Single crystal thin films with compositions from the AlN- InN-GaN system were grown via metal-organic chemical vapor deposition on single crystal 6H-SiC substrates. AlGaN containing high and low fractions of Al was grown directly on the SiC for use as a buffer layer. Subsequent epitaxial layers of GaN and AlGaN were doped with Mg and Si to achieve p-type conductivity, respectively. N-type InGaN layers with In compositions up to approximately 50 percent were also achieved. Room temperature photoluminescence on these films exhibited single peaks in the spectral range from the UV to green. Various layers were combined to form light emitting diode (LED) and laser structures. Blue LEDs with both insulating and conductive buffer layers exhibited an external quantum efficiency of 2-3 percent with a forward operating voltage of 3.4-3.7 V. Laser diode structures having a separate confinement heterostructure multiple quantum well configuration were optically and electrically pumped. Photopumping resulted in stimulated emission at 391 nm. Electrically pumped structures resulted in a peak emission at 393 nm and a bandwidth of 12 nm. No lasing was observed.
A pulsed electrochemical technique for etching MOCVD-grown InGaN/GaN light-emitting diode material at room temperature is reported. The p-GaN and InGaN layers can be etched away in minutes providing access to the n-GaN layer below. Movement of the etch front through the p-n junction region can be observed by monitoring changes in the current pulse shape with time on an oscilloscope.
We have studied electroluminescence and photoluminescence for Nichia single-quantum-well Al0.2Ga0.8N/In0.45Ga0.55N/GaN green light-emitting diodes over a broad range of temperatures and currents. The most striking behavior is an anomalous temperature shift of both photo- and electroluminescence, with the emission peak moving towards higher energies with increasing temperature. This blue shift is opposite to that of the energy gap of the active layer, which practically excludes standard interband transitions as responsible for the observed optical transitions. We have also observed a very large blue shift of the emission peak with the increased current. This shift is an order of magnitude larger than one might expect from calculations of the conduction band filling. We show that the observed anomaly can be accounted for within the framework of the band tail model. In addition, our measurements of temperature-dependent voltage-current characteristics show that electrical transport through the p-n junction in these devices is dominated by carrier tunneling, which seems to be omnipresent in GaN-based optoelectronic devices.
InGaN single-quantum-well (SQW) structure blue/green light- emitting diodes (LEDs) were fabricated. At 20 mA, the output power and the external quantum efficiency of the blue SQW LEDs were 5 mW and 9.1 percent, respectively. Those of the green SQW LEDs will a 10 degree cone viewing angle was 10 cd at 20 mA. A white LED made by combining a blue InGaN SQW LED and yttrium aluminium garnet phosphor, which is less expensive than a white LED composed of three primary color LEDs, was developed.
The competition between bandgap and the 2.2 eV yellow luminescence of epitaxial GaN is studied for excitation densities ranging from 5 X 10-6 W/cm-2 to 50 W/cm-2. The ratio of the peak intensities of the bandgap-to-yellow luminescence changes from 4 to 1 to 3000 to 1 as the excitation density is increased by seven orders of magnitude. At room temperature, the bandgap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1/2, 1 and 3/2. The theoretical dependences are in excellent agreement with experimental results. It is shown that the intensity of the yellow luminescence line is negligibly small at typical injection currents of light- emitting diodes.
LEDs are important devices in communications and optical displays. One of the most promising methods of improving the LED comes in the form of Resonant Cavity Light Emitting Diode, or RCLED. A planar microcavity changes the fundamental emission properties of the light emitting material between two planar mirrors. The intensity and spectral purity of emission can be considerably increased over that of a normal LED. We present results for InGaAs/GaAs/AlGaAs optical fiber communications RCLEDs operating at 910nm to 950nm wavelength. These substrate- emitting devices consist of a silver back mirror and contact and a strained layer InGaAs multiple quantum well active region. The output mirror consists of an AlGaAs.GaAs distributed Bragg reflector.These devices exhibit higher intensities on axis at low input current than a perfect internal efficiency conventional LED. The narrower spectral emission coupled into a fiber results in less chromatic dispersion over longer fiber distances. Such devices are also attractive in optical interconnect applications where high efficiency, low power consumption, and high speed are important. We demonstrate that with simple speed up electronics, our RCLEDs can easily communicate at over 622 Mbit/second over short interconnects, and work best at only 5 to 10 mA of input current. Long term measurements indicate no degradation after 14000 hours of room-temperature operation. Mention will be made of the requirements of performance, reliability, and cost for RCLEDs to become viable commercial products.
