Increasing emission power and efficiency in green light emitting diodes is one of the big challenges towards all-solid-state lighting. The prime challenge lies in the combination of extension of wavelength from 470 nm blue to 525 nm green while maintaining the emission power level. Commonly a steep decrease in power is observed. In a broad development effort we have been able to ameliorate that decrease significantly and obtain bare die performance at 525 nm of 1.6 mW at 20 mA for 350x350 μm² dies. Here we discuss critical die performance and wafer yield aspects of our optimization approach to the active layer of the GaInN/GaN quantum wells.

Deep levels in n-type GaN grown by molecular beam epitaxy, metalorganic chemical vapor deposition, and hydride vapor phase epitaxy were characterized for comparison between the different methods of growth. The deep level energies, capture cross sections, and concentrations were determined for each using deep level transient spectroscopy on Schottky diodes from 80 K to 700 K, to characterize traps up to ~1.2 eV. The capture kinetics and bias dependence were also measured for the main traps in each, in order to determine if they are related to threading dislocations, and if they are donor-type traps. Several traps were detected in samples from each growth method. The field dependence and the capture kinetics were not the same for peaks appearing in the same temperature in deep level spectra, associated with different growth method. Traps in HVPE GaN at 0.212 eV and 0.612 eV uniquely showed field dependence indicating singly charged donors. Overall, the thick hydride vapor phase epitaxy GaN samples showed the lowest concentration of traps.

The junction temperature of red (AlGaInP), green (GaInN), blue (GaInN), and ultraviolet (GaInN) light-emitting diodes (LEDs) is measured using the temperature coefficients of the diode forward voltage and of the emission-peak energy. The junction temperature increases linearly with DC current as the current is increased from 10 mA to 100 mA. For comparison, the emission-peak-shift method is also used to measure the junction temperature. The emission-peak-shift method is in good agreement with the forward-voltage method. The carrier temperature is measured by the high-energy-slope method, which is found to be much higher than the lattice temperature at the junction. Analysis of the experimental methods reveals that the forward-voltage method is the most sensitive and its accuracy is estimated to be ± 3°C. The peak position of the spectra is influenced by alloy broadening, polarization, and quantum confined Stark effect thereby limiting the accuracy of the emission-peak-shift method to ±15°C. A detailed analysis of the temperature dependence of a tri-chromatic white LED source (consisting of three types of LEDs) is performed. The analysis reveals that the chromaticity point shifts towards the blue, the color-rendering index (CRI) decreases, the color temperature increases, and the luminous efficacy decreases as the junction temperature increases. A high CRI > 80 can be maintained, by adjusting the LED power so that the chromaticity point is conserved.

The near-ultraviolet (nUV) white LED approach is analogous to three-color fluorescent lamp technology, which is based on the conversion of nUV radiation to visible light via the photoluminescence process in phosphor materials. The nUV light is not included in the white light generation from nUV-based white LED devices. This technology can thus provide a higher quality of white light than the blue and YAG method. A typical device demonstrates white luminescence with Tc=3,700 K, Ra > 93, K > 40 lm/W and chromaticity (x, y) = (0.39, 0.39), respectively. The orange, yellow, green and blue OYGB) or orange, yellow, red, green and blue (OYRGB) device shows a luminescence spectrum broader than of an RGB white LED and a better color rendering index. Such superior luminous characteristics could be useful for the application of several kinds of endoscope. We have shown the excellent pictures of digestive organs in a stomach of a dog due to the strong green component and high Ra.

Recently, the applications of light emitting diodes (LED) in illumination are up roaring, and the solutions with high color rendition under various color temperature are in urgent need. Therefore, an effective method for improving lighting properties is very critical and necessary. To study new material of phosphor to combine with the LED chip or new LED structure to emit different spectrum could improve them. In addition, to combine with different kind of LEDs could achieve the same purpose and even with much lower cost. In this work, it has developed such an optimization method for a common white LED combined with a color LED. This method allows for finding the optimal mixing ratio of luminance of both LEDs in order to obtain the best CRI for illumination. By combining with a specific color LED, a white LED whose color temperature is approximately 6500K and CRI is 70, can be improved its color rendering index better than 80 and its correlated color temperature down to near 3000K simultaneously. Moreover, it could produce one which is more close to the warm-white lighting, and it is possible to substitute for the incandescent lamp. Thus, the mixing light is more suitable for illumination in housing applications. In the same way, with more LEDs in combination, CRI could be enhanced much better.

