ACTIVITIES
Nanophotonic Quantum Well LEDs: Analysis of Light Extraction Efficiency in GaN-Based
Coaxial Microwall Light Emitting Diodes.
REU Student: Michael Wabaunsee
Graduate Mentor: Mohsen Nami, Ashwin Rishinaramangalam
Faculty Mentor: Dr. Daniel Feezell
Introduction
This research project is aimed at the study of improvement of external quantum efficiency
(EQE) in gallium nitride (GaN) based light emitting diodes (LEDs). The EQE of an LED, ηe can
be represented as follows: ηe = ηi × ηext
where, ηi is the internal quantum efficiency (IQE) and ηext represents the extraction
efficiency. IQE is defined as the percentage of photons emitted for every electron-hole pair
recombining in the active region of an LED. IQE of an LED depends on the material parameters
like threading defect density, extent of electron-hole wavefunction overlap and material
composition of the active region. The extraction efficiency depends on the effective transmission
of the photon generated in the quantum well active region of an LED into the epoxy enclosing it.
The research in this project is aimed at studying the improvement of the light extraction
efficiency of LEDs using techniques employed in industry like surface roughening and also
design based techniques by modifying the LED architecture from a planar to a three-dimensional
coaxial structure. This study has been conducted using simulation software (LightTools®),
which employs ray tracing through predefined materials to estimate the percentage of photons
generated within the LED active region that are extracted to the outside.
.
Background
An LED is a semiconductor based light source that are used as indicator lamps in many devices
and are increasingly being used for general lighting purposes too. Gallium nitride material
system has been efficiently used for the manufacture of LEDs since 1990. The alloy of GaN with
indium (InGaN) or aluminum (AlGaN) with a band gap dependent on ratio of In or Al to GaN
allows the manufacture of light-emitting diodes (LEDs) with colors that can go from red to ultraviolet.
+ve contact
p-GaN
InGaN/GaN
QWs
n-GaN
-ve contact
Figure 1. A cross-section of a blue GaN based planar light emitting diode indicating the three
regions of any semiconductor light emitter.
Any LED has three major regions, as shown in figure 1, a positive contact (also called the
p-side), a negative contact (also called n-side) and a light-emitting region (also called active
region or quantum well(s) region). The first high-brightness blue LED was demonstrated by Dr.
Shuji Nakamura of Nichia Corporation in 1994. The LED efficiency was measured ~15% in
1994. One major reason for this low LED EQE was the absence of efficient extraction of emitted
light.
In most cases, the light from an LED is coupled into a much lower-index medium like
epoxy. The refractive index of most LED semiconductor materials is higher than air or epoxy
(2.5 vs. 1.5 epoxy or 1 for free space). This large index difference makes the reflection of
emitted light quite substantial. The light generated in the active region gets partially reflected
back into the semiconductor, where it may be absorbed and turned into heat energy, being one of
the dominant causes of LED inefficiency. Another portion of emitted light is reflected back at the
LED-package and package-air interfaces.
A few conventional techniques to enhance extraction efficiency of LEDs include the
roughening of the top surface, use of a reflector on the bottom LED surface, using a domeshaped (half-sphere) package with the LED in the center to help outgoing light rays strike the
surface perpendicularly, and the use of a transparent top metal contact to decrease light
absorption in the contact layers.
In this project, we are studying the enhancement of light extraction efficiency by means
of modifying the LED architecture (using a coaxial LED design, as shown in Figure 2) and the
possibility of surface texturing on the coaxial structures to improve extraction.
active
region
Figure 2. A cross-section of a coaxial GaN based LED showing the n-side, p-side and the active
region
Methodology
The light extraction efficiency for an LED depends on many parameters:
 The optical properties of the different compounds. Including index of refraction and
absorption coefficient.
 Geometrical Properties of the microwall structure including length, aspect ratio
(height/width), number of adjacent microwalls, thickness of different layers and shape of
epoxy dome.
 Surface property of the sapphire substrate and ITO.
Description of Simulation:
We started with a simple simulation by putting a point light source in a bulk GaN surrounded by
air shown in figure 3.
Figure 3. A cube of GaN with a point source in the middle
Next we simulated the same Structure encased in epoxy. The simple simulation gives an
approximate result that can be compared to actual results. The simulation of the model showed
very close results that would be in agreement with theoretical results. After the results from the
first simulation were in close proximity, it was possible to move to other complicated models.
The first model was the Flip-chip Planar LED (shown in figure 4).
n-side contact
+
InGaN/GaN
QW
p-side
contact
n GaN
p GaN
silver
Silicon Substrate
Figure 4. A cross-section of a Flip-chip Planer LED
This structure required more variables and more structured layers. The first variable is the
thickness of InGaN material and textures. The refractive indexes of the materials are constant.
The refractive index for GaN is 2.5 and for InGaN 2.7. Also is another varied setting is the type
of light source to be used for the simulation. There are three settings in LightTools for the light
source. In all the simulations the volume source option is used. Simulations were conducted
using various light source thickness and various GaN thicknesses. Also simulations were
conducted to obtain light extraction efficiency for different lengths and texture. The next model
was the Coaxial Micro structured LED (show in figure 5).
Figure 5. A cross-section Coaxial Microstructured LED
This model is closer to the actual model that will be the finished product. The structure of this
model is more complex and has more optical properties applied to the simulation. This model
used different sizes of coaxial micro-wall, different styles of texturing, and the height was also
varied in the simulations. The behavior between the extraction efficiency and various
geometrical properties of the Micro-walls where studied. The aspect ratio, length and the relation
between extraction efficiency and optical properties like absorption coefficient of the Indium Tin
Oxide (ITO) were also examined. The ITO layer is located on the surface area of the structure.
The simulation showed how ITO absorption could affect the extraction efficiency for different
length of Micro-walls. It also demonstrated that the texturing of ITO could increase the
extraction efficiency to a greater extent.