Optical cavity light-emitting diode structures with 'buried' mirrors, and their fabrication by lateral epitaxy are described. Single-crystal, high-quality epitaxial layers are formed over substrates coated with patterned, reflective masks using liquid-phase or vapor-phase epitaxial lateral overgrowth processes. The reflecting mask acts as a backside mirror and forms an optical cavity leading to enhanced external quantum efficiencies. An AlGaAs optical cavity LED incorporating a refractory metal 'buried' mirror is assessed: a greater than 3-fold increase in output optical power is measured compared to control devices with no buried mirror. Application of the epitaxial overgrowth techniques to LED structures utilizing electron-beam deposited dielectric/semiconductor 'buried' mirrors and to other semiconductor materials, such as InGaAsSb, SiC, and ZnSe is described.
Polymer light-emitting diodes, based for example on MEH-PPV, are known to be susceptible to oxidative degradation. This leads to loss of conjugation, i.e. lower carrier mobility and higher operating voltage, and to the formation of carbonyl species, i.e. to luminescence quenching. In-situ FTIR has revealed that ITO can act as the source of oxygen. In order to explore further the mechanism of oxidation and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with a variety of conducting polymer anodes including polyaniline and polythiophene derivatives cast from various solvents and with various molecular and polymeric dopants. In all cases examined, it is found that polymer anodes lead to significant improvement in lifetime over devices with ITO as the anode contact. Moreover, in contrast to the variability observed for ITO anodes, conducting polymers with polymers with polymeric dopants yield consistently good devices with power efficiencies of about 0.5 percent at 5 volts and brightness in excess of 1000 cd/m2. Anodes prepared with small molecule dopants are more variable and exhibit short term behavior which suggests interfacial electrochemistry. We describe the device characteristics in the context of a model of hole-dominated bipolar charge injection with Langevin recombination.
We report microcavity efficiency enhancement of organic electroluminescent devices based on the hole transporter bis(triphenyl)diamine and the electron transporter and light emitter tris(8-hydroxyquinoline)aluminum. Microcavity organic light emitting diodes are described which emit 4 times the light measured in the forward direction, or almost twice the total light of a non-cavity organic light-emitting diode for identical electrical drive conditions.
We developed a new type of light-emitting-diodes in which the emission layer is confined inside an aluminium channel. A 1200 angstrom aluminium layer was first deposited on glass by vacuum evaporation and then carved using microlithography techniques. Channels were typically 1.5 micrometers width. A 2000 angstrom thick electroluminescent polymer film was spin coated on top of the Al channel. The diode emits light under alternative voltages. It can be seen under room illumination. We studied current-voltage characteristics and LED quantum efficiency. Electroluminescence results from a field ionization and recombination process. No charge injection at electrodes takes place in such device.
A spin-coated doped-polymer light emitting diode is studied. Blue electroluminescence decays within one hour. X-ray reflectivity analysis of the aged diode shows the formation of an interfacial layer made of the ITO semi-transparent electrode indiffused into the polymer. X-ray reflectivity stands as a powerful tool for aging studies of organic semiconducting devices.
A new class of LEDs based on the AlGaInP material system first became commercially available in the early 1990's. These devices benefit from a direct bandgap from the red to the yellow-green portion of the spectrum. The high efficiencies possible in AlGaInP across this spectrum have enabled new applications for LEDs including automotive lighting, outdoor variable message signs, outdoor large screen video displays, and traffic signal lights. A review of high-brightness AlGaInP LED technology will be presented.
A model of optical processes in LEDs was created that takes into account device geometry, light absorption in contacts and cladding layers, photon recycling, light randomization due to surface scattering and the benefit from encapsulation of the device into epoxy. Based on the results of our modeling, an optimization of the LED was proposed. Also, photoluminescence measurements of internal quantum efficiency were performed on the epi-layers used for LED fabrication.
