High-efficient light emitting diodes (LEDs) emitting red, amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 680 nm which emission energy was smaller than the band-gap energy of InN were fabricated mainly resulting from the piezoelectric field due to the strain. The localized energy states caused by In composition fluctuation in the InGaN active layer seem to be related to the high efficiency of the InGaN-based emitting devices in spite of having a large number of threading dislocations. InGaN single-quantum-well- structure blue LEDs were grown on epitaxially laterally overgrown GaN and sapphire substrates. The emission spectra showed the similar blue shift with increasing forward currents between both LEDs. The output power of both LEDs was almost the same, as high as 6 mW at a current of 20 mA. These results indicate that the In composition fluctuation is not caused by dislocations, the dislocations are not effective to reduce the efficiency of the emission, and that the dislocations from the leakage current pathway in InGaN.

The light emitting diode (LED) is the dominant type of compound semiconductor device in terms of the epitaxial area of material produced as well as the number of devices fabricated and sold. Recent breakthroughs have resulted in dramatic performance increases for visible LEDs. Very high performance devices are commercially available using the AlGaInP materials system for red, orange and yellow and the InGaN system for green and blue. External quantum efficiencies greater than 10% are available for most colors, with greater than 20% having been achieved in red to orange. Currently, the luminous performance of LEDs exceeds that of traditional incandescent lamps for colors from red to green. As a result of these advances, LEDs are becoming competitive in applications such as large area signs, traffic signals and automobile lighting. By mixing red, blue and green LEDs or by using phosphor-converted blue or ultraviolet devices, the creation of white light can be achieved, opening up additional applications. A review of the applications for high-brightness LED technology will also be presented.

In the extensive research dedicated recently to metal- organic chemical vapor deposition (MOCVD)-grown high- efficiency GaN LED device design, a significant effort has been made to increase the conductivity of p-GaN layers, while n-GaN layers received relatively little attention. We demonstrated, both experimentally and theoretically, that the resistivity of n-GaN layers has a profound effect on blue InGaN LED performance. Optimization of n-GaN epitaxial layers allows the achievement of device series resistances below 15 Ohms and forward voltages as low as 2.9 Volts at 20 mA. We have also shown that contactless measurements of sheet resistivity of the entire LED epitaxial structure closely correlate with the ohmic resistance of the GaN layer measured in the fabricated devices. This provides an excellent non-destructive characterization tool for n-GaN optimization. Insufficient n-GaN conductivity is shown to trigger a distinct degradation mechanism by initiating current crowding in a localized device area. InGaN LED lamps with optimized n-GaN layers had a high external quantum efficiency and a good long-term reliability.

We describe the growth and characteristics of GaN based light emitting diodes grown on Si(111) substrates. We show that the UV electroluminescence of such diodes can be used to generate fluorescence in organic color converters so that multicolored hybrid nitride-organic light emitting diodes that emit in the visible can be prepared.

As more advances are made in the performance of GaN-based devices, a trend toward the use of large scale MOCVD reactors for epitaxial growth of GaN-based device structures is clear. In this paper we describe the use of Emcore's SpectraBlue^TM reactor for large-scale manufacturing of Blue and Green LEDs. The high throughput growth of GaN based LEDs is demonstrated without compromising LED uniformity or overall performance. In-situ control of key parameters critical to the production of high quality LEDs, such as buffer layer growth is now feasible using in-situ reflectance spectroscopy. Film properties as well as LED device performance are discussed.

InGaAsN is a new semiconductor alloy system with the remarkable property that the inclusion of only 2% nitrogen reduces the bandgap by more than 30%. In order to help understand the physical origin of this extreme deviation from the typically observed nearly linear dependence of alloy properties on concentration, we have investigated the pressure dependence of the excited state energies using both experimental and theoretical methods. We report measurements of the low temperature photoluminescence energy of the material for pressures between ambient and 110 kbar. We describe a simple, density-functional-theory-based approach to calculating the pressure dependence of low lying excitation energies for low concentration alloys. The theoretically predicted pressure dependence of the bandgap is in excellent agreement with the experimental data. Based on the results of our calculations, we suggest an explanation for the strongly non-linear pressure dependence of the bandgap that, surprisingly, does not involve a nitrogen impurity band. Additionally, conduction-band mass measurements, measured by three different techniques, will be described and finally, the magnetoluminescence determined pressure coefficient for the conduction-band mass is measured.

