAG crystals with a grain size of approximately 20μm were uniformly dispersed in the glass matrix. The yellow emission, around 540nm in wavelength, was observed from the glass-ceramic phosphor, when it was excited by a blue LED (465nm). The white light due to the mix of yellow and blue light was observed from the glass-ceramic plate with a thickness of 0.5mm. The YAG glass-ceramic phosphor showed a high-temperature resistance and a good performance in a damp heat test. Moreover, a higher thermal conductivity of 2.18 Wm^-1K^-1 and bending strength of 125MPa were observed compared with a conventional soda-lime glass or an epoxy resin. In addition, since the YAG glass-ceramic phosphor can be formed in a plate-like shape, there is no need to be sealed in resins for the fabrication of the LED devices. Therefore, it is expected that this newly developed glass-ceramic phosphor is a promising candidate for the realization of resin-free, high-temperature and high-humidity resistant, long-life white LED devices.

Optical properties of the Ce:YAG glass-ceramic (GC) phosphor for the white LED were investigated. Concentration dependence of fluorescence intensity of Ce³⁺:5d→4f transition in the GC showed a maximum at 0.5mol%Ce₂O₃. Quantum efficiency (QE) of Ce³⁺ fluorescence in the GC materials, the color coordinate and luminous flux of electroluminescence of LED composite were evaluated with an integrating sphere. QE increased with increasing ceramming temperature of the as-made glass. The color coordinates (x,y) of the composite were increased with increasing thickness of the GC mounted on a blue LED chip. The effect of Gd₂O₃ substitution on the optical properties of the GC materials was also investigated. The excitation and emission wavelength shifted to longer side up to Gd/(Y+Gd)=0.40 in molar composition. As a result, the color coordinate locus of the LED with various thickness of the GdYAG-GC shifted to closer to the Planckian locus for the blackbody radiation. These results were explained by partial substitution of Gd³⁺ ions in the precipitated YAG micro-crystals, leading to the increase of lattice constant of unit cell, which was confirmed by X-ray diffraction.

This study will discuss the heat dissipation effect of light emitting diode (LED) device applied a commercial miniature heat pipe (MHP). For lowering the thermal resistance of LED, the MHP can reduce the working temperature and raise the allowable input power of LED chip obviously. By comparing with a copper rod, the LED temperature was decreased about 19% at 1.59W input power and the LED power was increased about 43% under 118°C chip temperature. On the other hand, the thermal resistance of LED also can be reduced by using a thinner slug. Moreover, the results showed that the thermal spreading effect was significant. The MHP could be used to avoid the hot spot of LED packaging due to its excellent heat spreading property. Simultaneously, a LED thermal simulation was carried out to verify the optimum value of slug thickness.

Silicone based materials have attracted considerable attention from Light Emitting Diode (LED) manufacturers for use as encapsulants and lenses for many next generation LED device designs. Silicones can function in several roles that include protective lenses, stress relieving encapsulants, mechanical protection and light path materials. The key attributes of silicones that make them attractive materials for high brightness (HB) LEDs include their high transparency in the UV-visible region, controlled refractive index (RI), stable thermo-mechanical properties, and tuneable hardness from soft gels to hard resins. The high current and high operating temperatures of HB-LEDs present a significant materials challenge for traditional organic materials such as epoxies, acrylics and cyclo olefin copolymers (COC) that lack the thermal and molecular stability needed to provide optical clarity and mechanical performance required for next generation devices. In addition, the retention of optical clarity over the lifetime of the device, which involves long term exposure to high flux in the UV-visible wavelength region, is a critical requirement. Silicones have been demonstrated to provide the required stability. This paper will describe recent silicone materials development efforts directed towards providing LED manufacturers with silicone materials solutions for LED device fabrication. Injection molding of novel silicone resin based materials will be discussed as a surmountable challenge for high throughput LED device manufacturing.

