LEDs are fast becoming the light source of choice for automotive signal lamps. Before an LED can be used in an automotive signal lamp, however, a study must be made of the LEDs that are available for these applications. This is required to determine the correct LED for the particular lamp being designed. Significant parameters that must be evaluated to determine this LED's suitability for the selected application are the wavelength of the emitted light, the radiation pattern of the light and the usable Lumen output. One additional criteria that must also be considered is the ratio of the cost per Lumen of the targeted LED. These criteria are a few of the parameters that are measured in and out of the laboratory to determine if the LED being considered can be used in an exterior automotive signal lamp application. The selection process of the LED involves the characterization by accurately measuring the light output of the LED at a specified electrical current and thermal resistance. Once this has been completed and it has been established that the selected LED is a possible candidate for an automotive exterior signal lamp, the LED is subjected to durability performance testing. This involves testing the LED for its ability to meet the environmental, electrical and optical requirements. Since there are many manufacturers of LEDs it is necessary to choose an LED that has the highest quality along with the greatest amount of usable light output. It is this testing and characterization process that will enable us to select only the brightest and the highest quality LEDs for the automotive signal lamp. In addition each customer has their own unique set of requirements that the light source must meet. Therefore, the light source must meet or exceed the most stringent requirements that have been set forth.
This paper describes optics designed for the application of edge emitting LEDs (EELEDs) in retinal scanned displays (RSD). Directly modulated semiconductor light sources are scanned onto the retina to generate high-resolution displays. Red visible laser diodes produce high brightness monochrome scanned displays, or when combined with blue and green sources, generate full color displays. Green CW, room temperature visible laser diodes with any appreciable lifetime are currently unavailable for such applications. Blue and Green InGaN EELEDs have sufficient radiance and modulation speed for RSD applications, but lack the optical gain and stimulated emission required for laser diode optical power levels. Consequently, bright EELED-based displays require designs for maximizing the system optical collection efficiency. This paper describes anamorphic collection optics designed for optimizing the brightness and resolution of retinal-scanned microdisplay systems incorporating EELEDs. This design utilizes the laser-diode-like characteristics of EELEDs to maximize the collected useable optical power.
Very high external efficiencies have been reported from surface-textured thin-film light-emitting diodes. We have developed a novel process for the wafer-scale fabrication of surface-textured thin-film LEDs, avoiding the use of wet thermal oxidation and epitaxial lift-off. The LEDs consist of a double-mesa structure with a structured gold reflector serving simultaneously as a p-contact. The light emission occurs on the side of the original GaAs substrate, which is removed by selective etching after glueing the sample with the processed side onto a carrier substrate. The light emission of the devices is fully confined within the diameter of the LED itself. In comparison to our previously reported LEDs, the series resistance has been significantly reduced by the current injection through the mirror. 85(mu) 2m diameter GaAs/AlGaAs LEDs reach maximum external quantum efficiencies of 42% before and 51% after encapsulation. Encapsulated devices reach a maximum wallplug efficiency of 47% at a current of 3.5 mA. At an operating current of 20 mA, they emit 14 mW of light. As a first result on 650 nm GaInP/AlGaInP LEDs we obtained external quantum efficiencies of 28% for un-encapsulated devices with a diameter of 75micrometers . At a drive current of 8 mA the LEDs emit 3.4 mW of light.
A new type Bell Shaped Light Emitting Diode(BS-LED) with a circular 45 degree(s) corner reflector, deep side-wall and microlens is proposed and fabricated. Because the light of in-plane radiation in the active layer of Surface Emitting LED(SE-LED) can be extracted to emission surface by a circular 45 degree(s) corner reflector, the output power saturation phenomena that occur due to the in-plane superluminescence can be considerably improved. So, the light output power and the linearity of light-current curve can be improved efficiently by the corner reflector. The deeply etched side-wall can dramatically improve the external quantum efficiency of LED by side-wall reflection and photon recycling mechanism. Microlens is formed on light emission surface to improve the beam pattern. The fabricated BS-LED shows the dramatically improved external quantum efficiency up to about 8 times than that of conventional LED. The output power improvement is simulated as device design parameters. The BS-LED is characterized using spectrum, near-field pattern and light-current measurement.