We have used time-resolved photoluminescence (PL) to examine light-emitting diodes made of InGaN/GaN multiple quantum wells (MQWs) before the final stages of processing. The time-resolved photoluminescence from a dim MQW was quenched by nonradiative recombination centers. The PL kinetics from a bright MQW were not single exponential but stretched exponential, with the stretch parameter (beta) equals 0.59 +/- 0.05. The emission lifetime varied with energy, within error (beta) was independent of the emission energy. The stretched exponential kinetics are consistent with significant disorder in the material. Related results for an InGaN film and InGaN/GaN MQWs are also reported. We attribute the disorder to fluctuations of the local indium concentration.

Generalized variable angle spectroscopic ellipsometry (VASE) and Raman scattering have been employed to study the optical anisotropy of GaN/Sapphire structures. The GaN films were grown hydride vapor phase epitaxy and molecular beam epitaxy on both m-plane and c-plane sapphire ((alpha) -Al₂O₃) substrates, respectively. Anisotropic optical phonon structure of sapphire have been measured, based on which the optical axis of sapphire substrate has been determined. A 541 cm^-1 TO phonon of GaN grown on m-plane sapphire substrate has been discovered experimentally which is due the coupling of A₁ and E₁ TOs. Optical axis orientation of GaN film on m-sapphire has been fully determined by the anisotropic angular dependence of the coupled TO phonon. Off-diagonal elements A_pst and A_spt of transmission VASE (TVASE) are very sensitive parameters related to the optical anisotropy. The optical axis orientation of GaN on m-sapphire has also been accurately determined by TVASE at two special sample positions. The optical anisotropy due to GaN film and sapphire substrate has been successfully separated at 90 degree(s) samples position allowing to study the optical anisotropy of GaN film only.

We present the development of high-quality In_xGa_1-xP graded buffers on GaP substrates (In_xGa_1-xP/GaP) for use in epitaxial transparent-substrate light-emitting diodes. The evolution of microstructure and dislocation dynamics of these materials has been explored as a function of growth conditions. The primarily limiting factor in obtaining high-quality In_xGa_1-xP/GaP is a new defect microstructure that we call branch defects. Branch defects pin dislocations and result in dislocation pileups that cause an escalation in threading dislocation density with continued grading.

Several issues related to the organic electroluminescent devices are reviewed. Numerical simulation results are presented for the trap-charge limited conduction processes. Several experimental observations are explained based on our simulations. Current-voltage characteristics, band and charge profiles are obtained to elucidate the conduction mechanisms with large trap densities. Recombination strength dependence, doping effects, and the interplay among the free, discrete, and exponential trap charges in single- and double-carrier devices are discussed.

We report our progress to date in the materials and processing aspects of developing organic light emitting diode technology based on high molecular weight polymers. A portfolio of fluorene-related polymers are prepared through the coupling of 9,9-disubstituted 2,7-bis-1,3,2- dioxaborolanyl-fluorene with a variety of aromatic dibromides. In the case of fluorene homopolymers, the polyphenylene main chain provides the mechanical, electrical and electronic properties and the C-9 maintains coplanarity of the biphenylene unit and a site for property modification without altering effective conjugation. In the case of alternating polymers, the optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic unit into the system. These polymers are soluble in common organic solvents and are readily processed into uniformed films of high quality by spin casting. Unlike PPV and related materials, LED devices with fluorene polymers in a conventional configuration appear to have electrons as the majority carrier and their performance is markedly improved when modified with an appropriate polymeric hole transporting layer. Bright green light with a luminance of 10000 Cd/m² is achieved at a very low drive voltage of < 6 V attributable in part to the high hole mobility of fluorene-based polymers.

Organic semiconductors have been intensively studied over the past decades. The potential of this new class of materials for photonic and electronic device applications is demonstrated by successful fabrication of organic and organic-on-inorganic heterostructures for electroluminescent devices, photodetectors, and microwave diodes. The fabrication technology of organic semiconductor devices for photonic applications is discussed. In contrast to spin-on or dipping techniques for fabrication of polymeric films, organic compounds with low molecular weight are sublimated under ultra high vacuum conditions. The organic molecular beam deposition technology employed allows the reproducible growth of complex layer sequences with a defined thickness of various organic semiconductors in combination with dielectric films, different metallizations, and indium-tin- oxide layers.

