We are progressively approaching the physical limits of microcavity LEDs (MC-LEDs) for high brightness, high efficiency LEDs. They are promising high efficiency devices and they offer the very attractive prospect of full planar fabrication process. However, to compete with other high efficiency LED schemes, they need to approach or surpass the 50 % efficiency mark. We first explore the limits of planar MC-LEDs in both the GaAlInAsP and GaInAlN materials systems, and show that the single-step extraction limit is in the 40 % range at best, depending on the materials system used, with the largest part of the non-extracted light being emitted into guided modes. The waveguided light can itself be extracted by photon recycling, when the internal quantum efficiency is high. Otherwise, another extraction scheme for that light is provided by various photonic-crystal-assisted extraction schemes. Simple photonic crystals (PCs) appear to lack the omnidirectional extraction properties required. However, more rotation-invariant PCs like Archimedean tilings allow to obtain such extraction with added efficiencies already in the 10% range. We discuss the further improvements to such structures.

Optical cavity effects have a significant influence on the extraction efficiency of InGaN/GaN quantum-well-heterostructure flip-chip light-emitting diodes (FCLEDs). Light emitted from the quantum well (QW) self-interferes due to reflection from a closely placed reflective metallic mirror. These interference patterns couple into the escape cone and cause significant changes in the extraction efficiency as the distance between the QW and the metallic mirror varies. In addition, the radiative lifetime of the QW also changes as a function of the distance between the QW and the mirror surface. Experimental results from packaged FCLEDs, supported by optical modeling, show that a QW placed at a neighboring position corresponding to a minimum in overall light extraction. Furthermore, the optical model and experimental data are used to estimate the absolute internal quantum efficiency.

We report on the fabrication and characterization of Si/SiO₂ Fabry-Perot microcavities. These structures are used to enhance the external quantum emission along the cavity axis and the spectral purity of emission from Rare earth doped and undoped SiOx (x <= 2)films that are used as active media to fabricate a Si based RCLED (Resonant Cavity Light emitting Devices). These structures are fabricated by chemical vapour deposition on a silicon substrate. The microcavities are tuned at different wavelengths: 540nm, 980nm, 1540nm, 780nm and 850nm (characteristic emission wavelength respectively for Tb, Yb and Er and Silicon Rich Oxide (SRO)). The reflectivity of the microcavities is of 97% and the factor quality ranges from 50 (for the cavity tuned at 540nm) to 95 (for the cavities tuned at 980nm and 1540nm) and 150 (for the cavity tuned at 780nm and 850 nm). These cavities have been characterized by TEM analysis to evaluate films uniformity, thickness and densification after annealing process for temperature ranging from 800° to 1100°C. The reflectivity and photoluminescence spectra show resonant wavelengths in agreement with the calculated values. A new structure to electrically pump the active media has been designed. The electrical properties of the active media have been analysed. An enhancement of the photoluminescence signal of twenty times have been achieved for the selected emission wavelength.

Future in-house Multimedia networks, based on the IEEE 1394b standards, require low cost and robust optical transmission system in the range of 100 meter. In this paper, we presented the state of the art 650 nm micro-cavity light emitting diodes (RCLEDs) for such application. We had made RCLEDs with diameters of the emission window of 84, 60, 40μm for different requirements. Because of excellent epitaxy quality and structure design. Our RCLEDs perform record high power and efficiency. With expoxy encapsulated, the 84μm devices give an efficiency of 12% and yield more than 3.5mW at operation current 20mA. Our 40μm devices exhibit high small-signal modulation-bandwidths (f_-3D) as 310MHz at bias current of 20mA. The output power of 40μm devices is still as high as 1.5mW, which is suitable for IEEE 1394b s400 standard. On the other hand, we had developed metal bonding RCLEDs (MBRCLEDs) to improve the high temperature performance of RCLEDs. By proper design the structure and process, the MBRCLEDs can have very low power decay as 0.6dB from 20°C to 100°C.

