GaN homojunction and InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) were fabricated and characterized. The blue LED has a typical operating voltage of 3.6 V at 20 mS. Temperature dependence of the emission characteristics of the GaN-based LEDs was studied from 25 degrees C to 130 degrees C. The emission intensity of the InGaN/GaN SQW LED decays exponentially with the increase of temperature. The temperature coefficient L_c is 2.5 X 10^-2/degrees C. The emission wavelength of the InGaN/GaN SQW LED was found to be relatively independent of the LED operation temperature while the UV emission of the GaN homojunction LED has a red-shift with the increase of temperature. The temperature coefficient (alpha) of the bandgap energy of Si-doped n-type GaN derived from the EL measurement is 8.5 X 10^-4/K. The low temperature coefficient of emission wavelength of the InGaN/GaN SQW LED indicates that the recombination processes involves localized states. The localized states are attributed to excitons localized at the potential minima in the quantum well due to In content fluctuation.

InGaN double heterostructure light emitting diodes (DH-LEDs) were fabricated on hydride vapor phase epitaxy (HVPE) GaN- on-sapphire substrates. These substrates consisted of a thick HVPE GaN layer grown directly on sapphire and eliminated the need for the growth of a low-temperature buffer layer for GaN epitaxy on sapphire. Homojunction and DH-LEDs have been fabricated with various composition InGaN active regions resulting in strong electroluminescence in the blue, green, and yellow portion of the visible spectra. These devices had turn-on voltages as low as 3.6 volts.

Our studies of device lifetime and the main degradation mechanisms in Nichia blue LEDs date back to Spring 1994. Following the initial studies ofrapid failures under high current electrical pulses, where metal migration was identified as the cause of degradation, we have placed a number ofNichia NLPB-500 LEDs on a series oflife tests. The first test ran for 1000 hours under normal operating conditions (20 mA at 23 °C). As no noticeable degradation was observed, the second room temperature test was performed with the same devices but with a range of currents between 20 and 70 mA. After 1600 hours, some degradation in output intensity was observed in devices driven at 60 and 70 mA, but it was still less than 20%. The subsequent tests included stepping up the temperature by 10 °C in 500 h intervals up to a fmal temperature of 95 °C using the same currents applied in the second test. This work reviews the failure analysis that was performed on the degraded devices and the degradation mechanisms that were identified.

We report on the growth of GaN, InGaN and GaN/InGaN/GaN pn- junctions grown on sapphire by RF-plasma assisted MBE. MBE allows us to grow high quality nitrides with growth rates around 1 micrometers /h at relatively low temperatures. Thereby p- type doping with Mg and the incorporation of In in InGaN are greatly facilitated. Device-typical n- and p-type doping levels yield room temperature mobilities of 220 cm²/Vs and 10 cm²/Vs, respectively. InGaN with In contents of more than 40 percent is readily achieved. LEDs fabricated from heterostructures with a 4 nm InGaN layer show bright blue or green electroluminescence depending on the In content. Various effects in the electroluminescence caused by fluctuations in the conduction and valence band will be discussed, the most striking one a reduction in linewidth with increasing temperature.

Motivated by the potential advantages that device-quality zinc blende nitrides would offer over their wurtzite counterparts for optoelectronic applications, the optical properties of state-of-the-art cubic GaN grown by molecular beam epitaxy on GaAs(001) are investigated in detail. In view of the high densities of structural defects being caused by the large lattice mismatch to the substrate, special attention is paid to the influence of nonradiative recombination processes on the room temperature band-edge luminescence. A detailed analysis of the temperature and excitation density dependence of the band-edge recombination allows to determine transition and activation energies.In conjunction with model calculations, the internal quantum efficiency and the non-radiative lifetimes are estimated. The decay times as well as the observed phenomenon of defect saturation are verified by time-resolved photoluminescence studies. Upon pulsed optical pumping, optical gain exceeding 100 cm^-1 at an excitation energy density of 20 (mu) Jcm^-2 is obtained at room temperature.