Nowadays, the high power GaN-based LED has attracted serious attention for the lighting application. One of key issues for high power GaN-base LED to achieve sufficient lighting efficiency over the traditional light sources, such as, white incandescent and halogen light bulb is the efficiency of heat dissipation. Typically, GaN epi-layer is grown on sapphire substrates. The poor thermal conductivity of sapphire substrate has been identified to be the main limitation for the application of high power GaN LED. To improve the heat dissipation and lighting efficiency, we report a thin GaN structure by using Au-Si wafer bonding and Laser lift-off (LLO) technique. The GaN wafer was first deposited with a Au bonding layer and bonded onto a good thermal conduction substrate, i.e., heavy-doped Si. Then, 248nm KrF excimer Laser was used to strip the original sapphire substrate. To assure a successful GaN epi-layer transferring, Raman spectrum on the transferred GaN layer was performed and the result shows no quality change in the transferred GaN layer. In this work, we also fabricated the vertical LED devices on the transferred GaN epi-layer. Therefore, L-I-V result was obtained which will be presented in this talk. Moreover, we will discuss the effects and advantages of Au-Si bonding on the efficiency of lighting.

The temperature distribution of a dual Multi-Quantum Well (MQW) light emitting diode (LED) has been investigated using both infrared imaging and micro-Raman Spectroscopy; mean values over the device yielded temperatures ranging from 30-75°C. The InGaN/GaN based LED, grown by Metal Organic Chemical Vapor Deposition (MOCVD), was also studied using the 3ω method in order to determine an effective thermal conductivity of the MQW stack in the temperature range from 300-540K. The LED structure under investigation showed effective thermal conductivities in the range from 82-140 W/mK with the peak conductivity occurring at 440K, well above room temperature. Using temperature dependent properties determined experimentally, a numerical model of the LED structure was developed in order to study the effect that the package thermal resistance and input power has on the temperature of the device.

The ubiquitous downlight inhabits our ceilings by the millions. Hot, inefficient, and electrically wasteful, it is next in line for replacement by the latest high-brightness, high-efficacy white LEDs. The conventional downlight configuration of a large incandescent spotlight in a low-cost, ceiling-recessed metal can, represents the culmination of old technology, fated never to improve significantly. Incandescent downlights add greatly both to direct and indirect electrical consumption, with the lamps requiring relatively frequent replacement. The small size of LED emitters means small optical elements can produce much higher-quality beams than incandescent spotlight-lamps can produce. Herein we introduce compact high-luminosity LED downlights with lenses that deliver uniform illumination to delimited targets such as tables. One version utilizes circular lenses and micro-diffuser films to deliver square outputs. The other uses lenses cut to the target shape. In particular, one of these lenses is the first to offer a semicircular spot suitable for gambling tables.

Operating at altitude and often in turbulent, low visibility conditions, in-flight refueling of aircraft is a challenging endeavor, even for seasoned aviators. The receiving aircraft must approach a large airborne tanker; take position within a "reception window" beneath and/or behind the tanker and, dependent upon the type of receiving aircraft, mate with an extended refueling boom or hose and drogue. Light is used to assist in the approach, alignment and refuel process of the aircraft. Robust solid state light emitting diodes (LEDs) are an appropriate choice for use in the challenging environments that these aircraft operate within. This paper examines how LEDs are incorporated into several unique lighting applications associated with such aerial refueling operations. We will discuss the design requirements, both environmental and photometric that defined the selection of different LED packages for use in state-of-the-art airborne refueling aircraft Formation Lights, Hose Drum/Drogue Unit lights and Pilot Director Lights.

As LEDs continue to improve in efficacy and total light output, they are increasingly finding their way in to new applications in the aviation industry as well as adjacent markets. One function that is particularly challenging and may reap substantial benefits from this new technology is the fuselage mounted anti-collision light. Anti-collision lights provide conspicuity for the aircraft by periodically emitting bright flashes of light. The color, light distribution and intensity levels for these lights are all closely regulated through Federal Aviation Regulation (FAR) documents. These lighting requirements, along with thermal, environmental and aerodynamic requirements, drive the overall design. In this paper, we will discuss the existing technologies used in anti-collision lights and the advantages and challenges associated with an LED solution. Particular attention will be given to the optical, thermal, electrical and aerodynamic aspects associated with an LED approach. A specific case study will be presented along with some of the challenges that have arisen during the design process. These challenges include the addition of an integrated covert anti-collision lighting.

