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InGaN light-emitting diodes (LEDs) on sapphire substrate in the shape of cuboid and circular disk are diced via ultraviolet nanosecond laser micromachining. The geometrical influence on light extraction and emission pattern are evaluated. Our postulation on the optical characteristics of circular LEDs is qualitatively confirmed according to ray-tracing simulation. The enhancement in light extraction over conventional cuboid chips is more than 10% according to measurements of our fabricated devices. The fair consistency between the simulation result and measurements indicates that the circular design is indeed effective for improving light extraction from InGaN LEDs. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a direct-write lithographic technique for the fabrication of micro-lens arrays with an ultraviolet (UV) micro-light-emitting diode (LED) array, serving as an exposure source. Owing to the Lambertian emission distribution of LEDs, the exposed UV sensitive coating (UV epoxy) adopts the same profile, resulting in spherical lenses being formed naturally, without further reflow. This technique is distinctive from conventional UV curing method and possesses many advantages, including simplicity of processing, allows control of the dimensions of micro-lenses through process parameters, and is readily scalable. Micro-lens arrays of high optical quality have been fabricated by this method. The properties of the lenses are evaluated by optical microscopy and atomic force microscope. The determined focal length is close to the predicted value. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

While longer wavelength emission from InGaN/GaN light-emitting diodes can be achieved by increasing the Indium (In) content in the qunatum wells, the increased In content gives rise to side-effects to the material and device performance and reliability. It was found that the induced strain in the wafer and the density of threading dislocations increases with increasing In content. From current-voltage and 1/f noise measurements, it was observed that the leakage currents, static resistance and noise magnitudes rises monotonically with increasing emission wavelength (In composition), which can be attributed to higher defect concentrations. After undergoing a 1000-hr reliability test, it was discovered that the optical degradation rates for the longer wavelength green LEDs were significantly higher than those of shorter wavelength. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, the fabrication and characterization of nano-pillar arrays on GaN substrates using the technique of microsphere lithography have been demonstrated. Self-assembled hexagonally-packed silica microsphere array were formed on GaN wafers by the methods of spin coating and tilting. By precisely controlling the process parameters, a monolayer of microsphere array can be formed over a wide region, acting as a hard-mask for pattern transfer of the nanostructures. After dry etching, arrays of nano-pillars were formed on the surface of the wafer. The structure properties of the ordered nanostructures were characterized by SEM images, while photoluminescence (PL) measurements revealed a twofold enhancement of light emission intensity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel design and implementation of a single-chip wavelength tunable light emitting diode (LED) is introduced in this paper, with varying emission spectral characteristics ranging between 470 nm (blue) and 525 nm (green). The emission wavelength of green fluorescent microspheres (525 nm), acting as a colour conversion agent, is chosen to be efficiently excited by a GaN LED of 470 nm. It is selectively-coated onto designated micrometer-scaled pixels, which are interconnected via metal lines. As a result, the LED allows separate control of optical output either 470 nm, 525 nm or even a mixture of both wavelengths. Since the pixels are of micron-scale which are non-resolvable by the unaided human eye, the output appears optically mixed. This type of device, combining the application of micro-light-emitting diode and fluorescent microsphere technologies, offers a colour tunable output suitable for a wide range of applications including such as outdoor panel displays. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This journal issue entitled: Special Issue: 7th International Conference on Nitride Semiconductors (ICNS-7)

Light-emitting diodes with vertical stacking geometry were fabricated and characterized. Color-tunable emission and white-light emission has been demonstrated in Stacked-LED (SLED). Emissions of different colors, ranging from green-blue, to pink-purple, to yellow-orange and white light emission have be achieved by adjusting the relative intensity of two or three individual emission bands, which in turn are controlled by adjusting voltage supply. Compared to conventional lateral side-by-side geometry, the shift of white color chromaticity with respect to viewing angles is less in the case of SLED device. The farthest shift of white CIE coordinate point away from the pure white CIE coordinate is (0.29, 0.29) in SLED compared to (0.37, 0.35) in the commerical available lateral packaged structure. This vertically-stacked LED is poised to open new frontiers in high performance color-tunable LEDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)