The market entrance of thinfilm based, substrate-less LEDs has stimulated the field of high-brightness LEDs. One of the most prominent advantages of thinfilm LEDs is the possibility to achieve a high light extraction efficiency independently of the chip area. This feature is particularly suitable for large-area, high-flux devices. In this paper, we report on high-power LEDs with a chip-area of 1 mm2 for red and infrared emission. Mounted in packages with improved heat sinking and operated at a continuous-wave (cw) current of 800mA, the devices achieve an output power of 440 mW both for red (λ = 615 nm) and infrared (λ = 850 nm) wavelengths. Together with Osram's ThinGaN chips, a family of devices is available with very similar emission characteristics, performance and geometry, which allow the assembly of powerful light engines for a number of advanced applications.
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.
Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.5P light-emitting devices (LEDs) is analysed experimentally and theoretically. A test series of LEDs was grown by MOCVD with identical layer sequence but different Aluminum content x in the active AlGaInP layer resulting in devices emitting light between 644 nm and 560 nm. The analysis yields the wavelength dependence of both the nonradiative recombination constant A as well as the carrier leakage parameter C of devices before and after aging. While test devices with λ>615 nm are very stable, LEDs with shorter emission wavelengths exhibit both an increase of A and a slight decrease of C upon aging. Possible degradation mechanisms are discussed.
The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized.
Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected.
Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.
The combination of wafer soldering using metal layers and the introduction of buried micro-reflector structures has proven to be a promising approach to fabricate high brightness, substrate-less LEDs in the AlGaInP material system. In addition to the enhanced light output, the scalability of this approach has been predicted as a major advantage. In contrast to other approaches, larger area LEDs can be fabricated without altering the epitaxial structure and thickness of layers simply by offering a larger area for light generation. First samples of amber (λ = 615 nm) buried micro-reflector LEDs with side-length up to 1000 μm have been realized. Devices mounted in packages with improved heat sinks are capable of low voltage CW operation with currents as high as 600 mA (Vfw≤ 2,8 V) without significant thermal flattening of the light-current characteristics. The maximum luminous flux achieved at these oeprating conditions is 46 lumen. Already these first experiments demonstrate the potential of the concept of buried micro-reflector LEDs not only for high-brightness but also for high-current operation. The results are among the best values of high-flux LEDs in this wavelength range.
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.
We report on an optoelectronic receiver consisting of a special photoconductive n-i-p detector and a n-i-p reference diode fabricated from the same structure. The receiver is illuminated by two surface-normal light beams using a dual rail code. All applied DC biases are compatible with normal silicon CMOS logic, no AC biasing is required. The photoconductive gain of the receiver allows for output currents of more than 10 mA without any further amplification. The active device area of the smallest detectors has been scaled down to 120 micrometers 2, resulting in a total optical switching energy for this receiver as low as 348 fJ. This optical switching energy is constant over a wide range of input power, resulting in fast switching times at sufficiently high input power while still retaining well-defined, but slower switching characteristics at lower input powers. Using input beams with 0.6 mW optical power at a wavelength of 787 nm, high speed measurements with a 3 dB frequency in excess of 400 MHz have been made. At these measurements a photoconductive gain of 6 times the p-i-n photocurrent was achieved, but using input powers of about 7 nW a gain in excess of 106 has been demonstrated. Due to their simple design and biasing demands these photoconductive receivers are well suited for smart pixel applications and optical interconnects. For demonstration we present results for new a monolithically integrated smart pixel with a high-efficiency non-resonant cavity LED as emitter.
We present a high-bandwidth parallel optical link that operates from CMOS chip to CMOS chip, for applications like chip-to-chip, board-to-board and rack-to-rack interconnects. The optical channel is an oversampling imaging fiber bundle of 1.9 mm diameter. The light sources are a 10 X 10 2D array of high-speed, high-efficiency light-emitting diodes, design for flip-chip mounting onto CMOS driver circuits. Detectors and receivers are integrated together in standard CMOS, as an array of 10 X 10 detector/receiver cells.