It is of great technological importance to develop high quality III-Nitride layers and optoelectronic devices on Si substrates due to its low cost and wide availability as well as use of the highly matured Si microtechnology. Here we report on a novel scheme of substrate engineering to obtain high quality GaN layers on Si substrates. An ion implanted defective layer is formed in the substrate that partially isolates the III-Nitride layer from Si substrate and helps to reduce the strain in the film. The experimental results show substantial decrease in crack density, indicative of high interfacial tensile strain, with an average increase in the crack separation of 190 μm with crack free regions of 0.18 mm² for a 2 μm thick GaN film. The optical quality and strain reduction in GaN film show strong dependence on the implantation conditions and the thickness of buffer layer. Moreover the GaN film grown on implanted AlN/Si substrate has better optical properties as compared to non implanted AlN/Si. In this paper we will show how the above mentioned scheme can resolve the issues related to cracks and dislocation density in the film that are detrimental to GaN based optoelectronic devices.

Silicon quantum dot (QD) based luminescent structures can emit throughout the visible region by controlling their size and/or the host matrix. Consequently, multiple sized Si QDs embedded in thin films could be used to produce efficient white light sources, when integrated with a blue/UV LED, and film structures designed for high light extraction. In this paper, we report strong red photoluminescence from Si QDs embedded in films prepared by thermal evaporation of SiO in vacuum or an O₂ atmosphere. The SiO_x film composition (1.0< x <1.9) was controlled by varying the deposition rate and the oxygen flow rate in the chamber. After annealing at 1100°C, silicon nanocrystals of 20nm to 2nm in size were formed in films with different stroichiometry, as indicated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization. Red photoluminescence was observed from films with Si QDs smaller than ~5nm, and attributed to confined carrier radiative recombination in the Si QDs. The emission peak shifted from 840nm to 745nm with increasing O₂ flow rate due to a decrease in the size of the Si QDs.

Several aspects of the Color Rendering Index (CRI) are flawed, limiting its usefulness in assessing the color rendering capabilities of LEDs for general illumination. At NIST, we are developing recommendations to modify the CRI that would overcome these problems. The current CRI is based on only eight reflective samples, all of which are low to medium chromatic saturation. These colors do not adequately span the range of normal object colors. Some lights that are able to accurately render colors of low saturation perform poorly with highly saturated colors. This is particularly prominent with light sources with peaked spectral distributions as realized by solid-state lighting. We have assembled 15 Munsell samples that overcome these problems and have performed analysis to show the improvement. Additionally, the CRI penalizes lamps for showing increases in object chromatic saturation compared to reference lights, which is actually desirable for most applications. We suggest a new computation scheme for determining the color rendering score that differentiates between hue and saturation shifts and takes their directions into account. The uniform color space used in the CRI is outdated and a replacement will be recommended. The CRI matches the CCT of the reference to that of the test light. This can be problematic when lights are substantially bluish or reddish. Lights of extreme CCTs are frequently poor color renderers, though they can score very high on the current CRI. An improved chromatic adaptation correction calculation would eliminate the need to match CCT and an updated correction is being considered.

The goal of this study was to characterize the chromaticity shift that mixed-color and phosphor-converted white LED systems undergo when dimmed. As light-emitting diodes continue to rapidly evolve as a viable light source for lighting applications, their color shift while being dimmed should meet the current requirements of traditional lighting sources. Currently, LED system manufacturers commonly recommend pulse-width-modulation or PWM dimming schemes for operation of LED systems. PWM has the ability to achieve lower intensity levels and more linear control of light intensity compared to continuous current dimming methods. However, little data has been published on the effect dimming has on chromaticity shift of white LED lighting systems. The primary objective of this study was to quantify chromaticity shifts in mixed-color and phosphor-converted white LED systems due to continuous current dimming and pulse-width-modulation dimming schemes. In this study, the light output of the LED system was reduced from 100% to 3% by means of continuous current reduction or PWM methods using a PC white LED system and a mixed-color RGB LED system. Experimental results from this study showed that the PC white LED system exhibited very little chromaticity shift (less than a 4-step MacAdam ellipse) when the light level was changed from 100% to 3% using both dimming schemes. Compared to PC white LEDs, the mixed-color RGB LED system tested in this study showed very large chromaticity shifts in a similar dimming range using both dimming schemes. If a mixed-color RGB system is required, then some active feedback system control must be incorporated to obtain non-perceivable chromaticity shift. In this regard the chromaticity shift caused by the PWM method is easier to correct than the chromaticity shift caused by the continuous current dimming method.