Highly luminous polymers have been successfully prepared by copolymerization of various arylene vinylene units. Among the copolymers synthesized, poly(phenylene vinylene) (PPV) and its derivatives have been studied most intensively because they show fairly strong fluorescence and have a wide structural variety. In this paper, light-emitting polymers and traditional organic LED technology are briefly reviewed at first. A novel and unique point source based on the organic LED fabricated on an optical fiber is then proposed. As proof of concept, some preliminary experimental results are shown. Two light-emitting materials needed for our experiments are synthesized. It is illustrated that the unique point OLED source is feasible, which can lead to many practical applications, such as biomedical imaging.

Non-fully conjugated 6F-PBO graft copolymers of poly(A_m-co-B_(1-m)) with poly[2,2-(m-2-hydroxyl-phenylene)- 4-4’-hexafluoroisopropane-bibenzoxazoles] (A_m) and poly[2,2-(1,4-didecylhydroxyl-phenylene)-4-4’-hexafluoroisopro-pane-bibenzoxazoles] (B_(1-m)) were synthesized with molar fraction m from 0, 0.25, 0.5, 0.75 to 1. The 6F-PBO random copolymers had identical benzoxazole with trifluoromethyl-ethyl as the backbone and pendants of mono-hydroxl (6F-PBO-OH, A_m) and/or di-decyloxyl (6F-PBO-di(OC₁₀H₂₁), B_(1-m)) on their phenylene rings. The copolymers were investigated by ultraviolet-visible (UV-Vis) light absorption covering 185 nm to 800 nm and photoluminescence (PL) emission excited at 363 nm. The PL exhibited a chromatic tuning range from green to white emission as m decreased. Light emitting devices (LEDs) of Al/copolymer/indium-tin-oxide were fabricated. A threshold voltage of 6 V was achieved for all 6F-PBO copolymer LEDs. The photometric coordinates of their electroluminescence (EL) emission were from (0.25, 0.53) to (0.24, 0.31) covering a visible range including white light emission. Another non-fully conjugated coil-like copolymer of poly(A_m-co-C_(1-m)) with 6F-PBO-OH (A_m) and poly[2,2’-(2-hydroxy-o-phenylene)- 5,5’-bibenzimidazole] (Pbi-OH, C_(1-m)) were synthesized and fabricated into LEDs. UV-Vis showed superposition of individual absorption from the 6F-PBO-OH and the Pbi-OH components. The PL and the EL had a red shift with increasing Pbi-OH content seemed to suggest that Pbi-OH was more charge delocalized than 6F-PBO-OH. PL and EL showed a green emission which was not tunable according to m. The threshold voltages were about at 2 ~ 3 V. Composites of poly(A_m-co-C_(1-m)) and multi-wall carbon nanotube (MWNT) were in situ synthesized for dispersion of the nanotubes. Few MWNT aggregations were observed via scanning electron microscope. The threshold voltages were 2 ~ 5 V. There was a red shift with MWNT addition in the PL and the EL but remained a green emission. Thus, up to 2 wt.% of MWNT was inconsequential on the PL and the EL emissions of the copolymer poly(A_m-co-C_(1-m)).

The inkjet printed micro-rings is formed by pinned contact line which pulls the ink solution outwards to compensate solvent evaporation. This phenomena is called “coffee drop effect,” which causes uniformity problem in the inkjet printing. Our attempt is to exploit this effect for fabrication of high resolution polymer LEDs by increasing the number of pixels per unit length. Here we present fabrication of high resolution (> 200 pixels per inch) polymer LEDs using inkjet printing and reactive ion etching. We also show possible way of further increasing the resolution of polymer LEDs by modifying the inkjet printing dispensing parameters.

