Che-Yu Liu

High efficiency GaN-based light-emitting diodes with embedded air voids/SiO2 nanomasks

We fabricated high efficiency LEDs with embedded micro-scale air voids and SiO2 nanomask exhibit smaller reverse-bias current and great enhancement of the light output (65% at 20mA) compared with the conventional LEDs.

Improvement of emission uniformity by using micro-cone patterned PDMS film

Micro-patterned PDMS film was fabricated and combined with LED chip on board (COB) package to improve the emission uniformity of LED chip. The micro scale patterned sapphire substrate (PSS) was used as a mold to fabricate micro-cone patterned PDMS (MC-PDMS) film. A strong scattering effect from this MC-PDMS film can be verified by the high haze ratio and the Bi-directional Transmission effect. The angle dependent color temperature measurement system was used to measure the ΔCCT of COB with and without MC-PDMS. The measurement results indicate that the ΔCCT was reduced from 1025K to 428K. This improvement can effectively eliminate the yellow ring effect of LED chip. This technology can be thus considered as a cost-effective way for the next generation of light source packages.

Efficiency and droop improvement in green InGaN/GaN light-emitting diodes on GaN nanorods template with SiO2 nanomasks

This study presents the green InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) grown on a GaN nanorods template with SiO2 nanomasks by metal–organic chemical vapor deposition. By nanoscale epitaxial lateral overgrowth, microscale air voids were formed between nanorods and the threading dislocations were efficiently suppressed. The electroluminescence measurement reveals that the LEDs on nanorods template with SiO2 nanomasks suffer less quantum-confined Stark effect and exhibit higher light output power and lower efficiency droop at a high injection current as compared with conventional LEDs.

Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Crown-Shaped Patterned Sapphire Substrates

In this letter, we report the high performance GaN-based light-emitting diodes (LEDs) with embedded air void array grown by metal-organic chemical vapor deposition. The donut-shaped air void was formed at the interface between crown-shaped patterned sapphire substrates (CPSS) and the GaN epilayer by conventional photolithography. The transmission electron microscopy images demonstrate that the threading dislocations were significantly suppressed by epitaxial lateral overgrowth (ELOG). The Monte Carlo ray-tracing simulation reveals that the light extraction of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.

Collective Lasing Behavior of Monolithic GaN–InGaN Core–Shell Nanorod Lattice under Room Temperature

We demonstrated a monolithic GaN-InGaN core-shell nanorod lattice lasing under room temperature. The threshold pumping density was as low as 140 kW/cm2 with a quality factor as high as 1940. The narrow mode spacing between lasing peaks suggested a strong coupling between adjacent whisper gallery modes (WGM), which was confirmed with the far-field patterns. Excitation area dependent photoluminescence revealed that the long-wavelength lasing modes dominated the collective lasing behavior under a large excitation area. The excitation-area-dependent lasing behavior resulted from the prominent optical coupling among rods. According to the optical mode simulations and truncated-rod experiments, we confirmed that the fine-splitting of lasing peaks originated from the coupled supermodes existing in the periodic nanorod lattices. With wavelength-tunable active materials and a wafer-level scalable processing, patterning optically coupled GaN-InGaN core-shell nanorods is a highly practical approach for building various on-chip optical components including emitters and coupled resonator waveguides in visible and ultraviolet spectral range.

High-Performance Ultraviolet 385-nm GaN-Based LEDs With Embedded Nanoscale Air Voids Produced Through Atomic Layer Deposition and Al 2 O 3 Passivation

This letter developed the high-performance GaN-based light-emitting diodes (LEDs) for ultraviolet (UV) light emission at 385 nm with the light output power enhanced by up to 52% compared with that of the conventional UV-LEDs. The high-power-efficiency UV-LEDs include sidewall-passivated Al2O3 dielectric nanoscale air voids, which were created using the atomic layer deposition method and act as intermediate media for improving the internal quantum efficiency, reducing threading dislocation, and relaxing strain. The fabricated air void nanostructure also enhanced the light extraction efficiency by ~32.7%, resulting in a high-power UV light-emitting device.

Color-conversion efficiency enhancement of quantum dots via selective area nano-rods light-emitting diodes

A large enhancement of color-conversion efficiency of colloidal quantum dots in light-emitting diodes (LEDs) with novel structures of nanorods embedded in microholes has been demonstrated. Via the integration of nano-imprint and photolithography technologies, nanorods structures can be fabricated at specific locations, generating functional nanostructured LEDs for high-efficiency performance. With the novel structured LED, the color-conversion efficiency of the existing quantum dots can be enhanced by up to 32.4%. The underlying mechanisms can be attributed to the enhanced light extraction and non-radiative energy transfer, characterized by conducting a series of electroluminescence and time-resolved photoluminescence measurements. This hybrid nanostructured device therefore exhibits a great potential for the application of multi-color lighting sources.

Highly efficient light extraction GaN-based light emitting diode with nano-rods in micro-holes compound structure

We demonstrated a high-power GaN-based light emitting diodes (LEDs) which have micro-hole array and nano-rods compound structure by nanoimprint lithography (NIL). The nanorods structure inside the micro-hole could efficiently guide the trapped light from the GaN epilayer. Therefore, the light output power of LED with micro-hole array and nanorods was as high as 1.27 times, as compared with standard LED.

Highly efficient InGaN-based LED with embedded cubic airvoids

Highly efficient InGaN-based LEDs with embedded sidewall passivation cubic airvoids made by nanoimprint lithography were demonstrated. The LEDs with embedded airvoids exhibit a 45% enhancement of light output at 20 mA compared with conventional LEDs.

Effect of InGaN/GaN multiple quantum wells with p-n quantum barriers on efficiency droop in blue light-emitting diodes

In this study, the structures of InGaN/GaN multiple quantum wells (MQWs) with p-n quantum barriers in various positions were proposed to investigate the efficiency droop behavior for blue light-emitting diodes (LEDs). The simulated electric field diagrams showed that the quantum well (QW) sandwiched by the p-n quantum barriers had a less electric field than the other QWs due to the original polarization-related electric field was partially balanced off by the built-in electric field of the p-n quantum barriers. In addition, the simulation results demonstrated that by selecting suitable position of p-n quantum barriers, the distribution of carriers could be effectively improved; hence the droop behavior could also be suppressed.