The theory, structure, and current manufacturing technologies for InGaAlP high brightness light emitting diodes (HB-LED) emitting in the range of 650 to 585 nm are described in this paper. A state-of-the-art HB-LED MOCVD reactor designed for high volume manufacturing (42 - 2' or 16 - 3' wafers) is demonstrated. Data for thickness and compositional uniformity and reproducibility are presented showing the material quality and reactor stability that can currently be achieved. In addition, device data for InGaAlP HB-LEDs is reported, including brightness, forward voltage, and emission wavelength with excellent intra and inter wafer uniformity and run-to-run reproducibility.

We report on electroluminescence and photoluminescence studies of arsenic rich InAs_1-xSb_x heterostructure LED's for the MIR region. Single-quantum- well LED's have demonstrated 300 K of approximately 24 (mu) W and approximately 50 (mu) W and approximately 8 micrometers , respectively, with corresponding internal quantum efficiencies of 0.8% and 1.6%. We also demonstrate 4.2 micrometers , 300 K emission from strained-layer superlattice (SLS) LED's with AlSb electron confining barriers with output powers > 0.1 mW. In reverse bias, these SLS devices exhibit negative luminescence efficiencies of approximately 14% at 310 K.

This paper reports the temperature dependence of the magneto-optical properties of Zn_1-xMn_xSe samples prepared by molecular beam epitaxy method. In the magneto- optical spectra, there exist several Faraday rotation peaks. The peaks located at approximately 2.18, approximately 2.36 and 2.45 - 2.57 eV are attributed to Mn²⁺ d yields d* transitions. The peak located at approximately 2.7 eV is attributed to the interband transitions and higher order Mn²⁺ d yields d* transitions, which are blue-shifted with decreasing temperature. The positions of the rotation peaks induced by Mn²⁺ d yields d* transitions show weak temperature dependence.

Since several years light emitting diodes are in use to generate white light. Pixels with green, red and blue LED's are arranged to get any coordinate in the CIE--diagram with matched current for each diode. For instance Siemens Opto Semiconductor now OSRAM Opto Semiconductor offers multi chip LED's (LHGB T676) especially for the application above. A far better solution for producing white light represents luminescence conversion. The emitted light of blue diodes is used as a primary source for exciting organic or inorganic fluorescent. By conversion, `Stokes Shift', red, green, yellow and mixed colored light can be generated.

Microcavity light emitting diodes (MCLEDs) present several interesting features compared to conventional LEDs such as narrow linewidth, improved directionality and high efficiency. We report here on MCLEDs with a top emitting geometry. The MCLED layers were grown using molecular beam epitaxy on GaAs substrates. They consist of a 3-period Be- doped distributed Bragg reflector (DBR) centered at 950 nm wavelength, a cavity containing three InGaAs quantum wells and a 15-periods Si-doped DBR. Different values for the wavelength detuning between spontaneous emission line and Fabry-Perot cavity mode were explored, between -40 nm and +10 nm. Devices sizes ranged from 420 X 420 micrometers ² to 22 X 22 micrometers ². As expected from simulations, the higher efficiencies are obtained when the detuning is in the -20 to 0 nm range. The devices exhibit then up to 10% external quantum efficiency, measured for a 62 degree(s) collection half-angle. After correction for the surface shadowing due to the grid p-contact, the efficiency increases to 14% and is practically independent of device size.

Efficient, cheap, and simple, LEDs are used in many applications and make up the bulk of the opto-electronic component market. Due to the small critical angle at the semiconductor-air interface, relatively little light escapes per facet. The conventional route is to collect light from all six facets and redirect it, using external reflectors into a useful direction. While this increases external quantum efficiency it does little to increase brightness. In the last few years the microcavity approach has been used to persuade the light to leave by just one facet, thus increasing the brightness considerably. Although remarkable efficiencies have been achieved, microcavity LEDs (MCLEDs) have yet to surpass conventional LEDs. We present here a single mirror LED, grown by MBE, which falls between the conventional LED and the planar MCLED.

The increasing demand for sophisticated laser devices for high speed telecommunication systems, CATV, multimedia or printing markets requires the application of multiwafer MOVPE systems. To meet the targets of these markets, the Planetary Reactor^TM as well proven production tool was used to fabricate InGaAsP single and multilayer test structures with outstanding uniformity.

We studied enhancement and suppression of spontaneous emission in thin-film InGaAs/InP photonic crystals at room temperature. Angular resolved photoluminescence measurements were used to determine experimentally the band structure of conduction band of such a photonic crystal and overall enhancement of spontaneous emission. We demonstrated spontaneous emission enhancement in thin slab photonic crystals. It was shown that emission into the leaky conduction bands of the crystal has the same effect as cavity-enhanced spontaneous emission provided these bands are flat enough relative to the emission band of the material.

