This PDF file contains the front matter associated with SPIE Proceedings Volume 7231, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

During the last few decades the efficacy and luminous flux of LEDs have developed fast. Also the color quality of white LEDs and LED illumination systems has improved considerably. Thanks to the performance improvements and the continuously declining cost per lumen, it is now possible to create LED lighting systems with high luminous fluxes that can be applied in downlights for general lighting and in spot lights for accent lighting. One of the important requirements on lighting systems in indoor lighting applications is the color consistency. For all systems the chromaticity, or color point, of the light should be the same, i.e. within well-defined small tolerance areas. For down lighting, LED modules with high optical efficiency have been developed based on the concept of mixing light from multiple LEDs and luminescent materials, and emitting the mixed light through a translucent window. This concept is ideal for down lighting and other general illumination applications since it enables the design of luminaires with high optical efficiencies and low glare. In addition, it enables high color uniformity and excellent color consistency between modules. The module concept enables forward compatibility by well-defined interfaces and optical properties that are decoupled from the actual performance and number of LEDs. In this paper the properties with respect to color consistency of the various concepts will be discussed. By applying a phosphor remote from the blue LEDs, we have developed mediumbrightness (100-200 kCd/m²) LED-modules with high system efficacy. This is the basis of the Philips Fortimo downlight system. Based on mixing of multiple colors, the color tunable Lexel downlight module has been developed. The systems comprising multiple LED colors have feedback loops to comply with color consistency requirements. In all systems a color consistency within 5 SDCM is achieved.

Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs) with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by 80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations. Keywords: light-emitting diodes, photonic crystal, cavity, InGaN

Staggered InGaN quantum wells (QWs) are analyzed as improved active media based on a 6-band k•p method taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. The growths of staggered InGaN QW emitting at green regime were conducted by employing graded temperature profile. The electroluminescence measurement shows a significant enhancement of the output power of the staggered InGaN QW LED as compared to that of the conventional InGaN QW LED.

Challenges for the MOVPE growth of LED heterostructures for emission in the UV-A and UV-B spectral range are discussed. Special attention is given to the effects of strain in the In(Al)GaN active region as well as in the complete layer stack. Here in-situ monitoring of wafer bowing is shown to be an important tool for optimization of the growth sequence. We will compare different buffer layer technologies, in particular GaN/sapphire for LEDs emitting at 380 nm and AlN/AlGaN buffer for shorter wavelength LEDs. By increasing the aluminum content in the InAlGaN multiplequantum- well active region and by optimizing the composition and doping profile of the electron blocking layers UV LEDs with emission wavelength between 380 nm and 318 nm are demonstrated.

Phototherapy is a very effective method for treating most of the incurable skin diseases. A fluorescent light bulb is used as a conventional UV light source for this type of therapy. However, infrared radiation from the light source sometimes causes serious problems on patient's health. In addition, the normal part of the skin is irradiated when a large fluorescent light bulb is used. Moreover, a conventional UV irradiation system is heavy and has a short lifetime and a high electrical power consumption. Therefore, a new UV light source for solving the problems of phototherapy is required. To realize low-power-consumption, lightweight and long-lifetime systems, group III nitride-based UV-A1 light-emitting diodes (LEDs) were investigated. We examined the UV LED irradiation of Jurkat cell, which is a tumor cell and more sensitive to UV light than a healthy cell. The numbers of apoptotic and necrotic cells were confirmed to be the same using a UV LED and a conventional lamp system. The UV LED showed the possibility of realizing a new UV light source for phototherapy.

Near-UV LEDs emitting at around 400 nm can be used e.g. as pump light source in tri-phosphor RGB white luminescence-conversion LEDs with high color rendering.¹ Although non-thermal roll-over decreases towards shorter emission wavelengths in GaInN-based LEDs, this effect still limits the efficiency of 400 nm emitting LEDs at current densities above 50 A/cm². One way to overcome non-thermal roll-over is to combine a GaInN wide-well active region with the growth on low dislocation density (DD) substrates. Single-well LEDs with GaInN layer widths between 3 nm and 18 nm were grown (a) directly on sapphire substrates with a resulting DD of 10⁹ cm^-2, (b) on low DD GaN templates on sapphire (DD of 10⁸ cm^-2), and (c) on freestanding GaN substrates (FS-GaN, DD of 4×107 cm^-2). At low current densities (pulsed mode operation) the LEDs with a 3 nm GaInN QW active region showed the highest efficiency, irrespective of the substrate. However, the electroluminescence (EL) efficiency peaks at around 50 A/cm² and shows a clear non-thermal roll-over towards higher current densities. The efficiency of LEDs with well widths >3 nm grown on sapphire decreases with increasing well width over the whole range of current densities (≤300 A/cm²). However, when grown on low DD GaN templates or FS-GaN, the efficiency of the LEDs with 11 and 18 nm wide GaInN layers surpasses that of the conventional LEDs (well widths ≤6 nm) for current densities ≥250 A/cm², yielding the highest EL efficiency of all LED-structures.

