C.W. Jeon

0.5-W single transverse-mode operation of an 850-nm diode-pumped surface-emitting semiconductor laser

We report the power scaling of a diode-pumped GaAs-based 850-nm vertical external-cavity surface-emitting laser, by use of an intracavity silicon carbide (SiC) heatspreader optically contacted to the semiconductor surface. To our knowledge, this is the first demonstration of bonding of SiC to a III-V semiconductor structure using the technique of liquid capillarity. High output power of >0.5 W in a circularly symmetric, TEM/sub 00/ output beam has been achieved with a spectral shift of only 0.6 nm/W of pump power. No thermal rollover was evident up to the highest pump power available, implying significant further output-power scaling potential using this approach.

Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 μm 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 (WG) modes in particular] which appear to play a role in enhancing th...

Spectral conversion of InGaN ultraviolet microarray light-emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films

We report the fabrication of hybrid organic/inorganic semiconductor light-emitting devices that operate across the entire visible spectrum. The devices are based on a series of blue-, green-, and red-light-emitting polyfluorene materials that convert the emission from an array of micron-sized ultraviolet InGaN light-emitting diodes. We also demonstrate white-light-emitting versions of these hybrid devices by employing single films of carefully adjusted polyfluorene blends in which cascade energy transfer occurs between the constituent materials. The spectral and operating characteristics of the devices are described in detail. Such organic emission layer/inorganic light-emitting diode (LED) array based devices may provide a promising route to the fabrication of low-cost full-color microdisplays and other instrumentation devices.

Fabrication and performance of parallel-addressed InGaN micro-LED arrays

High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.

High-density matrix-addressable AlInGaN-based 368-nm microarray light-emitting diodes

We report on the fabrication of ultraviolet (UV) microarray light-emitting diodes, toward applications including mask-free photolithographic exposure. Devices with 64 /spl times/ 64 elements have been fabricated in matrix-addressed format, generating directed output powers of up to 1 /spl mu/W per 20-/spl mu/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 i-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.

Mask-free photolithographic exposure using a matrix-addressable micropixellated AlInGaN ultraviolet light-emitting diode

We report the integration of a UV-curable polymer microlens array onto a matrix-addressable, 368-nm-wavelength, light-emitting diode device containing 64×64 micropixel elements. The geometrical and optical parameters of the microlenses were carefully chosen to allow the highly divergent emission from each micropixel to be collimated into a narrow beam of about 8-μm diam, over a distance of more than 500μm. This device is demonstrated as a photolithographic exposure tool, where the pattern-programmable array plays the role both of light source and photomask. A simple pattern comprised of two disks having 16-μm diam and 30-μm spacing was transferred into an i-line photoresist.

Fabrication of matrix-addressable InGaN-based microdisplays of high array density

We describe the fabrication and characterization of matrix-addressable microlight-emitting diode (micro-LED) arrays based on InGaN, having elemental diameter of 20 /spl mu/m and array size of up to 128 /spl times/ 96 elements. The introduction of a planar topology prior to contact metallization is an important processing step in advancing the performance of these devices. Planarization is achieved by chemical-mechanical polishing of the SiO/sub 2/-deposited surface. In this way, the need for a single contact pad for each individual element can be eliminated. The resulting significant simplification in the addressing of the pixels opens the way to scaling to devices with large numbers of elements. Compared to conventional broad-area LEDs, the micrometer-scale devices exhibit superior light output and current handling capabilities, making them excellent candidates for a range of uses including high-efficiency and robust microdisplays.

Beam divergence measurements of InGaN∕GaN micro-array light-emitting diodes using confocal microscopy

The recent development of high-density, two-dimensional arrays of micrometer-sized InGaN∕GaN light-emitting diodes (micro-LEDs) with potential applications from scientific instrumentation to microdisplays has created an urgent need for controlled manipulation of the light output from these devices. With directed light output these devices can be used in situations where collimated beams or light focused onto several thousand matrix points is desired. In order to do this effectively, the emission characteristics of the devices must be fully understood and characterized. Here we utilize confocal microscopy to directly determine the emission characteristics and angular beam divergences from the individual micro-LED elements. The technique is applied to both top (into air) and bottom (through substrate) emission in arrays of green (540nm), blue (470nm), and UV (370nm) micro-LED devices, at distances of up to 50μm from the emission plane. The results are consistent with simple optical modeling of the expected ...

InGaN nano-ring structures for high-efficiency light emitting diodes

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1cm−1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated.

InGaN microring light-emitting diodes

The fabrication and performance of an InGaN light-emitting diode (LED) array based on a microring device geometry is reported. This design has been adopted in order to increase the surface area for light extraction and to minimize losses due to internal reflections and reabsorption. Electrical characteristics of these devices are similar to those of a conventional large-area LED, while the directed light extraction proves to be superior. In fact, these devices are found to be more efficient when operated at higher currents. This may be attributed to improved heat sinking due to the large surface area to volume ratio. The potential applications of these devices are also discussed.