A spectrally tunable light source using an integrating sphere with a large number of LEDs has been designed and constructed at the National Institute of Standards and Technology (NIST). The source is designed to have a capability of producing any visible spectral distribution, mimicking various light sources in the visible region by feedback control of the radiant power emitted by individual LEDs. The spectral irradiance or radiance of the source is measured by a standard reference instrument; the source will be used as a transfer standard for colorimetric, photometric and radiometric applications. A series of simulations have been conducted to predict the performance of the designed tunable source and source distributions have been realized for a number of target distributions.

A type of chromatic measuring system, which was based on the spectroradiometric method, controlled by a computer was developed. Spectra, chromaticity coordinates and correlated color temperature (CCT) of light-emitting diode (LED) can be real time measured by this system. It has high resolution of wavelength with 0.2nm, fast measuring speed and friendly operation interface. The measurement data indicate that the system has higher accuracy than CS-100. Difference between measured result and standard data of the system is within ± 2%, while repeatability is better than 98%.

We report on the study of heat 2D-distribution in InGaN LEDs with the stress made on local device overheating and temperature gradients inside the structure. The MQW InGaN/GaN/sapphire blue LEDs are designed as bottom emitting devices where light escapes the structure through the transparent GaN current spreading layer and sapphire substrate, whereas the LED structure with high-reflectivity Ni/Ag p-contact is bonded to the thermally conductive Si submount by a flip-chip method. The measurements are performed with an IR microscope operating in a time-resolved mode (3-5 um spectral range, <20 μm spatial and 10 μs temporal resolution), while scanning a heat emission map through a transparent sapphire substrate. We show how current crowding (which is difficult to avoid) causes a local hot region near the n-contact pads and affects the performance of the device at a high injection level.

The idea of light recycling is rather simple. Assume that part of the light emitted by a light source is returned to the light source itself. If the light source does not completely absorb this light then the part which is not absorbed, is still available for further use. The hidden virtue of light recycling is that the recycled light is superposed in the same phase space (etendue) as the original radiation. Thus the average radiance of the source is increased albeit at the price of a reduction of total radiant power. This seems to violate the Second Law of Thermodynamics because the temperature of the radiation is related to the spectral radiance. Increasing the radiance amounts to reducing the entropy. However, radiating into free space is an irreversible process in which entropy is created. Light recycling reduces the entropy carried by the radiation by reducing the entropy production rate in the emission process itself. We show that the maximum radiance which can be achieved by light recycling is marked by the equilibrium radiance. The equilibrium luminescent spectrum diverges as photon energies approach the splitting of the quasi Fermi levels. The familiar spectrum of LEDs clearly does not diverge because the absorptivity/emissivity approaches zero in this regime. These features render light recycling particularly attractive. We report on preliminary laboratory measurements which show encouraging results.

In this work, we investigate the optical and electrical properties of inserting a Ni thin barrier between contact layer, NiO-Au, and reflective layer, Al after sequent elevated annealing in air ambient. The reflectivity of NiO-Au/Ni/Al p-GaN contact configurations is 61% in 470nm which is 10% higher than NiO-Au/Al p-GaN contact configurations, after 500°C annealing. By inserting a Ni barrier layer, the specific contact resistance of the NiO-Au/Ni/Al was maintained on the order of 10^-2 Ω-cm², up to an annealing temperature of 500°C. The XPS results confirmed the function of the Ni barrier layer, and it shows relatively low atomic level of Al was detected in the GaN epi-layer. It was found that both the electrical and optical characteristics of NiO-Au/Ni/Al p-GaN contacts exhibited good thermal stability. This high thermal stable P-GaN enables the fabrication of thin-GaN LED device.

