Increased-pressure metalorganic vapor phase epitaxy (MOVPE) system with high speed switching valves is found to be effective for growing In-rich GaInN at a high temperature. High-speed switching valves enable the atomic layer epitaxy of high quality AlGaN at a low temperature, by which we can grow thin AlGaN capping layer without the thermal decomposition of underlying In-rich GaInN. This new growth technology sheds light on the digital alloy growth for the development of high-efficiency nitride-based visible long -wavelength light emitters.

We present a 7.5 cm x 7.5 cm white PHOLED^TM lighting panel that delivers 1,000 cd/m² with 68 lm/W efficacy, CRI > 80 and lifetime to LT70 ≈ 15,000 hrs. A simple all-phosphorescent device architecture, including a highly stable light blue phosphorescent emitter-host system, is used to reduce panel power consumption, extend operational lifetime and demonstrate exceptional emission color stability with aging.

A novel direct-emitting LED backlit for LCDs was demonstrated. Unlike the conventional white LED schemes for display applications, proposed blue light excited planar lighting (BLPL) exploits blue LED chip to remotely excite the YAG-phosphor film and thus render a uniform planar source, where the YAG-phosphor acts as the diffuser film and wavelength down converter simultaneously. Based on the diffusing characterization of YAG-phosphor layer, we examined the optical properties of the BLPL system in viewpoints of uniformity, luminance and mixing capability. Consequently, a prototype 10-mm-thickness BLPL module was demonstrated with 86% uniformity and 9800 nits without using any diffuser film or light guiding plate.

In this paper, based on Monte Carlo ray tracing we simulate light extraction efficiency and directionality of the light pattern of GaN LEDs implanted with micro-pyramid structure with or without lens encapsulation. We have shown that micro-pyramid structures at some specific slanted angles in the LEDs are useful to increase effective flux utilization through enhancement of both the directionality and light extraction efficiency.

Metalorganic vapor phase epitaxy (MOVPE) nucleation studies of GaN on planar sapphire and nano-patterned AGOG (Deposition of Aluminum, Growth of Oxide, and Grain growth) sapphire substrates were conducted. The use of abbreviated GaN growth mode, which utilizes a process of using 15nm low temperature GaN buffer and bypassing etchback and recovery processes during epitaxy, enables the growth of high-quality GaN template on nano-patterned AGOG sapphire. The GaN template grown on nano-patterned AGOG sapphire by employing abbreviated growth mode has two orders of magnitude lower threading dislocation density than that of conventional GaN template grown on planar sapphire. The use of abbreviated growth mode also leads to significant reduction in cost of the epitaxy. The growths and characteristics of InGaN quantum wells (QWs) light emitting diodes (LEDs) on both templates were compared. The InGaN QWs LEDs grown on the nano-patterned AGOG sapphire demonstrated a 24% enhancement of output power enhancement over that of LEDs grown on conventional GaN templates.

Over the last years, important efforts have been done in order to understand the degradation mechanisms of GaN-based LEDs submitted to forward-bias stress tests. On the other hand, only little work has been done to understand the degradation of LEDs submitted to reverse-bias stress. However, this topic is of high interest, since (i) the reverse-bias robustness of the LEDs is strongly correlated to their stability under Electrostatic Discharge (ESD) events and (ii) the analysis of the reverse-bias degradation can provide important information on the role of high electric fields and reverse current in limiting the reliability of the LEDs. Therefore the aim of this paper is to describe a detailed investigation on the reverse-bias degradation of GaN-based LEDs. The results described in this paper indicate that: (i) under reverse bias, LEDs can show a weak luminescence signal, due to the recombination of carriers injected in the quantum-wells; (ii) reverse-bias stress can induce the degradation of the electrical characteristics of the LEDs (increase in reverse-current, decrease in breakdown voltage), due to the generation of point defects in proximity of pre-existing defective regions. (iii) Furthermore, our tests indicate that the defective regions responsible for reverse-current conduction can constitute weak points with respect to ESD events: ESD failures are determined by the shortening of the junction in proximity of one of the defective sites responsible for reverse-current conduction.