We report on half-Watt level single spatial mode superluminescent laser diode at 1335 nm. Output optical power in excess of 500 mW from a single facet of angle-striped waveguide was realized at 10°C of heatsink temperature with peak electro-optical efficiency of 28%. To our knowledge this is the highest optical power and electro-optic conversion efficiency in a SLED device reported so far in the literature. Further optimization could lead to revolutionary result: 1) the creation of a high power optical device (SLED) with electro-optical efficiencies approaching and/or exceeding that of Fabry-Perot lasers (counting both facet outputs) with absolute optical power levels compared to that of Fabry-Perot lasers, 2) Electro-optical efficiencies approaching internal quantum efficiencies which could well exceed the 70-80% range observed in present commercial semiconductor laser and light-emitting structures.

We have remarkably improved the Metal Bonding (MB) AlGaInP LED luminous efficiency in the dominant wavelength range form 570 nm to 630 nm. Micro Shaping technology is fabricated on the Surface of LED Chip in order to enhance the extraction efficiency. As a result, the luminous efficiency of the new micro shaping Structure AlGaInP LED can achieve 80 lm/w, at 615 nm dominant wavelength under 20mA injection current. The luminous efficiency increased up to 50% than report valve before.

The growth, fabrication, and device characterization of the light-emitting diodes based on InP quantum-dot within a GaP matrix and on a GaP(100) substrate are described and discussed. The diode structures are grown using gas-source molecular beam epitaxy. Electroluminescence has been measured under a variety of bias conditions and temperatures. A green emission line at about 550 nm appears to result from carrier recombination in the strained InP wetting layer. Carrier recombination in the InP quantum dots results in red emission at about 720 nm.

The use of Germanium as an alternative substrate for the growth of AlInGaP LEDs provides several technical advantages such as lower substrate costs and the possibility of fabricating As-free AlInGaP devices. The LED layer structures are grown in a multiwafer MOVPE reactor on 4 inch Ge substrates. The growth conditions, such as temperature and substrate orientation, influence the LED external efficiency and its degradation behavior. In particular, it is found that during growth Ge is incorporated into the layers, which strongly affects the LED efficiency. Moreover a defect annealing occurs during regular operation resulting in an increased efficiency. Electrical characterization as well as deep level transient spectroscopy are performed in order to characterize the nonradiative recombination centers. In addition a quantitative analysis of the external quantum efficiency, before and after degradation, is carried out and the relative change in the nonradiative recombination rate is evaluated.

Electrical operation of III-Nitride light emitting diodes (LEDs) with photonic crystal structures is demonstrated. Employing photonic crystal structures in III-Nitride LEDs is a method to increase light extraction efficiency and directionality. The photonic crystal is a triangular lattice formed by dry etching into the III-Nitride LED. A range of lattice constants is considered (a ~ 270 - 340nm). The III-Nitride LED layers include a tunnel junction providing good lateral current spreading without a semi-absorbing metal current spreader as is typically done in conventional III-Nitride LEDs. These photonic crystal III-Nitride LED structures are unique because they allow for carrier recombination and light generation proximal to the photonic crystal (light extraction area) yet displaced from the absorbing metal contact. The photonic crystal Bragg scatters what would have otherwise been guided modes out of the LED, increasing the extraction efficiency. The far-field light radiation patterns are heavily modified compared to the typical III-Nitride LED’s Lambertian output. The photonic crystal affects the light propagation out of the LED surface, and the radiation pattern changes with lattice size. LEDs with photonic crystals are compared to similar III-Nitride LEDs without the photonic crystal in terms of extraction, directionality, and emission spectra.