We report on successful growth of zincblende ((beta) ) (In,Al,Ga)N heterostructures on GaAs(001) by means of rf plasma assisted molecular beam epitaxy. The composition of the samples under investigation is analyzed by secondary ion mass spectroscopy and x-ray diffraction. Cross-sectional transmission electron microscopy is used for studying the microstructure and selected area diffraction for verifying the phase purity of the epilayers. The surface morphology is investigated by atomic force microscopy. Temperature dependent transmission, reflectance, and photoluminescence investigations allow the determination of the band gap energy of (beta) -In_xGa_1-xN. It is shown that by using (beta) -In_xGa_1-xN blue and green band-edge related emission is obtained with respectively, x equals 0.17 and x equals 0.4 in contrast to wurtzite In_xGa_1-xN where In contents of about x equals 0.25 and x equals 0.55 are required for achieving the respective colors.

The near-band gap and the 'yellow' optical transitions in n- type GaN grown by MOCVD have been studied by photoluminescence experiments. The excitation density ranged from 5 X 10^-6 W/cm^-2 to 50 W/cm^-2. The UV PL intensity increases linearly in the entire range of excitation density. The yellow PL intensity exhibits a linear dependence oat low excitation densities and a square-root dependence at high excitation densities. A theoretical model is developed describing the intensity of the two radiative transition between continuum states and one defect level deep in the band gap as a function of the excitation density, free carrier and defect concentrations. The calculated dependences of the two luminescence channels follow power laws with exponents of 1/2 and 1 depending on excitation density. These dependences are in very good agreement with experimental results. The measured intensity of the yellow luminescence does not saturate at high excitation densities. This rules out the possibility that the yellow PL could arise from a sequential transition via two deep levels in the gap. It is shown that the intensity modulation that frequently appears in the PL spectra is caused by a micro-cavity which is formed by the semiconductor-substrate and semiconductor-air interfaces. Finally, the dependence of the yellow luminescence intensity on n-type doping concentration indicates that the deep center causing the yellow luminescence is an acceptor-like level.

A review is presented of the electrical and optical properties of nitride based optoelectronic devices, in particular AlGaInN light emitting diodes and laser diode structures. The III-nitride films and devices were grown by organometallic vapor-phase epitaxy on c- and a-face sapphire substrates. We will discuss the structural properties of GaN. InGaN and AlGaN films, heterostructures and InGaN/GaN quantum wells using x-ray diffraction and cross-sectional transmission electron microscope and describe their electrical and optical properties characterized by Hall effect, photoluminescence, and electroluminescence measurements.

Ink-jet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting polymers are solution processable, IJP technology becomes an ideal method for printing polymer light-emitting diodes with high resolution. In this manuscript, we present the first successful demonstration of patterning the polymer electroluminescent devices using the IJP technology. Unfortunately due to the dot form printing by the IJP, the polymer film printed from an ink-jet printer consists of pin-holes. This makes it unsuitable for fabricating high quality polymer electronic devices, particularly for devices in the sandwich structure. In this paper, we submit a hybrid structure, which consists of an ink-jet printed layer in conjunction with another uniform spin coated polymer layer, as an alternative to the regular ink-jet printed structure. The uniform spin coated polymer layer, as an alternative to the regular ink-jet printed structure. The uniform layer serves as a buffer layer to seal the pin hoe.s and the IJP layer is the layer consisting of the desired pattern, for example the red-green-blue dots for a multicolor display. To demonstrate, we applied this hybrid technology to fabricate efficient and large area polymer light-emitting logos. The use of this concept represents a whole new technology of fabricating polymer electronic device with lateral patterning capability.

Small-molecule-based organic light emitting devices (OLEDs) have been shown to have the brightness, range of colors and operating lifetime to be useful in full-color flat panel display applications. Recent technological advances have created exciting new opportunities for OLEDs in flat panel display applications. These include the development of a transparent OLED device for vehicular windshields, architectural windows, and 'head up' displays; a novel vertically-stacked OLED pixel structure that provides full- color tunability, minimum pixel size, and maximum fill factor for high-resolution displays; and an ultra- lightweight, flexible OLED for use in conformable or foldable displays. In addition, we discuss other development issues for the fabrication of high-resolution, full-color OLED displays.