In this paper, we would like to report the following two subjects:(1) Thermal decay mechanisms of phosphor-doped silicone in high power phosphor-converted white light emitting diode (PC-WLED) module and (2) Thermal aging variations of light profile and output power of blue LED modules having a polycarbonate lens and silicone as an encapsulant. Although silicone degradation attributed to the final thermal degradation, it is not a dominant factor until a much thicker silicone is employed in PC-LEDs. The major degradation mechanism of the PC-LEDs results from the higher doping concentration of Ce:YAG in silicone. However, the negligible difference of fluorescent lifetimes among the test samples before and after thermal aging (at 150°C for 500hrs) eliminated any significant quenching processes that existed in our aged samples. The emission spectra suggest that a higher doping concentration in silicone causes a higher degree of loss at the emission wavelength of Ce:YAG, namely 570nm. Therefore, minimizing any mismatch of the refractive index, thermal expansion , and chemistry between the phosphor and the silicone is a new sign of improving thermal reliability for high power PC-LEDs. Thermal aging variations of light profile and output power of LED modules fabricated by three manufacturers (namely, Type I, II, and III) were investigated experimentally and numerically. Both experimental results and simulation results suggested that improving the lens/ encapsulant materials and packaging designs are essential to not only greatly extend the product lifetime but also enhance the light quality of LED modules as illumination sources. Index Terms -High-powered phosphor-converted white-light-emitting diodes (PC-WLEDs), lumen loss, chromaticity shift, silicone, polycarbonate, thermal aging, etc.

Photoluminescence (PL) technique has demonstrated the powerful capability for practical application to wafer inspections in III-V compound semiconductor production. This paper describes the principle and the physics behind the variation of PL wavelength with laser excitation power density. We discuss the effect of GaN cap thickness on the PL measurements. This is especially relevant to LED manufacturers where thick GaN caps are commonly used and to PL metrology of InGaN MQW laser structures. A methodology was proposed to select the most appropriate laser and set of excitation conditions to achieve matching between PL and electroluminescence (EL) wavelengths for common Blue and Green LED structures.

We report on a novel high-performance active heat sink (AHS) offering unparalleled capacity in high-heat flux handling, heat spreading, and temperature control. AHS couples directly to standard high-brightness light emitting diodes (LED) packages. It efficiently removes high-flux waste heat from the LED slug and transfers it to ambient air. AHS has variable thermal conductance that can be electronically adjusted to compensate for changes in ambient temperatures or to adjust the light color. This paper presents AHS physics, engineering design, and results from numerical simulations. Applications to screw-in replacements for the R30 lamp are discussed.

Improvement of light extraction efficiency of InGaN light emitting diodes (LEDs) using polydimethylsiloxane (PDMS) concave microstructures arrays was demonstrated. The size effect of the concave microstructures on the light extraction efficiency of III-Nitride LEDs was studied. Depending on the size of the concave microsturctures, ray tracing simulations show that the use of PDMS concave microstructures arrays can lead to increase in light extraction efficiency of InGaN LEDs by 1.4 to 1.9 times. Experiments utilizing 1.0 μm PDMS concave microstructures arrays demonstrated 1.60 times improvement in light extraction, which is consistent with simulated improvement of 1.63 times. The enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to PDMS concave microstructures arrays, and reduced Fresnel reflection within the photon escape cone due to the grading of refractive index change between GaN / PDMS / air interface.

We have developed 4.26 μm room temperature resonant-cavity light-emitting diodes (RCLED) using III-V material system (InAs, InSb, and GaSb). The emission spectrum of RCLED closely matches the CO2 absorption profile. The wavelength of the peak of the emission spectrum is stable with respect to wide variations in the temperature and the drive current. Consequently we were able to develop a low cost RCLED based CO2 sensor operating over a wide temperature ranges of +5 to + 45 degrees C and minus 20 to + 60 degrees C (depending on the IR detector used) without a TE-cooler. These sensor devices consume very low power, at the level of mW, with a fast response time of 10 sec and better. It can measure CO2 levels in the range 0-10,000ppm with an accuracy of better than +-5%.