The bulk of LEDs sold today are still fabricated using older epitaxial techniques such as LPE and VPE, but have relatively low brightness and a limited color range. The newer high brightness LEDs are fabricated from the InGaAlP and III-Nitride systems, with MOCVD being the preferred growth technique for manufacturing. While these new materials represent a significant increase in performance, they are also more expensive to grow. In this paper we consider the reasons for this, which include a less mature growth technology, lower production volumes, expensive starting materials, process efficiency, equipment throughput and cost, and safety and environmental concerns. Addressing each of these issues in turn, we examine what has already been accomplished, and what may be improved by further advances in equipment and process. A realistic COO model is of great utility in comparing product cost for different device structures, staffing schemes, reactor sizes, etc. We demonstrate that for a dedicated LED manufacturing facility, the lowest epitaxial cost is achieved by running around the clock with the highest throughput reactor that is fully utilized for the desired production level. When maintenance tasks such as cleaning and test or calibration runs are minimized, then materials costs will dominate the epi cost, which leads to the desirability of achieving both the best reproducibility and increasing the process efficiency. We show how in-situ control techniques are now capable of increasing preproducibility and thereby lowering product costs for manufacturing scale MOCVD reactors.
The basic parameters of GaInAsSb mid-IR light emitting diodes designed for spectroscopic applications were shown. Two types of room temperature devices were presented: diodes for fixed emission wavelengths between 1.7 and 2.4 micrometers wavelengths and diodes tuned by drive current over wide wavelength range of 2.1-2.6 micrometers . The diodes were investigated for both continuous wave (CW) and pulse operation. The current-voltage characteristics, emission spectra, beam divergence, temperature shift of the emission band were presented and discussed. To find higher performance the output power was investigated versus pulse widths and repetition rates. It was reported that the GaInAsSb LED power was improved up to 2-3 times to the values as high as 3.7 mW CW and 82 mW pulse at (lambda) equals 1.94 micrometers as an example. It is shown that room-temperature operation, low electric power consumption, reduced cost and easy routine execution are the advantages to LEDs in comparison with spectrometers based on a diode lasers and thermal emitters. Sensitive and selective apparatuses for pollution detection, medicine and process control in the 1.7-2.4 micrometers wavelength range can be built on base of these LEDs as a source of radiation.
The advent of high luminance AlInGaP and InGaN LED technologies has prompted the use of LED devices in new applications formally illuminated by incandescent lamps. The luminous efficiencies of these new LED technologies equals or exceeds that attainable with incandescent sources, with reliability factors that far exceed those of incandescent sources. The need for a highly efficient, dependable, and cost effective replacement for incandescent lamps is being fulfilled with high luminance LED lamps. This paper briefly described some of the new applications incorporating high luminance LED lamps, traffic signals and roadway signs for traffic management, automotive exterior lighting, active matrix and full color displays for commercial advertising, and commercial aircraft panel lighting and military aircraft NVG compatible lighting.
The significant progress which as been made in the development of differential pairs and arrays of differential pairs of light-emitting thyristors has made the construction of optical computing systems with high speed interconnections a realistic possibility. In this paper we review our work on the practical implementation of these optoelectronic transceiver devices in systems and demonstrate most of the basic functionalities necessary to build a primitive digital parallel optical processor. We demonstrate the transcription of digital optical data between cascaded single elements and between 8 by 8 arrays of completely- depleted optical thyristor differential pairs. We also show results of digital optical logic NAND, NOR, AND, OR, NOT operations, logic plane to logic plane imaging with a diffractive fan-out and parallel digital data input with a computer controlled liquid crystal micro- display. As an example of a sub-system module which has reasonable complexity we focus on a demonstrator platform which combines optical thyristor logic planes, polarization- selective diffractive optical elements, liquid crystal variable retarders and large diameter gradient index lenses, and successfully demonstrate dynamically reconfigurable nearest neighbor interconnects. We conclude by discussing the future system performances in the light of system scalability.
Improvement of high-speed surface light emitting diodes has been demonstrated by a novel selective regrowth of circular etched mesas with a semi-insulating iron-doped indium phosphide layer. Inclusion of an interfacial layer of indium gallium arsenide phosphide between the circular dielectric mask and the underlying material produces a favorable smooth mesa profile by controlling the level of undercut during mesa etching. This combination of profile and undercut was found to be critical for successful selective regrowth and planarization. The semi-insulating indium-phosphide layer reduces the parasitic capacitance and improves the heat dissipation. These salient features make these devices suitable for high speed digital and analog communication applications requiring high linearity.