We present results on efficient InGaAlP light-emitting diodes using lateral outcoupling taper. This concept is based on light generation in the very central area of a circularly symmetric structure and, after light propagation between two mirrors, outcoupling in a tapered mesa region. We have demonstrated the suitability of this concept on As-based Light-Emitting Diodes emitting at 980 nm. Since the idea is not limited to a certain material system, we fabricated InGaAlP-based LEDs emitting in the red wavelength regime. By adjusting the process flow to the new material system we were able to achieve external quantum efficiencies in the range of 13% for unencapsulated devices. Additionally we present a new concept combining the idea of outcoupling tapers with a waferscale soldering technique. First samples show external quantum efficiencies in the range of 11%.
The Grating -Assisted Light-Emitting Diode, an LED design for high brightness based on a resonant cavity containing 1D or 2D periodically corrugated layers (grating), is subject of this presentation. The diffractive properties of the wavelength scaled periodic grating integrated in one or both of the interfaces of the resonant-cavity, can redirect the laterally propagating resonant guided mode to an extractable direction. Because of the high power fraction in these guided modes, the use of such gratings can result in a higher extraction efficiency than a homogeneously layered RCLED, in which the trapped guided mode is a loss of power. In this scope, a generally applicable rigorous electromagnetic analysis based on the Coupled Wave Theory for diffractive gratings, has been developed to calculate the extraction efficiency of spontaneous emission in a periodically corrugated layer structure. This general model has been applied to a GA-RCLED emitting at 980 nm. The structure consisting of a hybrid bottom-emitting cavity with metallic grating and bottom DBR mirror, shows simulated efficiencies of over 40%. Electrically pumped devices have been processed. Experimental data and simulation results, such as polarization selective emission, spectral behavior and efficiency are compared and discussed.
We report on the development of (AlGaIn)N quantum well LEDs covering the 380 to 430 nm wavelength range, which serve as the primary light source for tri-phosphor luminescence conversion white LEDs. Epitaxial layer growth was performed by low-pressure metal-organic chemical vapor deposition on sapphire substrates. Mesa LEDs were fabricated and either mounted in standard epoxy-based 5 mm radial LED packages or flip-chip bonded on ceramic submounts. Then, LED-chips with peak wavelengths matching the absorption spectrum of an appropriately chosen inorganic tri-phosphor blend, were used for the fabrication of single-chip tri-color luminescence conversion white LEDs. These de-vices allowed us to demonstrate the feasibility of the above concept for improved color rendering and tunability.
AlGaN-based highly efficient ultraviolet light emitting diodes (UV-LEDs) are reported. To enhance radiative recombination by suppressing the effect of the internal polarization field, we introduced an AlGaN quantum well as thin as 8 molecular layers, at which the optical transition is kept spatially direct. To improve the transparency and conductivity of p-type cladding layers, we introduced a short-period alloy superlattice. For stronger carrier confinement, high-Al-content carrier blocking layers are introduced. Further, to reduce nonradiative centers, we used high-quality bulk GaN substrate obtained by hydride vapor phase epitaxy (HVPE). The resultant UV-LED performance is intrinsically ideal. The maximum output power is 10mW at the injection current of 400 mA in spite of the optically absorptive GaN substrate, and the estimated internal quantum efficiency is over 80% at the wavelength of 352 nm. This wavelength has affords efficient excitation of fluorescent materials. We also demonstrate simultaneous and equivalent excitation of fluorescence material of three basal colors by this UV-LED.
The growth conditions for GaN/GaInN MQW structures have been studied in detail on AIX 2000 HT G3 Planetary Reactors. Major process variables, such as precursor supply, growth time and growth temperature have been varied. To describe the dependencies of MQW growth a second order polynomic model has been developed. The average prediction error within the model limits is 2.8 nm emission wavelength, in the range of 400 to 490 nm. The linear effects of the major growth parameters have been quantified. Additionally, statistically significant curvature factors have been identified as the product of growth time and temperature, the square of the TMIn molar supply rate and the square of the growth time. To increase the system throughput for mass production applications the reactor geometry has been scaled up to 11x2" in the AIX 2400 HT G3 configuration recently. Numerical simulation of thermal field and decomposition chemistry has been performed for the new reactor set up. From the simulation results initial values for the respective process parameters have been chosen. The numerical growth parameter model has been transferred to the larger configuration and is verified with experimental results. Employing the 11x2' Planetary Reactor configuration an excellent on wafer uniformity of better than 0.2% (1 nm) standard deviation at 487 nm average peak wavelength has been shown. Wafer-to-wafer reproducibility has been demonstrated with a variation of 1.4 nm standard deviation and run-to-run with a variation of 0.25 nm standard deviation at an average wavelength of 470 nm for a full load of 11 wafers. The uniformities obtained are compared with results from the 6x2" configuration.