Very high efficiency GaAs light-emitting diodes are based on surface-textured thin film structures. The technique relies on surface texturing by 'natural lithography', where a monolayer of randomly positioned polystyrene spheres acts as a mask for etching a random diffraction grating. We present result of a systematic experimental study on the influence of the surface-texturing parameters on the efficiency of these LEDs. The study was performed on GaAs/AlGaAs structures optimized for photoluminescence and electroluminescence, respectively. It shows that the maximum enhancement of the light output occurs for spheres of 200 nm to 300 nm diameter, which must cover more than 50 percent of the surface. The optimum etching depth is approximately 160 nm. Using these conditions, an external quantum efficiency for MBE-grown GaAs light emitting diodes of 10 percent was achieved for a device of only 50 X 75 micrometers ² in size.

We describe uncooled mid-IR light emitting and negative luminescent diodes made form indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The application of these devices to gas sensing, improved thermal imagers and imager testing is discussed.

Fabrication of various types of diamond LEDs, their characteristics and the operation mechanisms are reported. All the devices are based on the double injection electroluminescence: charge carriers are injected into intrinsic (i) diamond from adjacent semiconducting p- and n- type areas. The i-area is activated with appropriate optical centers. In the first step a survey of the appropriate optical centers and their spectra is given. The devices are fabricated in the form of either planar or vertical p-i-n or p-i-p structures. It is shown that the electrical and optical properties of the devices are controlled by the type, concentration, and spatial distribution of the deep traps in the i-region. The selection of diamond substrates suitable for the fabrication of LEDs is an essential point and it can be performed with a microwave photoconductivity technique. Several fields of possible application of diamond LEDs are shown: high temperature LEDs, optical sensors of magnetic field, color switches, color indicators of temperature, the high temperature applications being the most promising application area. Perspectives of creation of diamond based laser diodes is shortly discussed.

Mid-IR LEDs are being developed for use in chemical sensor systems. As-rich, InAsSb heterostructures are particularly suited for optical emitters in the mid-IR region. We are investigating both InAsSb-InAs multiple quantum well (MQW) and InAsSb-InAsP strained layer superlattice (SLS) structures for use as the active region for light emitting diodes. The addition of phosphorus to the InAs barriers increase the light and heavy hole splitting and hence reduces non-radiative Auger recombination and provides for better electron and hole confinement int eh InAsSb quantum well. Low temperature photoluminescence (PL) emission from MQW structures is observed between 3.2 to 6.0 micrometers for InAsSb wells between 70 to 100 angstrom and antimony more fractions between 0.04 to 0.18. Room temperature PL has been observed to 6.4 micrometers in MQW structures. The additional confinement by InAsP barriers results in low temperature PL being observed over a narrower range for the similar well thicknesses with antimony mole fractions between 0.10 to 0.24. Room temperature photoluminescence was observed to 5.8 micrometers in SLS structures. The addition of a p-AlAsSb layer between the n-type active region and a p-GaAsSb contact layer improves electron confinement of the active region and increases output power by a factor of 4. Simple LED emitters have been fabricated which exhibit an average power at room temperature of > 100 (mu) W at 4.0 micrometers for SLS active regions. These LEDs have been sued to detect CO₂ concentrations down to 24 ppm in a first generation, non- cryogenic sensor system. We will report on the development of novel LED device designs that are expected to lead to further improvements in output power.

