We report on blue and green light-emitting-diodes (LEDs) grown on (11-22)-GaN templates. The templates were created
by overgrowth on structured r-plane sapphire substrates. Low defect density, 100 mm diameter GaN templates were
obtained by metal organic vapour phase epitaxy (VPE) and hydride VPE techniques. Chemical-mechanical polishing
was used to obtain smooth surfaces for the subsequent growth of LED structures. Ohmic contacts to the p-type GaN
were obtained despite the lower activated acceptor levels. The LEDs show excellent output power and fast carrier
dynamics. Freestanding LEDs have been obtained by use of laser-lift-off. The work is the result of collaboration under
the European Union funded ALIGHT project.
The performance of a series of near-UV (~385 nm) emitting LEDs, consisting of high efficiency InGaN/AlInGaN QWs in the active region, was investigated. Significantly reduced roll-over of efficiency at high current density was found compared to InGaN/GaN LEDs emitting at a similar wavelength. The importance of optical cavity effects in flip-chip geometry devices has also been investigated. The light output was enhanced by more than a factor of 2 when the light-emitting region was located at an anti-node position with respect to a high reflectivity current injection mirror. A power of 0.49 mW into a numerical aperture of 0.5 was obtained for a junction area of 50 micrometers in diameter and a current of 30 mA, corresponding to a radiance of 30 W/cm2/str.
High brightness LEDs (HBLEDs) have been fabricated on GaN semiconductor material grown on sapphire substrate. These devices provide an optical output power in excess of 50 mW at a driving current of 1 amp. For this high current application, large size (1.8 mm × 0.6 mm) GaN LEDs are flip-chip mounted onto a heat sink to provide a low thermal resistance path from the junction to the ambient. For the flip-chip mounting, a Au/Sn/Au solder and a Au/Au thermal compression bonding process have been optimized. The bond strength of the Au/Sn solder joints and the Au-Au bonds is measured through shear testing. Good bond strength results of 224 g/f for the Au/Sn/Au solder and 288 g/f for the solid Au bonds have been achieved. The thermal modeling of the assembly is done with a finite element analysis and the optimum design has been adopted for this high current application. At present these assemblies are under lifetime test and so far nearly 6000 hours of continuous operation has been achieved.
We report on the development of resonant cavity LEDs (RCLEDs) for use in short distance datacommunication applications using the IEEE 1394 standard where plastic optical fibre (POF) is the physical medium. The devices are designed for 650nm and 500nm emission where POF has low attenuation. The red devices based on InGaAlP/GaAs are optimised for room temperature operation and 90μm diameter devices have a maximum coupled power to 1mm diameter POF of 1mW. At 10mA the coupled power is 0.4mW with a quantum efficiency of 2%. Current spreading is shown to be critical in optimising the output power. The devices function as resonant cavity detectors with a response FWHM of 4.2nm centred at 650nm. The blue-green RCLEDs are based on InGaN/GaN and use a hybrid metal-epitaxial mirror cavity. This wavelength is preferable for longer links. The substrate emitting devices have fibre-coupled powers of 200μW at 20mA. A datarate of 250Mb/s is measured. The resonant cavity is confirmed by angularly resolved spectral measurements. The tradeoff between green and red devices is discussed.
For optical networks, the operating life of optoelectronic components is expected to be over 20 years. Network designers therefore require components, which have been reliability tested in accordance with assured protocols, such as Telcordia Generic Reliability Assurance Practices (BellCore). In this paper, we report on the development of a system for thermal reliability studies of optoelectronic devices. The system incorporates an environmental test chamber programmed to provide differing temperature environments in the range (-180° to 300° C) as well as constant bias current or voltage to the device udner test. Case studies for preliminary screenign and temperature cycling tests on a wide range of novel active and passive devices fabricated at NMRC for short-haul networks markets are assessed and reported using this system.
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