InGaN-based LEDs suffer from a significant drop in quantum efficiency (QE) under high-current operation. We studied the Electroluminescence (EL) of InGaN-based multiple-quantum-well green LEDs on both Sapphire and free standing Bulk GaN, in an attempt to shed light on the underlying mechanism for the efficiency droop problem. The density of microstructural defects in the LED on GaN was substantially reduced, leading to a significant reduction in defectassisted tunneling currents and an improved injection efficiency under low bias. The LED on GaN outperformed the LED on sapphire at low injection currents and exhibited a ~65% peak internal quantum efficiency. However, it suffered from even more dramatic efficiency roll-off which occurs at a current density as low as 0.3 A/cm². The EQE roll-off is mitigated when the LEDs were tested at elevated temperatures. These results are explained as the combined result of efficient current injection and significant carrier overflow in a high-quality LED.

White light of superior quality is characterized by high efficiency, high color rendering, and suitable spectral shape. Color conversion from semiconductor light sources by use of phosphors appears to be a promising route to cover the customer needs, and in the past a lot of different approaches have been pursued. There are not so many really successful phosphor systems on the market, which is due to the industrial focus on blue LEDs as the excitation source, the increasingly high demands on thermal and photochemical stability (high-power LEDs provide enormous irradiances), and patent limitations. This paper reviews some concepts on color conversion including not only broadly used downconversion but also novel up-conversion principles and discusses the pro's and con's of these approaches.

The use of white LEDs for solid-state lighting to address applications in the automotive, architectural and general illumination markets is just emerging. LEDs promise greater energy efficiency and lower maintenance costs. However, there is a significant amount of design and cost optimization to be done while companies continue to improve semiconductor manufacturing processes and begin to apply more efficient and better color rendering luminescent materials such as phosphor and quantum dot nanomaterials. In the last decade, accurate and predictive opto-mechanical software modeling has enabled adherence to performance, consistency, cost, and aesthetic criteria without the cost and time associated with iterative hardware prototyping. More sophisticated models that include simulation of optical phenomenon, such as luminescence, promise to yield designs that are more predictive - giving design engineers and materials scientists more control over the design process to quickly reach optimum performance, manufacturability, and cost criteria. A design case study is presented where first, a phosphor formulation and excitation source are optimized for a white light. The phosphor formulation, the excitation source and other LED components are optically and mechanically modeled and ray traced. Finally, its performance is analyzed. A blue LED source is characterized by its relative spectral power distribution and angular intensity distribution. YAG:Ce phosphor is characterized by relative absorption, excitation and emission spectra, quantum efficiency and bulk absorption coefficient. Bulk scatter properties are characterized by wavelength dependent scatter coefficients, anisotropy and bulk absorption coefficient.

Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD and thermal CVD will be presented, along with their relation and difference to thin film counterparts. While there is a growing acceptance of the low band gap (0.6-0.7 eV) of InN, the optical properties of the actual samples still suffered, presumably due to the difficulty in obtaining high-quality samples and/or controlling their defect and carrier concentrations. However, the free-standing nanobelts can be nearly defect-free, allowing an excellent opportunity for fundamental investigations on unique dimensionality. InNNBs show photoluminescence (PL) in IR with peak width of 14 meV, the sharpest reported to date for InN. Interestingly, with increasing excitation intensity, InNNBs reveal an anomalously large blueshift in PL, compared to thin films; along with a decrease in the phonon frequencies as evident by Raman measurements. Surface band bending, converse piezoelectric effect, and photoelastic effects are employed to explain these behaviors. As for InGaN NWs, both In-rich and Ga-rich ternary nanowires have been synthesized by simply varying growth temperature. Morphological and structural characterizations reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming self assembled quantum dots (SAQDs) embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich nanowires, which has been explained by proposing a multi-level band schema.

As one of the most mature techniques for manufacturing free-standing GaN substrates, hydride vapor phase epitaxy (HVPE) always encounters problems associated with residue thermal stress, such as GaN bending and cracking during and after growth. This work presents a patterning approach and a non-patterning approach to reduce stress in thick GaN films grown on sapphires by HVPE. The patterning approach, forming dot air-bridged structures, adopted standard photolithography to fabricate hexagonally aligned patterns of dots on GaN templates. Following HVPE growth, regular voids were formed and buried in the GaN thick-films. These voids helped to relax the stress in the GaN thick-films. In the non-patterning approach, thick GaN films were simply grown at a specially set sequence of ramping temperatures during HVPE growth without any patterned structure. This temperature-ramping technique, gives crack-free high-quality 2"-diameter GaN films, thicker than 250 μm, on sapphires in high yields. These thick GaN films can be separated from sapphire using conventional laser-induced lift-off processes, which can be followed by subsequent HVPE regrowths. A 600 μm-thick free-standing GaN films has a typical dislocation density of around 4×10⁶ cm^-2 with a full width at half maximum (FWHM) in the high resolution X-ray diffraction (HRXRD) spectrum of GaN (002) of around 150 arcsec. The residual stress in the thick GaN films was analyzed by micro-Raman spectroscopy. The effectiveness of the patterning and the non-patterning techniques in reducing the strain in GaN films is discussed. The advantages and weaknesses of the patterning and the non-patterning techniques will be elucidated.