Research in erbium-doped silicon (Si:Er) is discussed in light of our effort to improve the luminescence performance of our LEDs and to demonstrate an integration scheme for a microphotonic clock distribution system. Excitation from Si:Er can occur int ow ways: (1) direct excitation of an Er ion by high energy electrons or (2) energy transfer from an injected electron-hole pair to an Er ion in the lattice. In an LED the first excitation mechanism corresponds to operation in reverse bias, and the latter corresponds to operation in forward bias. We have studied the forward bias case, and we use an energy pathway model to describe the excitation and de-excitation processes. The competing, nonradiative processes against excitation and spontaneous emission are discussed. Maximization of light output can be approached in three ways: (1) decreasing the number of nonradiative energy pathways, (2) enhancing the probability of the radiative pathway, or (3) simply increasing the concentration of active Er sties. We report specific methods that address these issues, and we discuss more device structures that can be used as emitters, optical waveguides, and optical switches in a fully integrated microphotonic system.

This paper reports the result of transmission experiments through 200 m of plastic optical fiber at 155 Mb/s and 622 Mb/s. Bit-error-rates of < 10^-9 has been obtained at received optical powers of -22 dBm at 155 Mb/s and -19 dBm at 622 Mb/s respectively. The measured power penalty due to model noise of < 1 dB is in agreement withthe calculated result.

The flexibility of diffractive optics can offer interesting solutions to a variety of sensing problems. Here, we consider two examples: (1) the determination of the color of LEDs and (2) correlation spectroscopy. In both cases, optical setups are matched to a specific task which results in efficient and impulse systems. This makes them interesting for sensing applications in process control and monitoring. Compact micro-optical systems result by using lithographic fabrication and integration of the free-space optical components on single substrates. We describe principles and basic applications of our concept.

High brightness visible light emitting didoes (LEDs) based on GaAlInP/GaAs are now in great demand. In order to meet the requirements for mass production with low unit cost, metalorganic chemical vapor deposition (MOCVD) is the primary deposition process for cost effective large area and multiple wafer growth of compound semiconductors. In this work low pressure MOCVD growth and non-destructive materials characterization on 100 mm wafer size epitaxial films of quaternary GaAlInP grown on GaAs substrates for LED applications is demonstrated. The ability to scale the deposition and fabrication process from the traditionally used 50 mm to 100 mm will be key in further reducing costs. MOCVD system design requires that growth to be laterally uniform, abruptly switchable, and robust against variations in process parameters that can be achieved so that production of high quality and high uniformity GaAlInP films are obtained. In parallel, to this effort there is the need to develop rapid while wafer and non-destructive mapping characterization techniques to investigate GaAlInP materials properties such as sheet resistivity, film thickness, photoluminescence (PL), Fourier transform IR and Raman scattering spectra for both material and for the on-going qualification of material during production. Typical uniformities of GaAlInP epitaxial film thickness, sheet resistivity, major PL band peak wavelength and width are 1-3 percent. For techniques without automatic mapping abilities, multiple point measurements and employed to obtain information over the entire wafer. Variations of these characteristic features, such as sheet resistivity, PL and Raman properties, with different Al compositions and doping are discussed in this work.

We have developed a new method for the fabrication of monolithic AlGaAs microlenses on the surface of GaAs/AlGaAs light emitting diodes by combing crystal growth, ion etching and steam oxidation with wet chemical removal of the oxide. Control over the precise processing parameters has resulted in the precise control over the shape, radius, position and smoothness of the microfabricated hemispheres. These microlenses can readily be used for the fabrication of highly efficient light-emitting diodes.

InAs/In(As,Sb) heterostructure LEDs are studied in forward (FB) and reverse (RB) bias where the phenomenon of 'negative luminescence' is seen for the first time in this materials system. Pseudomorphic 300K SQW LEDs, lattice matched to InAs and emitting at λ-5 micrometers and λ-8 micrometers , have internal conversion efficiencies of > 1.3 percent and > 0.83 percent respectively and maximum outputs in excess of 50 μW, in spite of an extremely low overall epilayer Sb content. Strain-relaxed InAs/In(As,Sb) SLS LEDs with AlSb barriers for electron confinement give 300K outputs in excess of 0.1mW at λ-4.2μm, approximately 3.5 times greater than control devices without the AlSb barrier. In RB the same SLS diodes exhibited efficient negative luminescence with output powers which increase with increasing device temperature to within 0.8 of the FB figures at 320 K.