In Thinfilm LEDs, the substrate absorption of the generated light is avoided by a metal reflector between the light emitting layer and the substrate. The light extraction can be further enhanced by buried microreflectors or surface texturing. We demonstrate that the combination of these technologies gives prospects equal or superior to all other known approaches in terms of luminous efficiency and luminance. At a peak wavelength of 617 nm, we have obtained a luminous efficiency of 95.7 lm/W at 20 mA. We further analyze the internal and light extration efficiencies of our LEDs using raytracing simulations as well as a theoretical model for the internal efficiency. This analysis shows quantitatively that the efficient light extraction from InGaAlP thinfilm LEDs becomes more and more difficult when approaching shorter wavelengths.

An electrically conductive omnidirectional reflector (ODR) is demonstrated as p-type ohmic contact for an AlGaInP light-emitting diode (LED). The ODR comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The SiO₂ dielectric layer, located between a GaP and a silver layer, is perforated by an array of AuZn micro-contacts thus enabling electrical conductivity. It is shown that the ODR-LED has a significantly higher light-extraction efficiency as compared to LEDs employing distributed Bragg reflectors (DBRs). For devices emitting in the red wavelength range, external quantum efficiencies of 18 % and 11 % are obtained for ODR- and DBR-LEDs, respectively. The performance of the ODR-LED can be further increased by replacing the SiO2 dielectric with materials having a refractive index << 1.45. Performance characteristics of such powerful reflectors will be presented.

A high brightness AlGaInP LED with a high reflectivity metal reflector structure was proposed. The AlGaInP LED layers with metal reflector is bonded to the high thermal conductivity silicon substrate by using indium as a solder. Because the light that would otherwise be absorbed by the opaque GaAs substrate is reflected by the high reflectivity metal reflector, the brightness is significantly improved. The high current operating characteristics are also improved by replacing the GaAs substrate with silicon substrate. The luminous efficiency of the new structure AlGaInP LED can achieve more than 40 lm/W in the dominant wavelength range from 585nm to 625nm.

Recently near UV conversion LEDs with an excitation wavelength of around 400nm have attracted increasing interest due to their high efficiency and output power. Favorably is also the great variety of efficient phosphors available for near UV excitation. For the generation of saturated colors this method is very efficient especially for dominant wavelengths from 480 nm to 570 nm. Direct light emission from InGaN chips show a strong temperature and current dependency increasing with the emission wavelength and may cause disturbing color shifts in applications. For the wavelength range between 530 nm to 570 nm the physical limitations of the InGaN system are reached and the efficiency of the InGaAlP system is not yet satisfying. Conversion LED's based on near UV-light emitting chips provide solutions for both problems. But up to now a disadvantage of these LEDs is the residual near UV-light emission which could lead to severe eye damages over time. OSRAM OS developed a eye safe solution by avoiding the near UV-peak while maintaining the high luminous efficiency. A luminous efficiency of 28 lm/W for λ_dom of approx. 560 nm was demonstrated, a value more than ten times higher than the efficiency of green emitting InGaAlP diodes. For these LEDs no more restrictions because of eye safety regulations are expected.

Solid state light sources based on integrating commercial near-UV LED chips with encapsulated CdS quantum dots are demonstrated. Blue, blue-green, and white quantum dot LEDs were fabricated with luminous efficiencies of 9.8, 16.6, and 3.5 lm/W, respectively. These are the highest efficiencies reported for quantum dot LEDs. Quantum dots have advantages over conventional micron-sized phosphors for solid state lighting, such as strong absorption of near-UV to blue wavelengths, smaller Stokes shift, and a range of emission colors based on their size and surface chemistry. Alkylthiol-stabilized CdS quantum dots in tetrahydrofuran solvent with quantum yields (QYs) up to 70% were synthesized using room temperature metathesis reactions. A variety of emission colors and a white spectrum from monodisperse CdS quantum dots (D~2 nm) have been demonstrated. The white emission was obtained from the CdS quantum dots directly, by altering the surface chemistry. When incorporated into an epoxy, the high solution phase QY was preserved. In contrast to other approaches, the white LED contains monodisperse CdS quantum dots, rather than a blend of different-size blue, green, and red-emitting quantum dots. The concentration of CdS quantum dots in epoxy can be increased to absorb nearly all of the incident near-UV light of the LED.