Creation of patterned, efficient, and saturated color hybrid organic/inorganic quantum dot light emitting devices (QD-LEDs) is dependent on development of integrated fabrication and patterning methods for the QD layer. We show that micro-contact printing can be applied to QD deposition, generating micron-scale pattern definition, needed in pixilated-display applications. We demonstrate saturated color QD-LEDs with external quantum efficiencies in excess of 1%. Combining this technique with the use of wide optical band gap host materials, and a new synthetic route for the creation of blue emitting (CdS)ZnS nanocrystals, it is now possible to fabricate QD-LEDs with saturated color emission in the red, green and blue regions of the spectrum.

Quantum-well and quantum-dot superluminescent diodes operating at 1.55 μm at high power have been developed. An optical output power of more than 600 mW in pulsed mode was produced from the device having 8 identical quantum-wells at room temperature, corresponding to the wall-plug efficiency of 28%. A continuous wave power was 26 mW (p-side up mounting) and the spectral modulation depth was 15% over the entire emission spectral width of 25 nm. For a device with 5 non-identical quantum-wells, a 255-nm spectral width centered at 1.55 μm was achieved. For a device with 5 closely identical layers of quantum-dots the gain medium exhibited a spectral width of 181 nm around 1.55 μm.

Large-area (1000×1000 μm²) p-side down InGaN light-emitting diodes (LEDs) have been fabricated by laser lift-off (LLO) technique. The p-side down LEDs with different geometric patterns of n-electrode were fabricated to investigate electrode pattern-dependent optical characteristics. Current crowding effect was first observed in in the p-side down InGaN LLO-LEDs. The LEDs with well designed n-electrode shows a uniform distribution of light-emitting pattern and higher out put power due to uniform current spreading and minimization of thermal effect. The output power saturation induced by current crowding in the LEDs with simplest geometric n-electrode was demonstrated. In absent of transparent contact layer for current spreading, the n-electrode pattern has remarkable influence on the current distribution and consequently the light output power of the large-area p-side down LEDs.

To investigate the high performance light source for high-speed plastic optical fiber (POF) communication application is important as high-speed short distance communication for the home networks becomes popular. It is straightforward to reduce the size of RCLEDs to increase the small-signal modulation bandwidth (f_-3dB). But reduce the size of RCLEDs not only reduce the output power but also decrease lifetime because higher current density flowed through active region. In this paper, we improve. f_-3dB of RCLEDs with the aperture of 84μm by reducing the number of quantum wells (QWs) in active region. We found the speed of RCLED inverse proportional to the number of QWs. By reducing the number of QWs to one, the device with standard aperture size exhibits high f_-3dB as 235MHz at bias current of 20mA without sacrificing the other performance like maximum output power, high temperature performance, etc. These devices can transmit data rate as high as 500Mb/sec through graded-index POF over 50 meters. Beyond 1Gbits/sec, we have investigated red VCSELs as suitable high-speed light sources. The structure of red VCSELs is similar to RCLEDs except more pairs of DBR yield high reflectivity. Our red VCSEL can have output power as high as 1.5mW at 5mA and transmission data rate up to 2.5Gbits/sec.

A sensor based on selective optical absorption allows monitoring of hazardous engine exhaust emissions such as gaseous hydrocarbons and carbon monoxide. The IR components presented here offer the potential to develop a compact, fast and selective sensor reaching the technical and cost requirements for on-board automotive applications. Optical gas monitoring requires light sources above 3 μm since most of the gas species have their fundamental absorption peaks between 3 and 6 μm. We report here on resonant microcavity light sources emitting at room temperature between 3 and 5 μm. The emitter combines a Cd_xHg_1-xTe light emitting heterostructure and two dielectric multilayered mirrors. It is optically pumped by a commercial III-V laser diode. The principle of the resonant microcavity emitter allows tailoring of the emission wavelength and the line width to fit the absorption band of a specific gas, ensuring a very good selectivity between species. Moreover, this kind of emitter allows fast modulation enabling high detectivity and short response time. We report performances of light sources in the range 3 - 5 μm allowing the detection of hydrocarbons and carbon monoxide. Association of emitters peaking at different characteristic wavelengths with a single broad band detector allows designing of an optical sensor for several gas species. Sensitivity and time response issues have been characterized: detection of less than 50 ppm of CH₄ on a 15 cm path has been demonstrated on synthetic gas; analysis of exhaust gases from a vehicle has allowed the resolution of a cylinder time. This optical sensor offers the potential of various on-board automotive applications.