Confocal imaging is a technique used in microscopy and sensing. The use of microoptic technology allows one to build compact integrated confocal systems. Diffractive elements are used to introduce chromatic dispersion into the imaging setup. This allows one to gain depth or spectral information. In this poster, we discuss the performance of a diffractive confocal imaging systems and its potential applications. In particular, we will focus on an application in spectroscopy whose impulse response is matched to the spectrum of a specific light source. Matched spectroscopy may be of use in environmental sensing and process control. For the realization of the optics we consider integrated free-space optics using a planar 2D layout.
The path for silicon materials development has been charted. By the year 2010 we will have fabricated integrated circuit chips contained 109 transistor with 40 angstrom thick gate oxides and 1000 angstrom minimum feature sizes running at 4GHz clock speeds. It is conceivable that incremental advances on the current chip architecture will satisfy the required materials and process improvements. The interconnection problem is the only challenge without a proposed solution. The signal propagation delay between devices is now longer than the individual device gate delay. The resistance and capacitance associated with fine line Al interconnects limit speed and increase power consumption and crosstalk. High power line drivers are limited by the reliability constraint of electromigration. There is no current paradigm for 4GHz electronic clock distribution. Optical interconnection can remove the electronic transmission bandwidth limit. The main challenge is development of a silicon-compatible, microphotonic technology. Rare earth doping has provided a means of sharp- line electroluminescence from silicon at (lambda) equals 1.54 micrometers . Silicons high index of refraction and low absorption in the near infrared yield an ideal optical waveguide. As with microelectronics, the silicon/silicon-dioxide materials system allows high levels of integration and functionality. The applications of silicon materials to light emission, optical waveguides, photonic switching and photon detection are reviewed. These developments are discussed in the context of systems applications to communications and computation.
The investigation of photo- and electroluminescence in erbium-doped silicon additionally co-implanted with oxygen, phosphorus and boron show an enhancement of the 1.54 micrometers line luminescence intensity in Si:Er:O and Si:Er:O:P and an intensity quenching in Si:Er:O:B as compared with Si:Er. A threshold in dependence of defect-related line electroluminescence signal on drive current is observed too. A model describing the observed variations of luminescence spectra in dependence on implantation and annealing conditions is presented. Optimization of technological regimes resulted in formation of light emitting erbium-doped silicon structures operating at room temperature.
A novel LED design employing a 2D periodic photonic crystal host is demonstrated theoretically to yield extraction efficiencies approaching 100 percent. Specifically, a thin slab of 2D photonic crystal is shown to alter drastically the radiation pattern of spontaneous emission. By eliminating all guided modes at the transition frequencies, spontaneous emission can be coupled entirely to free space modes, resulting in a greatly enhanced extraction efficiency. Such structures might provide a solution to the long-standing problem of poor light extraction form high refractive-index semiconductors in light-emitting diodes.
Optical interconnects for use in high speed computing and communication systems require dense optoelectronic integrated circuits (OEICs). Monolithic integration of III-V optoelectronics with VLSI optoelectronics with VLSI- complexity electronics will yield OEICs of the high density, performance, manufacturability, and reliability. The epitaxy-on-electronics (EoE) technique monolithically integrates optoelectronic devices with commercially- fabricated, fully-metallized GaAs VLSI integrated circuits. This manuscript reviews the EoE process and details the fabrication of integrated LEDs. This LED-OEIC process is being used by optical interconnect systems researchers on a prototype basis through the OPTOCHIP project: the current status of this effort is reviewed.
The high-efficient blue, green light-emitting diodes (LEDs) using GaInN/GaN multiple quantum wells (MQW) have been successfully developed. The peak wavelength of MQW blue and green LEDs are approximately 460nm and 520nm, respectively. The full width at half-maximum of the MQW blue LEDs is 40nm at a forward current of 20mA which is much narrower than 70nm of the conventional blue LEDs. The degrees of shift in peak wavelength of the MQW blue and green LEDs are less than 5nm in the current ranging from 5mA to 50mA.
The external QE of microcavity light emitting diodes strongly depends on the device size and operational current density. Our experiments reveal that spectral broadening of the optical spectrum emitted by the three InGaAs QWs as well as photon originally emitted into the guided mode of the cavity can explain these differences. An optimized microcavity layer design yields external QEs of 20 percent for substrate emitting light emitting diodes with diameters of 1.5 mm.