Use of deep ultraviolet (248 nm) KrF laser irradiation to roughen vertical GaN-based LEDs surface with volcanolike protrusions for light output (Lop) improvement was proposed and demonstrated. After pulse irradiations of KrF laser (750-850 mJ/cm²), the rate of electron-hole pair recombination at sites with dislocation defects is greater than for crystalline GaN, favoring for the formation of GaO_x, and in turn, resulting in a relatively lower etching rate therein and leading to a roughened surface with volcano-like protrusions. Typical diameter/height and density of protrusions are around 2~4 μm/2 μm and 10⁶ cm^-2. Through the use of KrF laser and KOH etching, an enhancement in the root-meansquare surface roughness by 250 times and an improvement in Lop by 25% at 750 mA were obtained. It is expected that the surface roughness of Gallium Nitride by KrF excimer laser technology would be a potential candidate for the fabrication of high power GaN-based LEDs for solid-state lighting in the near future.

The use of Polystyrene Spheres (PSs) to realize nano-roughened Indium-Zinc Oxide (IZO) surface and a high reflective ohmic p-contact to improve the optoelectronic properties of larger-area (1×1 mm²) vertical metallic-substrate GaN-based light-emitting diodes (VLEDs) were proposed and investigated. A metal system consisting of annealed- Pt/Al/Pt was employed to serve as a reflector and ohmic contact to p-GaN, which exhibits a good ohmic contact (1.84×10^-3 Ωcm²) and high reflectivity (88% at 465 nm). After the removal of sapphire using laser lift-off process (LLO) and etching of u-GaN by ICP, Ti/IZO film was then deposited to serve as a transparent conduction layer (TCL). After that, the polystyrene spheres (PSs) were dispersed on the IZO surface, followed by second sputtering-deposition of IZO film to fill the space between neighboring PSs. The PSs were then removed to form a nano-roughened IZO top-layer. Compared to regular VLEDs with Ni/Au ohmic contact and Ti/Al/Ti/Au as reflector layer, the fabricated VLED shows a typical increase in light output power (i.e., ▵Lop/Lop) by 72.2% at 350 mA and a decrease in forward voltage (Vf) from 3.43 V down to 3.33 V. It is expected that the proposed PSs nano-roughening technology and high reflection annealed- Pt/Al/Pt metal system for ohmic contact to p-GaN would be a potential candidate for the fabrication of high power GaNbased LEDs for solid-state lighting in the near future.

Propagation matrix method was used to calculate the surface plasmon dispersion for metal on top of InGaN / GaN quantum wells (QWs). Purcell enhancement factor related to the slope of the surface plasmon dispersion curve was calculated for metal on top of InGaN / GaN QWs. The use of double-metallic Au / Ag layers coupled to InGaN QWs results in wide-spectrum tuning of the Purcell peak enhancement of the spontaneous recombination rate for nitride lightemitting diodes.

Since high-power LEDs show great potential in reducing energy consumption worldwide, a great deal of research has been performed to understand their degradation rate. As reported in many publications, temperature is of critical importance so lifetests are mainly based on the internal temperature of the junction (Tj). A common testing method is to overdrive the LED with high current in order to cause self-heating. However, by doing so, it is assumed that current does not produce self-degradation. This topic is of great importance nowadays because of the recent development of LEDs used to increase operating current. We have conducted a lifetest on LEDs to isolate the influence of current by using a thermally-controlled heatsink to keep the same T_j for different driving currents. This paper presents the experimental setup with the associated protocol used in the experiment. We also present preliminary results obtained from two high-power white LEDs. These were stressed at currents ranging from 350 mA to 1000 mA and at temperatures ranging from 75°C to 150°C. To our knowledge, this type of measurement has not been reported in the literature. In the future, we would like to use a Weibull statistical model to study the combined effects of temperature and current on the degradation of LEDs.