In this paper, we overview the critical materials challenges in the development of AlGaN-based deep ultraviolet light emitting diodes (LEDs) and present our recent advances in the performance of LEDs in the 275-290 nm range. Our primary device design involves a flip-chip, bottom emitting, transparent AlGaN (Al = 47-60%) buffer layer structure with interdigitated contacts. To date, under direct current operation, we have demonstrated greater than 1 mW of output power at 290 nm with 1 mm x 1 mm LEDs, and greater than 0.5 mW output power from LEDs emitting at wavelengths as short at 276 nm. Electroluminescence spectra demonstrate both a main peak from quantum well emission as well as sub-bandgap emission originating from radiative recombination involving deep level states. The heterostructure designs that we have employed have greatly suppressed this deep level emission, resulting in deep level peak intensities that are 40-125X lower than the primary quantum well emission for different LED designs and applied current densities.

Fundamental electrical and optical properties of strained wurtzite InGaN/GaN-based quantum well light emitting diodes are calculated based on the Rashba-Sheka-Pikus Hamiltonian in the vicinity of the Gamma point. The theoretical results show an excellent correlation with experiments. A novel design of using AlInGaN as quantum barrier is proposed to realize efficient red emission, which is hard to achieve if GaN is used as barrier. To achieve high efficiency, the important factors relating to the oscillator strength are discussed in detail.

In this work, we investigate the absorption distribution in InGaN-on-sapphire based light-emitting diodes (LEDs). We observed by photothermal deflection spectroscopy (PDS) and transmission measurements that most of the absorption takes place in a thin layer close to the sapphire substrate. The lateral intensity distribution in the surrounding of LED emitters is determined by the photocurrent measurement method. Based on the observations by PDS and transmission, a model for the lateral light propagation in the LED-wafer containing also a thin, strong absorbing layer is presented. It is shown that interference of the mode profiles with the absorbing layer leads to different modal absorption which explains the non-exponential intensity distribution. We are able to estimate the optical thickness of the absorbing layer to be 75 nm. Furthermore, this layer can be identified as one of the major loss mechanism in InGaN-LEDs grown on sapphire substrate due to the large absorption coefficient which is effective at the emission wavelength.

The influence of the Mg doping profile on the electroluminescence efficiency of GaInN light emitting diodes (LED) has been investigated. The Mg doping profile is influenced by segregation as well as by diffusion during the growth. The diffusion of the Mg dopants into the active region can be controlled by the growth temperature of the Mg doped layers. An increase in Mg concentration close to the active region results in an improved hole injection and thus in a higher electroluminescence efficiency of the GaInN quantum wells. However an excessive spread of the Mg doping atoms towards the GaInN quantum well active region leads to nonradiative recombination and thus a lower output power of the LEDs. An LED test structure containing multiple quantum wells which differ in In content and emission wavelength was used to probe the spatial distribution of the radiative recombination of electrons and holes in the active region and to clarify the influence of Mg dopants in the active region on nonradiative recombination.

We report on the fabrication and performances of highly efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1540 nm electroluminescence at 300K with a 10% external quantum efficiency, comparable to that of standard light emitting diodes using III-V semiconductors. Emission at different wavelenghts has been achieved incorporating different rare earths (Ce, Tb, Yb, Pr) in the gate dielectric. RE excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light emitting MOS devices have been fabricated using Er-doped SRO (Silicon Rich Oxide) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 0.2%. In these devices different pumping mechanisms for the Er ions are simultaneously operating: Er can be excited by direct hot electron impact (like in stoichiometric oxide MOS) and by energy transfer from excited Si nanostructures, depending on the Si excess in the film. We propose a model to describe the electrical conduction mechanism in a Silicon Rich Oxide film. The electrical characteiristics can be fitted by a Schottky emission mechanism at low electrical fields and by a SCLC(Space Charge Limited Conduction) model for high elctrical fields. Data obtained from C-V measurements confirm the proposed model.