For non-destructive evaluation of sugar content in fruits, it has been proposed new measurement technique that uses three near infrared diode lasers. The reflectance R_i at the wavelength λ_i is defined by the ratio of two diffuse light powers reaching the two receiving fibers that were positioned at the different distance from a light emitting fiber. The value γ = ln (R_i/R_k)/ln (R_j/R_k) reaches the value that doesn't approximately depend on optical path length. As a simulation result of γ to a modeled apple, it was found that there are some combinations of three near-infrared wavelengths in which the correlation of γ and the sugar content of fruits raise. We actually evaluated the sugar content of apples by the use of three diode lasers 911nm, 936nm and 1055nm. As a result, it was confirmed that the measurement technique proposed here was usefulness.

Poly-p-phenylenebenzobisoxazole (PBO) and carbon nanotube (CNT) contain fully conjugated rodlike backbone entailing excellent mechanical properties, thermo-oxidative and solvent resistance. Rigid-rod PBO is commonly processed by dissolved in methanesulfonic acid or Lewis acid. A CNT of multi-wall carbon nanotube (MWNT) was dissolved in a Lewis acid solution of PBO for dispersion of nanotube, and then spun for thin film. MWNT concentration in the films was from zero up to 5 wt.%. Compared to that of pure PBO film, all PBO/MWNT composite films retained same but enhanced UV-Vis absorption peaks showing that MWNT and PBO do not have overlapping electron orbitals affecting their energy gaps. The composite films were excited at 325 nm using a He-Cd laser for photoluminescence (PL) response. All PL spectra had maximum wavelength peak at 540 nm indicative of yellow-green light emission. In the case of light emitting diodes, MWNT doped PBO would decrease threshold voltage for about 2 V and increase device emission current up two orders of magnitude than those without MWNT. This required a larger bias voltage leading to a shorter device lifetime.

The unique feature of color variability in light emitting diode (LED) sources made of red, green, and blue LEDs (RGB-LEDs) allows the user to select the desired color point of the lamp. The highest color uniformity is obtained using LED clusters with high-density packaging. However, packaging density of LED arrays is limited by cost, available space, and particularly by thermal problems. This paper presents an investigation of the effects on color uniformity of illumination due to different cluster configurations and packaging density of RGB-LEDs. We present a photometric analysis and experimental results that show the performance that can be achieved with a number of different cluster configurations of LEDs.

In this work, we investigate the thermal stability and surface morphology of Ti metal contact on unintentionally doped n-type aluminum gallium nitride (AlGaN). Different annealing temperatures (400°C - 800°C) and durations (1 - 30 minutes) are investigated, as thermally stable metal-semiconductor contacts are essential for high quality devices. Cryogenic quenching after heat treatment is also performed to determine the effects of this treatment on the characteristics of the contacts. Specific contact resistivity, ρ_c (SCR) determined using transmission line method (TLM) and scanning electron microscopy (SEM) measurements are carried out to the as-deposited, annealed (A), and annealed-and-cryogenically (A+C) treated contacts where the electrical behavior and the surface morphology of each of these conditions are compared. The result shows that cryogenic treatment is able to reduce the SCRs after annealing as most of the A+C samples exhibited lower SCR as compared to A samples. For relatively low annealing temperatures, i.e. 500°C and below, the difference of SCR values between A and A+C treated samples is insignificant, however, when the samples are thermally treated at higher temperatures, i.e. 600°C and above, substantial difference of the SCR values is observed between A and A+C treated samples. SEM images indicate that little difference of surface morphology is observed for all the samples regardless of the annealing temperatures, durations and treatments. The A+C sample annealed at 600°C for 2 minutes is found to be able to yield the lowest SCR in this study.