Kui Bao

Improvement of Light Extraction From Patterned Polymer Encapsulated GaN-Based Flip-Chip Light-Emitting Diodes by Imprinting

To improve surface light extraction of GaN-based flip-chip light-emitting diodes (FC-LEDs), we employed an imprint approach of thermosetting polymer for patterning microscale surface grating on the polymer encapsulant. One-dimensional (1-D) and two-dimensional (2-D) taper-like polymer gratings with a period of 6 mum were successfully realized on encapsulant above the sapphire backplane of GaN LED. By adopting the 1-D and 2-D taper-like grating encapsulant, the improvement of light extraction from the 1 mm times 1 mm FC-blue LED with a reflective Ag film on the p-side was about 18.5% and 31.9% compared to the LED encapsulated by flat polymer, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on 1-D rigorous coupled wave analysis with the supporting experiments.

Improvement of light extraction from GaN-based thin-film light-emitting diodes by patterning undoped GaN using modified laser lift-off

To improve light extraction from GaN-based light-emitting diodes (LEDs), we demonstrated an approach of modified laser lift-off (M-LLO) technique for patterning undoped GaN (u-GaN). The M-LLO consists in sequentially forming a two-dimensional triangular lattice pattern with a 4μm period on a polymer layer over a sapphire substrate backplane by UV imprint and delivering the pattern onto u-GaN accompanied with the removal of the sapphire substrate. The enhancement of light extraction from GaN-based M-LLO LED with a reflective Ag film on the 120nm deep u-GaN pattern was about 31% and 100% compared to that of a LLO-LED with a reflective film and a conventional LED with a sapphire substrate, respectively.

Light Extraction Improvement From GaN-Based Light-Emitting Diodes With Nano-Patterned Surface Using Anodic Aluminum Oxide Template

An improvement of light extraction from GaN-based light-emitting diodes (LEDs) was demonstrated by forming surface nano-structures using an anodic aluminum oxide (AAO) template as a dry etching mask. Nano-pores were simultaneously formed on indium tin oxide and GaN surfaces of LED with different pore depths by the same procedural steps, with a pore density and pore diameter of 1.1t109/cm2 and 163 plusmn 31 nm, respectively. The nano-patterned LEDs achieved a top-face light output power enhancemetop-face light output power enhancementnt of 72% compared to the conventional LEDs at 20 mA. The light extraction effects of the nano-structures on the trapped optical modes in LED were revealed by scanning electron microscopy and microscopic electroluminescence.

Light extraction improvement of GaN-based light-emitting diodes using patterned undoped GaN bottom reflection gratings

The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and compared with the non-grating (flat) case. A time monitor was also placed just above the top of the LED to measure both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three parameters that affected the structure of the micro-structure grating: unit cell period (Α) from 1 to 6 microns, unit cell width (w) from 1 to 6 microns, and unit cell grating height (d) from 50 to 200nm. The simulation results show that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109% light extraction improvement compared to the flat plate case.

Improvement of light extraction from micro-pattern encapsulated GaN-based LED by imprinting

We have demonstrated an improvement of light extraction from GaN based flip-chip LEDs by patterning encapsulant. Two dimensional (2D) micron-scale patterns of encapsulant were realized by using imprint technique of thermosetting polymer. This approach has several advantages such as technical simplification, low cost and freedom of material choice. In this work, we fabricated 2D micron-scale patterns with the triangular or sinusoidal profiles on the polymer encapsulated GaN-based flip-chip LEDs. The enhancement factors of light extraction of GaN LEDs with the patterned encapsulant comparing to the flat encapsulated LEDs are about 32% and 47% corresponding to the triangular and sinusoidal profiles, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on simplified one-dimensional (1D) rigorous coupled wave analysis (RCWA). The calculation reveals that the grating of sinusoidal profile has greater transmittance than that of triangular profile which is in the same trend with the experimental results. These results provide a guideline for improvement of the LED light extraction.

GaN-based light emitting diodes with large area surface nano-structures patterned by anodic aluminum oxide templates

We demonstrate a manufacturing approach of nanostructures on the large surface area of GaN-based LED chip to improve the light extraction efficiency. We prepared the nanoporous anodic aluminum oxide (AAO) template on an aluminum foil by the conventional two-step anodization. Using the AAO template as etching mask, we successfully transferred the nanoporous structures to the surfaces of GaN-based LEDs by inductively coupled plasma dry etching. About a quarter of two-inch GaN-based LED chip was patterned by the nanostructures. The pore spacing was modulated from 100 nm to 400 nm. The improvement of light extraction efficiency of the device was achieved. A light output power enhancement of 42% was obtained from the p-side surface nanopatterned LEDs compared to the conventional LEDs on the same wafer at 20 mA. This approach offers a potential technique of nanostructures fabrication on GaNbased LEDs with the advantages of large area, rapid process and low cost.

Light output enhancement of a GaN-based light emitting diode by polymer film imprinting

We report a simple and inexpensive method to enhance the light output efficiency of a GaN-based light-emitting diode (LED). The method employs polydimethylsiloxane (PDMS) films prepared by nanoimprinting on the surface. Two kinds of PDMS films were prepared: one without a pattern and the other with a triangular pattern. After covering with a PDMS film, the light output was increased by about 25% for the LED with a no-pattern film, and about 33% for that with a triangular pattern film. These results can be explained by the surface roughening between the PDMS/air interfaces, which give more opportunity for photons generated in the LED active layer to escape. The research suggests that it is applicable to prepare patterned films by a nanoimprint technique to improve the light output of a GaN-based LED.