An optically pumped emitter for the mid-infrared region around 4 µm based on narrow gap semiconductors is demonstrated. The pumping takes place in the near-infrared around 1 μm and the radiation is converted by the narrow ap semiconductor into the MIR region as spontaneous emission. IV-VI lead chalcogenide-based compounds, especially PbSe and III-V InAsSb-based quantum well systems are applied for frequency conversion. These materials are grown by MBE and characterized mainly by photo luminescence spectroscopy. For a high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest.

Performance of white light LEDs has improved significantly over the past few years. White LEDs are typically created by incorporating a layer of phosphor over the GaN-based blue emitter. Heat at the p-n junction seems to be the major factor that influences light output degradation in these devices. In an earlier paper, the principal authors of this manuscript demonstrated that the junction temperature of white LEDs could be measured from the (W/B) ratio, where W represents the total radiant energy of the white LED spectrum, and B represents the radiant energy within the blue emission peak. In that earlier study, the concept was verified using commercially available 5-mm type white LEDs. The goal of the study presented here was to evaluate whether the (W/B) ratio could be used to estimate junction temperature of new high-flux white LEDs. The results show that (W/B) ratio is proportional to the junction temperature of the high-flux white LED; however, the proportionality constants are different for the different white LED types.

Critical requirements including high brightness, high color rendering, and high wall plug efficiency for the most demanding LED applications such as surgical illumination and industrial and dental sealant curing are difficult to meet with the limitations posed by commercially available LED packages. The importance of optimizing an illumination system from the system-level perspective is presented. It is necessary to integrate efficient die, electrical drive conditions, heat dissipation, LED out-coupling optics and auxiliary optics. It is not sufficient to collect the maximum amount of light from an LED package; the light must be captured into a minimum aperture while maintaining maximal brightness. Commercially available LED packages, including the recently available 1 and 5 Watt emitters, suffer by varying degrees, in their applicability to today's most demanding applications. An optimized LED package is described that outperforms commercially available packages. Specific medical, commercial and industrial LED applications are described that can meet many of the most demanding requirements with today’s technology.

As light-emitting diode (LED) power levels and chip sizes increase, thermal management and thermal stresses, which affect performance, power conversion efficiency nad lifetime, are becoming increasingly serious problems. Traditional materials have serious deficiencies in meeting requirements for thermal management and minimization of thermal stresses in high-brightness (HB) LED packaging. Copper, the standard material for applications requiring high thermal conductivity, has a coefficient of thermal expansion (CTE) that is much larger than those of ceramics and semiconductor materials, giving rise to thermal stresses when packages are subjected to thermal excursions. Aluminum has a larger CTE than copper. Traditional materials with low CTEs have thermal conductivites that are little or no better than that of aluminum. There are an increasing number of new packaging materials with low, tailorable CTEs and thermal conductivities up to four times those of copper that overcome thise limitations. The ability to tailor material CTE has been used to solve critical warping problems in manufacturing, increasing yield from 5% to over 99%. Advanced materials fall into six categories: monolithic carbonaceous materials, metal matrix compsites, carbon/carbon composites, ceramic matrix composites, polymer matrix composites, and advanced metallic alloys. This paper provides an overview of the state of the art of advanced packaging materials, including their key properties, state of maturity, cost and applications.

In this work, AlGaN layers were grown on sapphire by metal-organic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, with the intention to investigate the effect of varying Al/MO and V/III ratios on the Al incorporation into the AlGaN layers. The parameters Al/MO and V/III describe the proportions of source material inside the reactor. With the help of optical transmission measurements, characteristic cut-off wavelengths of the AlGaN layers were determined. These wavelengths were used to calculate the Al content x of the layers, leading to values between 26.6% and 52.1%. Using the two process parameters Al/MO and V/III as input and the Al content of the AlGaN layers as a response variable, the experimental results were further investigated with the help of the software STATGRAPHICS. An estimated response surface for the variable x was generated. It was found that the Al incorporation is only tunable within a wide range for high V/III ratios of about 900. For constant Al/MO ratios and varying V/III ratios, two different growth characteristics were observed at high and low Al/MO values. This behavior is ascribed to the superposition of two oppositional effects.

