Jung-Tang Chu

Study of GaN light-emitting diodes fabricated by laser lift-off technique

The fabrication process and performance characteristics of the laser lift-off (LLO) GaN light-emitting diodes (LEDs) were investigated. The LLO-GaN LEDs were fabricated by lifting off the GaN LED wafer structure grown on the original sapphire substrate by a KrF excimer laser at 248 nm wavelength with the laser fluence of 0.6 J/cm2 and transferring it onto a Cu substrate. The LLO-GaN LEDs on Cu show a nearly four-fold increase in the light output power over the regular LLO-LEDs on the sapphire substrate. High operation current up to 400 mA for the LLO-LEDs on Cu was also demonstrated. Based on the emission wavelength shift with the operating current data, the LLO-LEDs on Cu show an estimated improvement of heat dissipation capacities by nearly four times over the light-emitting devices on sapphire substrate. The LLO process should be applicable to other GaN-based LEDs in particular for those high light output power and high operation current devices.

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.

Fabrication of InGaN/GaN nanorod light-emitting diodes with self-assembled Ni metal islands

We report the fabrication of InGaN/GaN nanorod light-emitting diodes (LEDs) using inductively coupled plasma reactive-ion etching (ICP-RIE) and a photo-enhanced chemical (PEC) wet oxidation process via self-assembled Ni nanomasks. An enhancement by a factor of six times in photoluminescence (PL) intensities of nanorods made with the PEC process was achieved in comparison to that of the as-grown structure. The peak wavelength observed from PL measurement showed a blue shift of 3.8 nm for the nanorods made without the PEC oxidation process and 8.6 nm for the nanorods made with the PEC oxidation process from that of the as-grown LED sample. In addition, we have demonstrated electrically pumped nanorod LEDs with the electroluminescence spectrum showing more efficiency and a 10.5 nm blue-shifted peak with respect to the as-grown LED sample.

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.

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.

Room-Temperature Operation of Optically Pumped Blue-Violet GaN-Based Vertical-Cavity Surface-Emitting Lasers Fabricated by Laser Lift-Off

Room-temperature optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) were demonstrated by laser lift-off. A VCSEL was fabricated by combining a GaN-based cavity with two dielectric distributed Bragg reflectors: SiO2/TiO2 and SiO2/Ta2O5. The Q factor of the VCSEL is 518 indicating a good interfacial layer quality of the structure. The laser emits blue-violet wavelength light at 414 nm under optical pumping at room temperature with a threshold pumping energy of 270 nJ. The laser emission has a narrow linewidth of 0.25 nm and a degree of polarization of 70%. The laser emission patterns clearly indicate a vertical lasing action of the VCSEL.

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.

Light–Output Enhancement of Nano-Roughened GaN Laser Lift-Off Light-Emitting Diodes Formed by ICP Dry Etching

In this paper, we report the fabrication and characteristics of nano-roughened GaN laser lift-off (LLO) light-emitting diodes (LEDs) with different scale surface roughness. The surface roughness of devices was controlled by inductively coupled plasma reactive ion etching. Using this fabrication method to form nano-scaled roughness, the electrical property was almost not degraded. Furthermore, the light-output power and wall-plug efficiency of LLO LED could be both significantly enhanced about two times using this simple method

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.

Emission characteristics of optically pumped GaN-based vertical-cavity surface-emitting lasers

The laser emission characteristics of a GaN-based vertical-cavity surface-emitting laser with two dielectric distributed Bragg reflectors were investigated under optically pumped operation at room temperature. The laser emitted wavelength at 415.9nm with an emission linewidth of 0.25nm and threshold pumping energy of 270nJ. The laser has a high characteristic temperature of about 278K and high spontaneous emission coupling factor of 10−2. The laser emission showed single and multiple spot emission patterns with spectral and spatial variations under different pumping conditions.