This paper explored the feasibility of using a LED-based bulb as the illumination light source for photolithography room. A no-blue LED was designed, and the prototype was fabricated. The spectral power distribution of both the LED bulb and the yellow fluorescent tube was measured. Based on that, colorimetric values were calculated and compared on terms of chromatic coordinates, correlated color temperature, color rendering index, and chromatic deviation. Gretagmacbeth color charts were used as a more visional way to compare the two light sources, which shows that our no-blue LED bulb has much better color rendering ability than the YFT. Furthermore, LED solution has design flexibility to improve it further. The prototype has been tested with photoresist SU8-2005. Even after 15 days of illumination, no effect was observed. So this LED-based solution was demonstrated to be a very promising light source for photolithography room illumination due to its better color rendering in addition to energy efficiency, long life time and design flexibility.

LED wavelength and luminosity shifts due to temperature, dimming, aging, and binning uncertainty can cause large color errors in open-loop light-mixing illuminators. Multispectral color light sensors combined with feedback circuits can compensate for these LED shifts. Typical color light sensor design variables include the choice of light-sensing material, filter configuration, and read-out circuitry. Cypress Semiconductor has designed and prototyped a color sensor chip that consists of photodiode arrays connected to a I/F (Current to Frequency) converter. This architecture has been chosen to achieve high dynamic range (~100dB) and provide flexibility for tailoring sensor response. Several different optical filter configurations were evaluated in this prototype. The color-sensor chip was incorporated into an RGB light color mixing system with closed-loop optical feedback. Color mixing accuracy was determined by calculating the difference between (u',v') set point values and CIE coordinates measured with a reference colorimeter. A typical color precision ▵u'v' less than 0.0055 has been demonstrated over a wide range of colors, a temperature range of 50C, and light dimming up to 80%.

Starting from a lab-curiosity, organic light emitting diodes have matured into a promising technology that has entered commercial markets. In particular for lighting applications, OLEDs can take advantage of their outstanding properties such as a high luminous efficacy, good color quality, and new design possibilities such as illumination by at light sources. In this contribution, new results on two approaches for highly efficient white OLEDs are presented: the all-phosphorescent concept and the triplet-harvesting approach.

One of the most promising market segments to mainstream OLED lighting is the Commercial and Institutional Segment. It account for 40% of the entire lighting market. 75% of all the fluorescent luminaires are sold into this market segment. In order to meet the future lighting energy allowances, it is recommended that OLED efficacy be designed to around 70+ lumens/watt initially and gradually increases to 100 lumens per watt and perhaps ultimately to 140 lumens per watt. Luminous Exitance of an OLED can be designed to 6400 - 8000 lumens per square meter (approximately 2000 - 2500 candelas per square meter). This level of performance will enable OLED to participate in most of the lighting applications found in commercial and institutional market segment. As for lifetime of an OLED, an initial lifetime of around 20,000 hours at L70 is reasonable. The performance will move toward the target of around 50,000 hours of effective operating life at L85. Proper lighting design with daylight harvesting and other means can be very helpful in accomplishing this target.

One major performance parameter for organic light-emitting diodes (OLEDs) in display and illumination applications is the overall efficiency of the device, which is directly affected by the emitter's internal luminescence quantum efficiency q. It is very desirable to determine q of the emitter in-situ, i.e. in electrically driven operation, since photoluminescence measurements of q provide a rather rough (over-)estimation. In a layered system (LS), the value q of the emitting material (EM) is associated with the coupling probability of emitted radiation into the different modes provided by the LS (air, substrate, guided, surface plasmon). We show the in-situ determination of q by optical means from a relative comparison of current efficiencies of OLEDs with varying emitter-cathode distance. As a prerequisite, we outline procedures for a complete characterization of the passive and active optical properties of the LS and the EM, respectively. Then, precise optical simulation allows determining q without additional assumptions.