Using a least squares technique we fit the measured normal-incidence reflectivity spectra of resonant cavity light emitting diode (RCLED) structures, using six fitting parameters-the thicknesses of the AlAs and AlGaAs layers in the top and bottom Bragg stacks, the thicknesses of the cavity region, and the Al concentration in the AlGaAs components of both Bragg stacks. We find that the fitting procedure indicates growth errors in these thicknesses and in the Al concentrations, and, in particular, gives a best fit when the Al concentration in the AlGaAs component of the Bragg mirrors is typically 61±1% instead of the intended 50%. Furthermore, the fitting reveals that the spatial period of the upper Bragg stack is typically 4% less than that in the lower stack, in these growth runs, a finding which is confirmed by a detailed analysis of scanning electron microscopy images of cleaved pieces of the RCLED wafers. This fitting method provides a useful and non-destructive tool to determine as-grown thicknesses and compositions of complex multilayer heterostructures which are otherwise difficult to ascertain.

In 1993, a laser light guiding water jet was successfully developed at the Institute of Applied Optics (EPFL, Lausanne, Switzerland) and patented as Laser Microjet. The laser beam is focused into a nozzle from which a thin low-pressure water jet is emitted. The laser beam is injected in the water jet and guided in it by total internal reflection at the water/air interface similarly to a standard optical fiber. Normally a pulsed laser is used, so the continuous water jet is able to immediately cool the cut, reducing efficiently the heat-affected zone. The result is a very narrow, parallel, burr-free, clean cut, without detectable thermal damage. LED manufacturing is one example where thin layers need to be removed from well-defined regions on a wafer without damaging the neighboring structures. Compared with diamond saw cutting for which chipping and delaminating of the wafer cannot be avoided due to the strong shear forces; or compared with conventional laser cutting where low power irradiation of nearby functional structures occurs, the laser Microjet offers better edge quality and high precision. Compared to the main competitor, etching techniques combined with subsequent sawing of the substrate, the water jet guided laser is faster at similar edge quality.

Lasers are an important tool in the fabrication of photonic components and in particular their use in scribing for separating LED dies on sapphire substrates. This paper describes scribing and cutting of sapphire and GaN using UV lasers (355nm and 266nm harmonics of Nd:YVO₄ and 255nm harmonic of CVL). Scribing of sapphire at speed of 30mm/s have been achieved and cutting of sapphire of up to 700 microns thickness has been demonstrated.

We report the high-density light-emitting diodes (LEDs) using lateral junction for LED printer and other applications. Semi-insulating GaAs (311)A substrates were patterned to create (100) sidewall. GaAs/AlxGa1-xAs epilayers were grown on the patterned substrate using the amphoteric silicon as a dopant, which forms the lateral p-n junction. For the first time, high-density (2400 dots per inch) LED arrays were fabricated using the lateral junction with device width of 10.6 micron. Light emission spectrum shows a single peak at a wavelength of 813 nm with FWHM of 56 nm at room temperature. The same method can be used to fabricate LED arrays with higher device densities for applications in high resolution LED printers, displays and other applications.

Compact ultraviolet light sources are currently of high interest for applications in solid-state lighting, short-range communication, and bio-chemical detection. Our nitride-based light-emitting diode (LED) includes AlGaN quantum wells with an emission wavelength of approximately 340 nm. In this paper, we analyze internal device physics by three-dimensional (3D) numerical simulation. The simulation incorporated a 3D drift-diffusion model for the carrier transport, the quantum well (QW) energy band-structure including interface polarization charges, the local QW spontaneous emission spectrum, as well as 3D raytracing for photon extraction. The simulation results showed good agreement with measurements. Internal physical mechanisms such as current crowding, carrier leakage, and carrier recombination were investigated. Nanoscale effects exhibited a strong influence on the LED performance.