Three photo-voltaic commercial panels based on poly-crystal (two panels) and amorphous (one panel) solar cells are analyzed. The behavior of the panels under transients, arising due to instantaneous load changes, is investigated. Two types of transients are analyzed: an instantaneous short-circuiting and an instantaneous open circuiting. It is observed that the solar cell panels behave differently under transients arising due to their short-circuiting and open-circuiting. The transients arising due to instantaneous short-circuiting from an open circuit condition have a character of damped oscillations for all analyzed solar cell panels. The transients arising due to instantaneous open-circuiting from a short circuit condition have a character of exponential growth for all analyzed solar cell panels. The processes developed under above transients are determined by the solar cell panel parameters only, therefore the results obtained cannot be explained by the currently used equivalent diagrams.

In this paper the three dimensional Laplace equation was solved for a three layer photoreceptor with t_i (i=1,2,3) thicknesses and dielectric ε_j(j =1,2,3) under suitable boundary conditions and the vertical component of electric field was obtained over the photoreceptor's photoconductive layer. Then, the behavior of the above equation was studied for different photoconductive layer's thicknesses and four thicknesses were chosen. For making the samples, the deposition of Al and formation Al₂O₃ in 2x10^-5 - 5x10^-7 mbar pressure, and coating of Se in 2x10^-7 mbar pressure, 250 °C boat temperature and 95 °C substrate temperature in 120 min were carried out after the design and manufacture all of subsystems. In this way four samples were made by different photoconductive layer's thicknesses. Electrical measurements showed a resistance above 10¹²Ω in dark and about zero in light which these values suitable for development of electrostatic latent image. To consider the development field of made photoreceptors, the samples were mounted in Xerox machine and the operation of them observed. The sample with 60 μm photoconductive layer's thickness showed the best development and less background effects.

We achieved p-(Zn,Mg)O by doping with phosphorous and the conduction type was confirmed by capacitance-voltage properties of metal/insulator/p-(Zn,Mg)O:P diode structures as well as Hall measurements. The p-(Zn,Mg)O:P/n-ZnO junction was grown by pulsed laser deposition on bulk ZnO doped with Sn. Without post-growth annealing, the phosphorous-doped ZnMgO showed p-type conductivity (hole density ~10¹⁶ cm^-3, mobility ~6 cm²V^-1s^-1) in the as-grown state. The metal contacts in top-to-bottom p-n junctions were made with Ni/Au as the p-ohmic and Ti/Au as the backside n-ohmic contact. The p-contacts showed improved characteristics after annealing up to 350 - 400 °C, but the n-contacts were ohmic as-deposited. The simple, low temperature growth (≤500 °C) and processing sequence (≤400 °C) shows the promise of ZnO for applications such as low-cost UV light emitters and transparent electronics.

We analyze the effects on color uniformity of the near-field light distribution due to different cluster configurations (at optimum packaging density for uniform irradiance) of light sources using mixed red, green and blue (RGB) light emitting diodes (LEDs). A photometric analysis and experimental results that show the near-field performance that can be achieved with several cluster configurations of multicolor LEDs is presented. Contour maps for the color variation (in reference to illuminant D₆₅) in function of spatial coordinates of light distribution are given.

The room temperature transport and optical properties of phosphorus-doped ZnO and (Zn,Mg)O thin films are studied. Pulsed laser deposition (PLD) has been employed to grow epitaxial and polycrystalline layers on c-plane (0001) sapphire substrate. The ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O₃/O₂ mixture is two orders of magnitude higher than the films grown in oxygen and O₂/Ar/H₂ mixture. The photoluminescence (PL) spectra of the as-deposited films are composed of both the near band-edge and broadband visible emission, which peak at 3.29 and 1.87 eV, respectively. It has been shown that growing in the O₂/Ar/H₂ mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The opposite effect on the PL emissions is shown for the films grown in pure oxygen and O₃/O₂ mixture. There is an inverse correlation between the intensity of the visible broadband emission and the carrier density. The enhanced UV emission in the films grown in O₂/Ar/H₂ mixture may result from hydrogen passivation of the deep level emission centers. For the P-doped (Zn,Mg)O grown at 500°C, increasing the oxygen partial pressure from 20 to 200 mTorr yields a carrier type conversion from n-type to p-type without post-annealing. The films grown at 150 mTorr oxygen partial pressure are p-type and exhibit a hole concentration of 2.7 x 10¹⁶ cm^-3, a mobility of 8.2 cm²/Vs and a resistivity of 35 Ω-cm. All the films exhibit good crystallinity with c-axis orientation. These results indicate the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films.