Hung-Wen Huang

Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface

This investigation describes the development of InGaN-GaN light-emitting diode (LED) with a nano-roughened top p-GaN surface which uses Ni nano-mask and wet etching. The light output of the InGaN-GaN LED with a nano-roughened top p-GaN surface is 1.4 times that of a conventional LED, and wall-plug efficiency is 45% higher. The operating voltage of InGaN-GaN LED was reduced from 3.65 to 3.5 V at 20 mA and the series resistance was reduced by 20%. The light output is increased by the nano-roughening of the top p-GaN surface. The reduction in the series resistance can be attributed to the increase in the contact area of nano-roughened surface.

Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface

This investigation describes the development of an InGaN/GaN light emitting diode (LED) with a nano-roughened top p-GaN surface using an Ni nano-mask and laser etching. The light output of the InGaN/GaN LED with a nano-roughened top p-GaN surface is 1.55 times that of a conventional LED, and the wall-plug efficiency is 68% higher at 20 mA. The series resistance of the InGaN/GaN LED was reduced by 32% by the increase in the contact area of the nano-roughened surface.

Light-output enhancement in a nitride-based light-emitting diode with 22° undercut sidewalls

We successfully fabricated nitride-based light-emitting diodes (LEDs) with ∼22° undercut sidewalls. The ∼22° etching undercut sidewalls were achieved by controllable inductively coupled plasma reactive ion etching. With a 20-mA current injection, the output powers of the LED with ∼22° undercut sidewalls and standard LED were 5.1 and 3 mW, respectively - a factor of 1.7 times enhancement. It was found that such undercut sidewalls could enhance the probability of escaping the photons outside from the LED in the near horizontal and in-plane directions. This simple and controllable method is beneficial to fabricate brighter LEDs.

Enhancement in light output of InGaN-based microhole array light-emitting diodes

InGaN-based microhole array light-emitting diodes (LEDs) with hole diameters (d) of 3-15 /spl mu/m were fabricated using self-aligned etching. The effects of size on the device characteristics, including current density-voltage and light output-current density, were measured and compared with those of conventional broad-area (BA) LEDs fabricated from the same wafer. The electrical characteristics of the devices are similar to those of conventional BA LEDs. The light output from the microhole array LEDs increases with d up to 7 /spl mu/m. However, the light output declined as d increased further, perhaps because of the combination of the enhancement in extraction efficiency caused by the large surface areas provided by the sidewalls and the decrease in area of light generation by holes in the microhole array LEDs. The ray tracing method was used with a two-dimensional model in TracePro software. The findings indicate that an optimal design can improve the light output efficiently of the microhole array LEDs.

High extraction efficiency GaN-based light-emitting diodes on embedded SiO2 nanorod array and nanoscale patterned sapphire substrate

In this paper, GaN-based LEDs with a nanoscale patterned sapphire substrate (NPSS) and a SiO2 photonic quasicrystal (PQC) structure on an n-GaN layer using nanoimprint lithography are fabricated and investigated. The light output power of LED with a NPSS and a SiO2 PQC structure on an n-GaN layer was 48% greater than that of conventional LED. Strong enhancement in output power is attributed to better epitaxial quality and higher reflectance resulted from NPSS and PQC structures. Transmission electron microscopy images reveal that threading dislocations are blocked or bended in the vicinities of NPSS layer. These results provide promising potential to increase output power for commercial light emitting devices.

Nitride-based LEDs with nano-scale textured sidewalls using natural lithography

This investigation describes the development of a InGaN/GaN light-emitting diode (LED) with textured sidewalls using natural lithography with polystyrene spheres (PSs) as the etching mask and dry etching the epitaxial layers of LEDs to achieve nano-scale textured sidewalls. The LED with textured sidewalls increased the output power of the InGaN?GaN multiple quantum well (MQW) LEDs by a factor of 1.3, indicating that the LED with nano-scale textured sidewalls had larger light extraction efficiency. The wall-plug efficiency of nitride-based LEDs was increased by 30% using textured sidewalls.

Enhancement of Light Output Intensity by Integrating ZnO Nanorod Arrays on GaN-Based LLO Vertical LEDs

Enhancement of light output intensity for GaN-based vertical light-emitting diodes (LEDs), combining wafer bonding and the laser lift-off (LLO) process, employing an omnidirectional extraction surface with synthesized single-crystal ZnO nanorod arrays in aqueous solution at room temperature is presented. The light output intensity and wall-plug efficiency of the GaN-based LLO vertical LED with the omnidirectional extraction surface by ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a vertical LED without ZnO nanorod arrays. The ZnO nanorod arrays not only support a current spreading layer but enhance the probability of photon escape through the omnidirectional extraction surface.

Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks

High-density (3.0×1010 cm-2) InGaN/GaN multiple quantum well (MQW) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850°C for 1 min. The influence of the thicknesses of the Ni metal film on the dimensions and density of the nanorods was also investigated. The structural and optical properties of the InGaN/GaN MQW nanorods were established using field emission scanning electron microscopy, transmission electron microscopy and photoluminescence measurements. The diameters and heights of nanorods were estimated to be 60 to 100 nm and more than 0.28 µm, respectively. The peak emission wavelength of the nanorods showed a blue shift of 5.1 nm from that of the as-grown bulk. An enhancement by a factor of 5 in photoluminescence intensity of the nanorods compared with that of the as-grown bulk was observed. The blue shift is attributed to strain relaxation in the wells after dry etching, the quantum confinement effect, or a combination of the two, which results in the enhancement of emission intensity.

Fabrication of Large-Area GaN-Based Light-Emitting Diodes on Cu Substrate

A large-area GaN-based light-emitting diode (LED) 1000×1000 µm2 in size with a p-side down configuration was fabricated using wafer bonding and laser lift-off (LLO) techniques. The thin GaN LED was transferred onto a copper substrate without peeling or cracks. The large-area LEDs showed a uniform light-emission pattern over entire defined mesa area without a transparent contact layer on the p-type GaN. The operating current of the large-area LEDs can be driven up to 1000 mA with continuously increasing light output-power. The light output-power is 240 mW with a driving current of 1000 mA. Large-area emission and high current operation make the LLO-LEDs applicable to high-power LED applications.

Effects of different n-electrode patterns on optical characteristics of large-area p-side-down InGaN light-emitting diodes fabricated by laser lift-off

Large-area p-side-down InGaN light-emitting diodes (LEDs) 1000×1000 µm2 in size have been fabricated by laser lift-off (LLO). The p-side-down LEDs with different geometric patterns of n-electrodes were fabricated to investigate electrode pattern-dependent optical characteristics. The current crowding effect was observed in the large-area p-side-down InGaN LLO-LEDs. A LED with a well-designed n-electrode pattern shows a uniform distribution of light emission and a higher output power due to uniform current spreading. The output power saturation induced by the current crowding effect was investigated. In the absence of a transparent contact layer for current spreading, the n-electrode pattern has a marked influence on the current distribution and the consequent light output power of the large-area p-side-down LEDs.