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 (<i>Α</i>) from 1 to 6 microns, unit
cell width (<i>w</i>) from 1 to 6 microns, and unit cell grating height (<i>d</i>) 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.
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.
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.