The extraction of the luminous power internally generated by organic light emitting diodes (OLEDs) is still the most severe limitation in the overall efficiency of these devices. We present a joint theoretical and experimental study aimed to quantitatively evaluate the light outcoupling limitations of planar p-i-n type small-molecule OLEDs, both in bottom and top emitting configuration. We discuss the physical origin of these limitations by analyzing internal optical losses and overall light conversion efficiency in OLEDs.

There has been increasing research interest in nonpolar and semipolar GaN for high brightness lightemitting application. Due to the very limited supply of GaN bulk substrates, the feasible way of obtaining large-area nonpolar and semipolar GaN material is still through heteroepitaxy on foreign substrates at present. This paper highlighted the major challenges in the heteroepitaxy of nonpolar and semipolar GaN on sapphire and presented the progress in reducing the defect density by a two-step growth technique according to the in situ optical reflectance and the ex situ x-ray analyses and transmission electron microscopy measurements. A defect reduction model was proposed based on the correlation between the morphological evolution and the microstructural development during the two-step growth of nonpolar aplane GaN. The material research status of nonpolar and semipolar GaN was summarized. A promising approach (orientation controlled epitaxy) was pointed out for a further improvement of nonpolar and semipolar GaN material quality.

II-VI semiconductors can exhibit strong photoluminescence throughout the visible spectrum and are excellent candidates for filling the so-called "green gap". We report on the performance of green color-converted LEDs fabricated by bonding CdMgZnSe multiple quantum well structures to high-efficiency blue-emitting GaInN LEDs. A device efficacy of 181 lm/W at 537 nm (dominant) is measured under room temperature, 350 mA/mm² quasi-cw conditions, more than twice as efficient as typical commercial green LEDs today. The thermal roll-off is shown to be comparable to that of typical green GaInN LEDs. Finally, the implications of the availability of high-efficiency, narrow-band, green and yellow emitters in display applications will be discussed.

Air-stable luminescence silicon nanocrystals (Si-NCs) were synthesized using a novel in-flight system composed of a Si-NC synthesis SiH4/Ar plasma and an SF6 plasma which etches and passivates the NCs. The etch plasma can efficiently tailor the Si-NC size and the surface functionalities by tuning the gas flow rate, applied power, and pressure of the plasma. Si-NCs based light emitting diodes (LEDs) were fabricated by using the Si-NCs as the recombination center for injected electron-hole pairs. Si-NCs were deposited in between two inorganic metal oxide layers, nickel oxide (NiO) and zinc oxide (ZnO), which served as the hole transport layer (HTL) and electron transport layer (ETL), respectively. NiO and ZnO have been chosen by considering their energy band offsets with respect to Si-NCs, and their band offsets to the electrodes which should produce roughly comparable carrier concentrations once the contacts are forward biased, to get charge balance at the Si-NCs. The as-prepared metal oxides were confirmed to be stoichiometric using Auger Electron Spectroscopy (AES). Four-point probes measurements show the oxide sheet resistances in the range of 2-5×10⁶ Ω/(see manuscript). The as-prepared etched Si-NCs generate orange photoluminescence at a peak intensity of 650nm with a quantum efficiency of 23%. I-V characteristics and light intensities of the Si-NCs LED without depositing the ZnO ETL have been studied with respected to the Si-NCs thickness. LEDs made using a two minute deposition of Si-NCs (approximately 250nm thick) showed an easily visible air-stable light emission; however, the light intensity decreased by 50% for thicker (1.5μm) Si-NC films. The LED performance was improved by using an ITO/ZnO/SiNCs/NiO/Al device structure. The turned on voltage increased to 7V but the current saturated to 0.1A very rapidly. The Si-NCs LED EL spectrum was collected at a bias voltage of 8.5V. The emission peaked at 653 nm for the Si-NCs LED in good agreement with the PL results. At the highest current densities some degradation of the device was observed, otherwise device operation was consistent and yield was good. The I-V characteristics of the Si-NC LED made using all inorganic metal oxides showed Schottky behavior as well as good light intensity.