We present a non-resonant light emitting diode with a novel concept of light outcoupling. Light is generated in the center of a radially symmetric structure and propagates between two mirrors of a tapered region where outcoupling occurs. Principles of outcoupling are given using a simple ray tracing model. Different process routes are developed resulting in on-substrate as well as substratless devices. Not yet optimized devices show quantum efficiencies of 12% and 15%, respectively.

In this paper, we present measurements of the switch-on times and of the switch-off times of non-resonant cavity light-emitting diodes, compared to those of conventional reference diodes. From this comparison, we infer that the high quantum efficiency of NRC-LED's is not achieved by photon recycling, but purely by efficient extraction of generated photons. This is further corroborated by the good matching that is achieved between the measured switch-on times and theoretical predictions of the switch-on times. The latter are calculated with a model that includes only the electrical charging of the active layer and assumes that photon recycling does not occur. It is furthermore shown that the switch-on can be made faster by switching the diode between a non-zero low-state and the required high state. Doing so, an open eye diagram is achieved at 622 Mbit/s for a NRC-LED having an external quantum efficiency of 17%.

Non-resonant cavity light-emitting diodes (NRC-LED's) are based on the combination of surface texturing and the application of a back mirror. With this concept, the extraction efficiency of LED's can be enhanced considerably. We fabricated NRC-LED's with a more sophisticated design employing an oxidized current aperture, which is similar to the commonly used for vertical-cavity surface-emitting lasers. In our NRC-LED's, it confines the injection current to the center of the device in order to reduce light generation below the top contact. We analyze the impact of the aperture size on the device performance, and we show that both the maximum efficiency and the injection current where it is reached are strongly dependent on the device size. Its correlation with the temperature in the active region and the current density is discussed. In addition, we demonstrate that a considerably fraction of the light can be extracted from lateral guided modes in the LED structure by extending the surface texturing beyond the device mesa. Devices fabricated by applying all of the above techniques result in record external quantum efficiencies of 31%.

Planar Resonant Cavity LEDs (RCLEDs) are suitable light sources for parallel interchip interconnect links, due to their high efficiency, zero-threshold, low voltage, high reliability and high speed characteristics. The through- substrate emitting RCLEDs, optimized for Polymer Optical Fiber (POF) coupling, consist of an InGaAs quantum sandwiched between a metal mirror and a distributed Bragg reflector. The RCLEDs are arranged in 8 X 8 arrays with 250 um pitch. The arrays have been mounted onto glass carriers, and the coupling efficiency into POF, the far- field pattern and the modulation characteristics are measured. The overall quantum efficiency of the devices with 50 um diameter was found to be 13.4%, the QE into POF was 3.7%. The large-signal transient behavior of the devices has been investigated. Using a high-speed pulse source, nanosecond rise and fall times have been measured. Wide open eye diagrams at 1 Gbit/s were obtained using voltage pulse drivers. These data were compared to theoretical results based on a non-linear rate equations model.

We report on resonant cavity light-emitting diodes, operating at 660, 880, and 1300 nm wavelengths. Some of the characteristic features of these devices will be discussed. The devices were grown by all-solid-source molecular beam epitaxy (SSMBE). The results provide clear-cut evidence that SSMBE is a viable method to growth of phosphorous containing semiconductors.

We propose LEDs based on geometrically deformed chips of which the horizontal cross-section is either rhomboidal or triangular and, in addition, side walls may also be slanted. In such deformed chips, the photon trajectory changes with each reflection off the wall and, as a result, the continued total internal reflections as observed in conventional rectangular cubic chips are suppressed.

We present the results of an ongoing investigation into the use of directly modulated light emitting diodes and laser diodes as light sources for a retinal scanning display. Devices studied include commercially available single-mode 635 nm laser diodes, custom fabricated red edge-emitting LEDs, custom fabricated gallium nitride-based green and blue edge-emitting LEDs, and commercial LEDs. The diodes were characterized in both DC (light vs. current/voltage), and high frequency (approximately 1 ns rise/fall pulse) regimes, and measured for luminance using a CCD camera in conjunction with a variable image aperture, variable NA light collection system. Results show that edge-emitting LEDs and laser diodes are each suitable for a particular range of display requirements.