The issues and challenges of growing GaN-based structures on large area Si substrates have been studied. These include Si slip resulting from large temperature non-uniformities and cracking due to differential thermal expansion. Using an AlN nucleation layer in conjunction with an AlGaN buffer layer for stress management, and together with the interactive use of real time in-situ optical monitoring it was possible to realise flat, crack-free and uniform GaN and LED structures on 6-inch Si (111) substrates. The EL performance of processed LED devices was also studied on-wafer, giving good EL characteristics including a forward bias voltage of ~3.5 V at 20 mA from a 500 μm x 500 μm device.

Al₂O₃ layers have been deposited by atomic layer deposition (ALD) on both silicon and zinc oxide (ZnO) substrates as a transition layer for MOCVD growth of GaN. These Al2O3 layers have been shown to reduce tensile strain and cracking in GaN thin films on Si, and they have also been shown to help suppress impurity diffusion from the ZnO substrate into the GaN layers. Surface morphology of the ALD-grown layers was investigated using scanning electron microscopy (SEM), and structural properties were studied using high resolution x-ray diffraction (HR-XRD). GaN thin films were then grown on these layers to determine the effects of the Al₂O₃ layer on subsequent GaN quality. The optical and structural properties of these films were studied, as well as surface morphology. GaN layers grown using the Al₂O₃ layers on Si in particular exhibit structural and optical properties approaching those of typical GaN thin films on sapphire, which shows significant promise for high performance GaN-based devices on Si substrates.

CCD based spectrometers are commonly used to characterize the optical performance of LEDs. All CCD based spectrometers exhibit varying amounts of stray light. This situation is exacerbated by using tungsten-halogen based sources for calibration. NIST has come up with methods to characterize the stray light associated with CCD based spectrometers using multiple lasers spanning the visible and near IR spectrum. At Sphereoptics we have developed methods to map out and correct for stray light using a double monochromator and software. We have been able to mathematically remove the effects of stray light on many common LED characterization measurements such as spectral flux for instance. These effects were found to be quite substantial especially in the UV part of the spectrum. In this paper we will present our findings on the nature of stray light found in a B&W BRC112E UV spectrometer as well as our progress in correcting the errors it causes.

The unique properties of light emitting diodes (LED) offer significant advantages in terms of lifetime, source control, and response time all of which are potentially very important for various illumination applications. However, a number of factors such as the initial variability of the optical properties in a batch, high temperature dependence of the intensity and color especially of the red and amber LEDs and spectral degradation causing gradual intensity decrease and shift in color, need to be considered when developing LED light sources. The degradation and temperature dependence of the LEDs can significantly affect the performance of a light source over the operating period in terms of intensity and color stability, and efficacy. This is especially critical for polychromatic LED light sources, consisting of different types of LEDs with different spectral degradation rates and temperature dependencies. Therefore, a method estimating the intensity and color as a function of the affecting factors could be a valuable tool for developing LED light sources for various applications. In this paper, we present a simulator employing model-based estimation of the LED color, intensity, and efficacy as a function of temperature, forward current, and spectral degradation. The models are derived from the measurements of the electrical and spectral characteristic of the individual LEDs as a function of the forward current, junction temperature and corresponding thermal properties of the packaging. The results show that the simulator provides accurate estimates of the intensity and color of the LED light sources as a function of the temperature and forward current.

We propose and demonstrate weak-microcavity organic light-emitting diode (OLED) displays that deliver both a high light-extraction efficiency and wide viewing-angle characteristics. A single pair of low- and high-index layers is inserted between indium tin oxide (ITO) and a glass substrate. The electroluminescent (EL) efficiencies of discrete red, green, and blue weak-microcavity OLEDs (WMOLEDs) are enhanced by 56%, 107%, and 26%, respectively with minimal changes viewing angle and EL spectra characteristics. The color purity is also improved for all three colors. Moreover, we fabricated full-color 128×160 passive-matrix bottom-emitting WMOLED displays to prove their manufacturability. This design is realized by simple one-step 20-nm etching of the low-index layer of red/green subpixels. The EL efficiency of white color in the WMOLED display is 27% higher than that of a conventional OLED display.