We demonstrated 222-282 nm AlGaN-based efficient deep-ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on low threading dislocation density (TDD) AlN template. Low TDD AlN on sapphire were realized by using ammonia (NH₃) pulse-flow multilayer (ML) growth technique. We obtained quite high IQE (~80%) from slightly-Inincorporated (0.3%) InAlGaN QWs and obtained over 10 mW CW output power for 280 nm-band InAlGaN based LED. The maximum output power obtained were over 10 mW for 264-282 nm LEDs, 1.2-5mW for 240-256 nm LEDs and sub-milliwatt for 222-237 nm LEDs. The maximum external quantum efficiency (EQE) of 280 nm-band LED was 1.2%.

In this study, two approaches using various insertion structures are proposed for the near-UV LEDs. One is through a single MOCVD process where a heavily Mg-doped GaN insertion layer (HD-IL) technique is employed to improve crystalline quality of the GaN layer and followed by rest of required GaN-based LED structure. Another approach was demonstrated by the near-UV LEDs with an embedded distributed SiO₂-disk structure. The periodically spaced hexagonal disk-shaped SiO₂ mask array was deposited on the GaN/sapphire template and followed by the MOCVD regrowth process. These improvements contribute the high-performance 380-nm LEDs with enhanced output powers by 20-40% in magnitude.

Reflective (thick) and semi-transparent (thin) Ni/Ag/Ni contacts were prepared on GaInN-based light-emitting diodes (LEDs) and p-GaN/p-Al_0.15Ga_0.85N layer sequences. A light output power enhancement of 41% and forward voltage reduction of 0.59 V were obtained compared to a Ni/Au contact for LEDs emitting at 400 nm with thick p-GaN contact layers. The specific contact resistance of the Ni/Ag/Ni contacts on p-GaN/p-Al_0.15Ga_0.85N with varying p-GaN thickness (5-20 nm) were determined by transmission line method and compared to Ni/Au contacts. Low resistive ohmic contacts were obtained for a p-GaN thickness of less than 10 nm. The p-GaN layer can be completely omitted for the reflective Ni/Ag/Ni contact. In addition, reflection and transmission of the Ni/Ag/Ni metallization schemes were investigated in the ultra-violet spectral range. Thick Ni/Ag/Ni and thin Ni/Ag/Ni covered by Al are promising to serve as reflective contacts for ultra-violet LEDs. The former for wavelength around 350 nm and the latter for wavelengths below 350 nm.

To date, the reason for high ideality factor, β, in GaN-based LEDs grown on sapphire substrate is not fully understood and explained. It is believed that β-factor exceeding 2.0 originates from the trap-assisted tunneling and charge carrier leakage inside the active MQW LED region or is due to additional junctions available in the LED circuit. In this research, we demonstrate that β values higher than those predicted by the classical theory may be related to the current crowding effect that is difficult to avoid in LEDs grown on the insulating substrates. By analyzing theoretical model and testing commercial lateral blue LEDs with two different p-electrode pattern, we show that β -factor could increase from 2.0 (current spreading geometry) up to 3.5 (current crowding geometry). This modification of β-factor occurs in the intermediate range of current (10 μA - 10 mA, the space charge region dominates in LED performance) and therefore could be erroneously treated as the change of carrier transport mechanism and charge carrier recombination nature. At higher current (series resistance dominates) even insignificant increase of β-factor makes the current value of efficiency rollover to decrease (from 35 mA to 15 mA) and the efficiency droop to increase by 10%.

Ce³⁺/Mn²⁺-codoped Ba_1.20Ca_0.8-2x-ySiO₄:xCe³⁺, xLi⁺, yMn²⁺ phosphors show two emission bands peaking at around 600 nm (red) and 400 nm (deep-blue) from the forbidden ⁴T₁-⁶A₁ transition of Mn²⁺ ions and the allowed 4f-5d transition of Ce³⁺ ions, respectively. Eu²⁺-doped Ba_1.20Ca_0.7SiO₄:0.1Eu²⁺ phosphor shows a broad green emission band from 430 to 550 nm from the allowed f-d transition in Eu²⁺ ions. The mixtures of both phosphors, excited by the near ultraviolet of 365 nm, show the various qualities of white lights depending on the mixture ratio; the correlated color temperatures from 3500 to 7000 K, and the color-rendering indices up to 95 %. Furthermore, they show a high quenching temperature of about 225 °C.