The external quantum efficiency of planar light-emitting diodes (LED's) can be increased significantly by the approach of a non-resonant cavity (NRC) LED, which consists of texturing the top surface and applying a rear reflector. We demonstrate this approach for the first time on 650-nm InGaP/AlInGaP LED's. The LED's are fabricated using the processing techniques developed previously for 860-nm GaAs/AlGaAs NRC-LED's, which include wet thermal oxidation for the formation of a current aperture. With un-encapsulated NRC- LED's, we report an external quantum efficiency of 24% for an emission wavelength of 655 nm. This is an 11-fold increase of the external quantum efficiency, as compared to conventional devices. Furthermore, the efficiency is demonstrated to increase to 31% by on-wafer encapsulation of the LED's. This results in an optical output power of 4 mW for a drive current of 7 mA.
Light-emitting diodes (LEDs) with high efficiencies can be fabricated by a combination of surface texturing and the application of a rear reflector. We demonstrate an external quantum efficiency of 43% for unencapsulated surface-textured thin-film LEDs, which increases to 54% after encapsulation. At low temperatures, the efficiency of unencapsulated devices increases up to 68%. We investigate the light extraction mechanism from such LEDs employing a Monte Carlo simulation of the light propagation inside the LED structure. One essential input parameter for the simulation are the light scattering properties of the textured surface, which have been investigated experimentally. For light incidence below the critical angle of total internal reflection, the transmission through a textured surface is reduced compared to a flat surface. However, due to surface texturing, transmission becomes possible for incident angles above the critical angle. As a result, the internal scattering during internal reflection at the textured surface is not necessary for an efficient extraction of the light generated inside the LED structure. In addition, the Monte Carlo simulation also explains the strong increase of the LED efficiency at low temperatures quantitatively by photon recycling effects. Photon recycling is also demonstrated to be partially responsible for the shift of the emission wavelength in thin- film LEDs, as compared to conventional LEDs.
The external efficiency of conventional light-emitting diodes (LED's) is limited by total internal reflection at the semiconductor-air interface. For conventional GaAs-based LED's, this results in an extraction efficiency of 2%. In non- resonant cavity (NRC) LED's, this problem is overcome by a combination of internal scattering at a textured top surface and reflection on a back mirror, which increases the probability of escape. Using this approach, we demonstrate external quantum efficiencies of up to 40% without encapsulation of the LED. To gain a more detailed understanding of the out-coupling mechanisms in NRC-LED's, the scattering properties of the textured surface are investigated experimentally. The optimum surface texture is found to randomize the direction of the internally reflected light almost perfectly. In addition, NRC-LED's also enables the enhancement of the external quantum efficiency for small and fast LED's. With efficiencies of about 15%, we demonstrate bitrates of more than 1.3 GBit/s. In order to modify the lambertian output characteristics, we have successfully applied microlenses, allowing 50% coupling efficiency into optical fibers with NA equals 0.5.
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%.
We propose a simple, low cost, parallel optical data bus for CMOS to CMOS optical interconnects. At the light emission side, NRC-LEDs are chosen for their high external quantum efficiency at low currents (10 - 20% at sub-mA level), their expected high yield and temperature insensitive operation. Image Fiber Bundles (IFBs) are the medium of choice for conducting the high number of parallel light channels. IFBs are a mature product of glass technology, being at the same time flexible, low cost, low risk and highly efficient. Light is received directly in CMOS, using the concept of Spatially Modulated Light detection in combination with a Sense-Amplifier receiver at a bitrate of 180 Mbit/s.
We present an array of GaAs-based light-emitting diodes emitting at 850 nm, which is designed to be flip-chip joined on carriers like a silicon CMOS circuit. The light-emitting diodes are grown by MBE. After processing of the array and flip-chip joining using indium-bumps, the substrate of the LED array is removed completely. Individual light-emitting diodes reach an external quantum efficiency of 3.3% after the complete process.
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.
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