Micro-light emitting diode ͑LED͒ arrays with diameters of 4 to 20 m have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes ͓whispering gallery ͑WG͒ modes in particular͔ which appear to play a role in enhancing the optical output.

GaN micro-light-emitting diodes (micro-LEDs) with monolithically integrated microlenses have been demonstrated. Microlenses, with a focal length of 44 µm and a root mean square roughness of ~1 nm, have been fabricated on the polished back surface of a sapphire substrate of an array of micro-LEDs by resist thermal reflow and plasma etching. The optical properties of the microlenses have been demonstrated to alter the emission pattern of the LED emitters. The cone of light emitted from this hybrid device is significantly less divergent than a conventional broad-area device. This combination of micro-LED and microlens technologies offers the potential for further improvement in the overall efficiency of GaN-based light emitters

Light-emitting diodes ͑LEDs͒ based on an interconnected array of GaN/InGaN micro-ring elements have been demonstrated. The devices have electrical characteristics similar to those of conventional broad-area devices. However, due to the large surface areas provided by the sidewalls, the extraction efficiency is greatly enhanced. Intense light emission at the periphery of the micro-rings is observed upon excitation by an electron beam, suggesting scattering of the photons which are extracted through the sidewalls. The devices provide a doubling in total light output compared to a broad-area reference LED of equal light-generation area.

Matrix-addressable arrays of InGaN micro-lightemitting diodes with 128 96 pixels and a resolution of 1200 dpi have been fabricated using a novel "sloped sidewall" process. The devices have been fabricated on InGaN blue and green wafers, emitting light at the wavelengths of 468 and 508 nm, respectively. A simple circuit, which enables the display of an arrow pattern with 60% of the pixels turned on, was used for device testing. At an injection current of 60 mA, the devices deliver 3.3 (blue) and 2.4 mW (green) of output power, corresponding to a luminance of more than 30 000 Cd/m 2. These high-brightness and highly versatile devices are certainly an attractive form of emissive micro-display.

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Arrays of pivoted GaN microdisks have been fabricated on a GaN / Si material by a combination of dry and wet etching. The Si material beneath the GaN microdisks is removed by wet etching, leaving behind a fine pillar to support the disks. Raman spectroscopy reveals substantial strain relaxation in these structures. Resonant modes, corresponding to whispering gallery modes, are observed in the photoluminescence spectra. Stimulated emission is achieved at higher optical pumping intensities.

A vertical cavity structure composing of an in situ grown bottom Al x Ga 1−x N / Al y Ga 1−y N distributed Bragg reflector and a top SiO 2 / HfO 2 dielectric mirror for ultraviolet ͑UV͒ emission has been demonstrated. Close-packed nanopillars with diameters of around 500 nm have been achieved by the route of nanosphere lithography combined with inductively-coupled plasma etching. Optically-pumped UV lasing at a wavelength of 343.7 nm ͑3.608 eV͒ was observed at room temperature, with a threshold excitation density of 0.52 MW/ cm 2. The mechanism of the lasing action is discussed in detail. Our investigation indicates promising possibilities in nitride-based resonant cavity devices, particularly toward realizing the UV nitride-based vertical-cavity surface-emitting laser.

We report on the fabrication of ultraviolet (UV) microarray light-emitting diodes, toward applications including mask-free photolithographic exposure. Devices with 64 64 elements have been fabricated in matrix-addressed format, generating directed output powers of up to 1 W per 20-m-diameter element at less than 1.0-mA drive current. The resistance of each elemental device was found to depend strongly on the n-GaN stripe length. The center wavelength of the emission was measured to be 368 nm, which is very close to that of an-line (365 nm) UV light source. To our knowledge, this is the first report detailing the fabrication and performance of such devices operating in the UV.