For the fabrication of III-nitride LEDs, the surface treatment and passivation using (NH4)2Sx solution were investigated. Using x-ray photoelectron spectroscopy (XPS) analysis, we found that the original native oxide on the III-nitride surface was effectively removed by the NH4)2Sx solution. Furthermore, the Ga-S bonds and the occupation of nitrogen-related vacancies by the sulfur allowed a more stable and a lower surface state density. By using capacitance-voltage and photoluminescence measurements, we investigate the Schottky barrier height and surface state density of the (NH4)2Sx-treated III-nitride layers. The reduction of the surface state density is due to the formation of Ga-S bonds. To improve the ohmic performance, the preoxidation process was used before (NH4)2Sx treatment. The oxidation mechanism was investigated. The interfacial mechanism in ohmic metals contact to (NH4)2Sx- treated III-nitride layers was investigated. The function of the (NH4)2Sx treatment was analyzed.
A newly installed molecular beam epitaxy system has been used to grow GaN epifilms on c-sapphire. Initial efforts are on the nitridation and the low-temperature buffer layer growth. Considerable nitridation with a high V/III flux ratio results in pure near-bandgap (NBG) transition without the usual yellow emission in photoluminescence spectra. Spotty reflection high-energy electron diffraction patterns and field emission scanning electron microscopy show that the surface is rough. No or less nitridation with a lower V/III flux ratio gives a streaky reflection high-energy electron diffraction patterns and a smooth surface. Photoluminescence measurements yield a narrower NBG emission with certain yellow emission. The images of transmission electron microscopy ensure the thickness of the low temperature GaN buffer layer to be around 200 Angstroms, and the diffraction patterns indicate increasing better quality of the GaN epifilm away from the buffer layer and a single crystal near the surface. Both van der Pauw and capacitance-voltage measurements reveal that the undoped GaN epilayers are natively n-type. X-ray diffraction in (theta) /2(theta) mode is performed, which gives a sharp GaN (0002) peak with a full-width-at-half-maximum of 150 arcsec. Other details on the growth and characterizations are discussed.
The fabrication of CdS-nanoparticle light emitting diodes (LEDs) on Si and their properties at room temperature and variant temperatures are reported. Due to passivation of p- hydroxyl thiophenol group around nanoparticles, 86-meV spectral shift of free exciton transition at room temperature is observed. Controlled conditions for the preparation of CdS-nanoparticle LED such as heat treatment and/or with oxygen-rich environment are found to have significant influences on emission spectra. Radiative recombination of carriers trapped in oxygen-impurity level of 273 meV presents in samples prepared in oxygen-rich environment. Coalescence of nanoparticles into bulk form also occurs to contribute to increased magnitude of luminescence. Spectral behaviors of electroluminescence with varied temperature are studied.
The delays and power requirements associated with long metal lines for 70nm and beyond technologies make on-chip optical interconnects an attractive option. The semiconductor industry has not enthusiastically responded to on-chip optical interconnects due to the difficulties associated with fabrication of optical emitters on Silicon substrate. Most emitters being researched use materials other than Silicon technologies - not compatible with conventional Si fabrication process. Hence, an optical inter-connect incorporating Si light emitter that can be fabricated shows excellent promise. A reverse biased Si-based p-n junction is known to emit broadband visible and infrared light emission in the range of 400-900 nm. Due to indirect band-gap in Si, the speed of operation and power requirement of Si based light emitting diodes has been considered to be non feasible for VLSI implementation. In this paper, the speed of operation of a reverse biased pn junction has evaluated using a streak camera. Power requirement for optical interconnects has been calculated using the experimental value of quantum efficiency. Results from the experiments clearly show that Si-based diodes are capable of operating in minimum of 10's of GHz and will require power less than conventional global metal interconnects on an IC. These results clearly indicate that optical interconnects are a viable option for the future IC technologies.
Bluish white light emission was obtained from a phospher doped light-emitting diodes(LEDs) prepared from a poly(9- vinylcarbazole)(PVK) matrix and an inorganic phosphor as a dopant. We used PVK as an emitter and long afterglow phosphor based on Strontium Aluminate as a guest emitter. The turn-on voltage of the phospher doped PVK LEDs is about 6.5 V, which was lower than the undoped PVK with a voltage of 12V. By doping with inorganic phosphor, the maximum luminescence is about much higher than that of the pure PVK LEDs. External quantum efficiency of the doped PVK LEDs was improved by two times compared to that of pure PVK LEDs. An additional emission observed in electrophosphorescence of the doped PVK LEDs at 630 nm is tentatively assigned to energy transfer from PVK to phospher.