A novel blue-emitting phosphor, Sr₃Ga₂O₅Cl₂:Eu²⁺, was synthesized by a two-step solid-state reaction. It has the monoclinic structure with six different cation sites. It shows an efficient broad absorption band around the 400 nm of the commercial near ultraviolet light-emitting diodes (LEDs), and an intense broad blue emission. Thus, it can be a promising blue phosphor for white LED for solid state lighting.

Although the general properties of the rare earths' electronic states and transitions are well understood, much less is known regarding the relationships between them and the electronic band states of a crystal lattice. These interactions can enhance or inhibit performance and provide mechanisms for manipulating the material's optical properties. Up-conversion ZrO₂:Tm³⁺, Yb³⁺, Er³⁺, Ho³⁺ nano-crystalline samples were synthesized by sol-gel method and emission properties were analyzed as function of different concentrations of rare earth ions. The samples were pumped at 970 nm with a semiconductor laser source. The introduction of different ion concentrations affects the shape and peak intensities of the measured blue, green and red bands. Results showed in this work tend to demonstrate a feasible control of the chromaticity coordinates of emission and present an approximation to the equipotential white chromaticity coordinates.

We proposed and demonstrated a simple approach for designing and developing blue-excitation-light passing and phosphor-yellow-emission-light reflecting dielectric multilayer to enhance the forward efficiency of Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) yellow phosphor on top of a blue InGaN LED cup. When inserting a modified quarter-wave films of alternate high- and low-refractive index dielectric films (TiO₂/SiO₂) into the interface between a YAG:Ce phosphor layer and a glass substrate, enhancements of the efficiency and luminous efficacy of the forward white emission become 1.64 and 1.95 times that of a conventional phosphor on top of a blue LED cup with a lower correlated color temperature (< 4000K).

The output power of a light-emitting diode (LED) not only is affected by aging but also by dirt buildup. Environment and surroundings are typically characterized by the presence of substances, dust, liquids or vapors that may stick to the LED, reducing its light output. Knowing the effect of dirt on light output, manufacturers and users can efficiently design a cleaning or maintenance program. In this work, both 5-mm LEDs and high-power LEDs were subjected to output power tests for different degrees and types of dirt. In particular, I measure the light flux changes due to deposition of dust (sand), drops of water, coal dust, oil drops, fat (soldering paste), and fingerprints.

We report here the electroluminescence in the range of 3-4.5 μm and 6-10 μm from Sb-based type II interband quantum cascade structure LED devices. We measured the light emission from the top surface of the device with different grating structures. We used different etch depths for the grating formation. The light-current-voltage (LIV) characteristics measured at both room and cryogenic temperatures show that the device with 45 degree angle grating and 1.0 μm deep etch onto the GaSb surface has the highest emission power.

The most common method of making white light emitting diode (LED) is to mix the blue light from the LED die and the wavelength converted yellow light from the phosphor layer. The color conversion efficiency depends on the geometry and concentration of the phosphor layer including phosphor material. Thus the optimization of the phosphor geometry and concentration make increase the luminous efficiency of the white LED. In this paper, the remote phosphor scheme is optimized focusing on increasing the luminous efficiency in high power. The phosphor layer is separated by the silicone resin from the LED die. The silicone resin covers the LED die with dome shape to increase the extraction efficiency. The phosphor layer has very large volume with dilute concentration. The separation of phosphor layer from LED die and very large volumetric dilute phosphor layer were great important role in increasing the luminous flux. The improved luminous flux was 15% for 1mm² LED die at 700mA.