We study nano-scale ITO top transmission gratings to improve light extraction efficiency (LEE).
We use the finite difference time domain (FDTD) method to measure light extraction from a
device with various grating structures and layer thicknesses. We simulate our device using a twodimensional
model with top triangular-gratings in a crystal lattice arrangement described by
grating cell period (Α), grating cell height (d), and grating cell width (w). We also define ITO
layer thickness (L) as the layer between the p-type GaN and the ITO surface layers. Simulation
models vary in grating period, grating width, and ITO layer thickness. Our simulations monitor
the amount of light emitted from the top, bottom, and sides of the LED model. We calculate the
total light extraction and determine which grating duty cycle maximizes LEE. We found that
adding a nano-scale grating with optimum duty cycle can achieve 165.67% and 136.77% LEE
improvement, respectively, for ITO layer thickness of 230nm and 78nm.
We study nano-grated surface GaN LED to improve light extraction efficiency by optimizing the device parameters. Our study is based on rigorous coupled wave analysis (RCWA) to obtain total transmission across a device. Our simulation results allow us to optimize the device parameters to maximize light extraction efficiency. We simulate our device using a two-dimensional model with square-grating cells in a crystal lattice arrangement whose parameters we define as follows: grating cell period (Λ), grating cell height (d), and grating cell width (ω). We also define grating layer location (L) as the distance between the multi-quantum wells (MQW) source and the grating surface layer. Each simulation varies in grating cell period, grating cell width, and grating layer location and provides a result of total transmission across the device. These results are used to calculate improvement over the non-grated surface GaN LED. Our preliminary study focused on 50% fill factor and showed that location of the grating as well as the grating period both strongly effect the total transmission across the device. In addition, we noticed that optimizing the surface grating location might affect the total transmission. Our study allowed us to improve the light extraction efficiency of nano-grated GaN LED by an average of 133% when fill factor is 50%. We also present our study in detail which includes fill factors ranging between 0 to 100%.
Today’s advanced technology allows engineers to fabricate GaN LEDs with various heights, widths, shapes, and materials. Total internal reflection is a key factor in GaN LED design, because all light that is created inside the LED is lost unless it approaches the chip to air interface at an angle less than 23.58° with respect to the normal. The narrow range of angles at which light can successfully escape the chip is a result of the large difference in refractive indices between GaN and air. Adding a layer of ITO to the GaN reduces the difference in refractive indices between steps and increases the critical angle to 28.4°. Transmitting from ITO into epoxy reduces this difference in refractive indices again, bringing the critical angle to 47.9°. Because a higher critical angle should allow more light to escape the LED, we focus on enhancing light extraction efficiency of GaN LED's that utilize an ITO to epoxy interface using FDTD simulations. The simulation results show us that increasing the critical angle to 47.9° improves light extraction by 40%, proving that the critical angle does play a significant role in light extraction. From this initial result, we then compare light extraction efficiencies of ITO and GaN gratings over varied grating periods, and show that adding an Ag reflection layer improves overall efficiency. Finally, we show that the light extraction for LED's utilizing an Ag reflection layer is highly dependent on the sapphire substrate thickness.
This study analyzes optical confinement factor and light emitting mode order for three different GaN LEDs:
a conventional LED, thin Film LED, and thin Film LED with a photonic crystal (PhC) grating. For the first
structure, we increase the thickness of AlxGa1-xN from 0 to 600nm, alter the x composition in AlxGa1-xN
from 0.05 to 0.2 in steps of 0.05, and adjust the p-GaN and n-GaN thicknesses each from 0 to 200nm. For
the second structure, we alter the n-GaN substrate thickness from 300-1000nm in steps of 100nm and 1000-
4000nm in steps of 1000nm. These simulations show that increasing the substrate thickness causes the light
emitting mode order to increase. The higher the mode, the more current is needed to make the device emit
light. Higher current leads to shorter device lifetime. The last structure contains a photonic crystal grating
with a period T = 100nm, 230nm, 460nm, 690nm, 920nm, 1500nm, 2000nm, 3000nm and 50% duty cycle.
For each grating period, we display the effects on optical confinement factor and optical field intensity. The
results show that changing the grating period does not affect the mode order, but does affect the optical
field intensity. A larger grating period corresponds to lower optical field intensity. Maximizing optical field
intensity increases the brightness of the device. The simulation method above can be used to improve the
efficiency, brightness, and lifetime of GaN LEDs by reducing the effects of transverse mode coupling and
maximizing the optical field intensity.
In this paper, we use a Finite-Difference Time-Domain GaN LED model to study constant wave (CW)
average power of extracted light. The structure simulated comprises of a 200nm-thick p-GaN substrate,
50nm-thick MQW, 400nm-thick n-GaN substrate, and a 200nm n-GaN two-dimensional Photonic
Crystal(2PhC) grating. We focus on optimizing three design parameters: grating period (A), grating height
(d), and fill factor (FF). In the primary set of simulations, we fix the fill factor at 50% and simulate ten
different grating periods (100 to 1000nm in steps of 100nm) and four different grating heights (50 to
200nm in steps of 50nm), and calculate the average power output of the device. The results from these
simulations show that for both conical and cylindrical gratings, the maxmium light extraction improvement
occurs when A =100nm. In the second set of simulations, we maintain a constant grating period A = 100nm
and sweep the fill factor from 25 to 75%. The results of these simulations show that the fill factor affects
clyindrical and conical gratings differently. As a whole, we see that the nano-structure grating mostly
depends on period, but also depends on height and fill factor. The grating structure improves light
extraction in some cases, but not all.
We study the top transmission grating's improvement on GaN LED light extraction efficiency. We use the finite
difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell's equations, to measure
light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define
materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy
p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit
cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell
height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We
also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating
has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection
grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a
transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed
structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes
in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.
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.
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.
Optical modes of AlGaInP laser diodes with real refractive index guided self-aligned (RISA) structure were analyzed theoretically on the basis of two-dimension semivectorial finite-difference methods (SV-FDMs) and the computed simulation results were presented. The eigenvalue and eigenfunction of this two-dimension waveguide were obtained and the dependence of the confinement factor and beam divergence angles in the direction of parallel and perpendicular to the pn junction on the structure parameters such as the number of quantum wells, the Al composition of the cladding layers, the ridge width, the waveguide thickness and the residual thickness of the upper P-cladding layer were investigated. The results can provide optimized structure parameters and help us design and fabricate high performance AlGaInP laser diodes with a low beam aspect ratio required for optical storage applications.