LED_Final_Paper

Light-Emitting
Diodes
Group 1
NMT Program
3 August 2015
Alex Galli
Rashad Farrakhan
Will Shelton
Chris Bowser
Matthew Rosenwasser
I declare that I have produced signiﬁcant and fair contribution to this project and
have earned the right to have my name on this report.
Alex Galli
Rashad Farrakhan
Will Shelton
Chris Bowser
Matthew Rosenwasser

(a) Matthew Rosenwasser (b) Rashad Farrakhan
(c) Chris Bowser
(d) Will Shelton (e) Alex Galli
Figure 1: Group 1
1

Table of Contents
1 Introduction 4
2 Comparison of LED, Incandescent, and Fluorescent Light Sources 4
2.1 Incandescent Lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Fluorescent Lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Light–emitting Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Compound Semiconductors in LED Technology 10
4 Metal Organic Chemical Vapor Deposition 14
4.1 MOCVD Fabrication Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 Gallium Nitride Nanowire Properties . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 RGB LEDs and Applications 17
6 Limitations of LEDs 22
7 Fabrication Process 25
7.1 Substrate Selection and Clean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.2 Substrate Preparation for GaN Nanowire Growth . . . . . . . . . . . . . . . . . . 27
7.3 Nanowire Growth using MOCVD Fabrication . . . . . . . . . . . . . . . . . . . . 29
7.4 SEM Imaging After Nanowire Growth . . . . . . . . . . . . . . . . . . . . . . . . 32
7.5 X-Ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.6 Polyimide Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.7 Polyimide Etch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7.8 Transparent Conducting Oxide Deposition . . . . . . . . . . . . . . . . . . . . . . 39
7.9 ITO/Polyimide Transmittance Test . . . . . . . . . . . . . . . . . . . . . . . . . . 44
7.10 AZO Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.11 Deposition of Metal Contact Electrodes . . . . . . . . . . . . . . . . . . . . . . . 47
7.12 Photoluminescence Spectroscopy Test of Completed Device . . . . . . . . . . . . 48
8 Conclusion 49
Group Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Glossary 51
Index 53
References 57
Feasibility Report 61
List of Figures
1 Group Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2

2 Depletion Zone Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Comparison of Different Light Sources . . . . . . . . . . . . . . . . . . . . . . . . 9
4 Periodic Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5 Circuit Diagram of an LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6 Band gap vs. Doping Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7 Growth and Fabrication of Nanowires . . . . . . . . . . . . . . . . . . . . . . . . 15
8 GaN Growth Process and Characteristics . . . . . . . . . . . . . . . . . . . . . . . 16
9 Spectra of GaN NWs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10 Patient Attached Unit for Phototherapy . . . . . . . . . . . . . . . . . . . . . . . . 19
11 LEDT Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12 Assembly of an SPL sytem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
13 Junction Temperature effects over Time . . . . . . . . . . . . . . . . . . . . . . . 22
14 Current Injection in an LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
15 Arrangement of Phosphors in an LED . . . . . . . . . . . . . . . . . . . . . . . . 24
16 Technics PE II-A Plasma System . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
17 Schematic of Patterned Mask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
18 Growth of GaN Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
19 GaN NWs with Various Injection Times of Ga . . . . . . . . . . . . . . . . . . . . 30
20 GaN NWs with Various Injection Times of N . . . . . . . . . . . . . . . . . . . . 31
21 Current-Voltage Curves of GaN p-n junction . . . . . . . . . . . . . . . . . . . . . 32
22 SEM Image of GaN Ensemble . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
23 X-Ray Photoelectron Spectroscopic Data for GaN NWs . . . . . . . . . . . . . . . 35
24 Synthesis of Kapton Polyimide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
25 YES-PB-HV Series High Vacuum Oven . . . . . . . . . . . . . . . . . . . . . . . 37
26 Trikon Omega 201 Asher and Etch Tool . . . . . . . . . . . . . . . . . . . . . . . 38
27 Resistivity of ITO v. Substrate Temperature . . . . . . . . . . . . . . . . . . . . . 40
28 Resistivity vs. Ar Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
29 Transmittance of ITO as a Function of Wavelength . . . . . . . . . . . . . . . . . 42
30 Absorption Coefﬁcient of ITO for Varying Oxygen Vacancies . . . . . . . . . . . . 43
31 Optical Transmittance of Polyimide . . . . . . . . . . . . . . . . . . . . . . . . . 44
32 Effect of Annealing on the Transmittance of ITO . . . . . . . . . . . . . . . . . . 45
33 Transmittance of 24:1 Zn:Al AZO Sample on Glass . . . . . . . . . . . . . . . . . 46
34 Flow Diagram for the Entire Fabrication Process . . . . . . . . . . . . . . . . . . . 48
List of Tables
1 Comparison of Incandescent, Fluorescent, and LED lights . . . . . . . . . . . . . . 9
2 Compound Semiconductor Table . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 ITO Recipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4 AZO Lab Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5 Grade Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3

1 Introduction
As society advances in technology and energy consumption increases, it becomes more impor-
tant to improve upon the efficiency and cost of light sources used across the world. Incandescent
light sources have been used for over a hundred years since their invention in the 1800s, but in-
candescent light sources are inefficient, hazardous to the environment, and have short life spans.
Fluorescent light sources have since replaced incandescent light bulbs as a major source of light-
ing, and even more efficient technology is emerging that has the potential to replace the use of
incandescent and fluorescent light sources. Light-emitting diodes (LEDs) are an example of a new
lighting source capable of replacing incandescent and fluorescent light sources in a number of ap-
plications. LEDs have been around for over a century, but research has only started to advance
significantly in the past couple decades. An example of this is the recent Nobel Prize recipients
who invented the first high-efficiency blue LED. The focus of current research is to produce LEDs
with higher efficiencies using novel nanofabrication techniques, such as the use of quantum wells
and nanowires.
The objective of this paper is to compare LEDs to other commonly used light sources, the
working principle of their function, their applications in society, and novel fabrication methods that
have potential to be used in industry. A detailed fabrication method of an InGaN/GaN nanowire
LED will be proposed and its potential for success will be evaluated. An analysis of an LED’s cost
of ownership will be evaluated to determine its viability in the current market.
2 Comparison of LED, Incandescent, and Fluorescent Light
Sources
2.1 Incandescent Lights
Incandescent light bulbs are electrical lights that produce light from incandescence. Although
often thought to be invented by Thomas Edison incandescent lighting and light bulbs were only
refined by him. His “version was able to outstrip the others because of a combination of three
factors: an effective incandescent material, a higher vacuum than others were able to achieve (by
4

use of the Sprengel pump) and a high resistance that made power distribution from a centralized
source economically viable” [1]. The typical operation of these light bulbs involves the use of a
tungsten filament encased within a bulb filled with an inert gas such as argon, which is then heated
up to a high enough temperature that it emits electromagnetic radiation in the visible spectrum.
The filament is typically heated up to a temperature of about 2,000 to 3,300 °F, and is heated up
by a current running through the filament [1]. The purpose of the inert gas is to keep the filament
from evaporating or oxidizing.
Incandescence is a very ineffective means of producing light. This is due to the fact that during
this process a continuous spectrum of light is emitted, but most of this energy is given off as heat
because most of the wavelengths are in the near-infrared range. Only 10 to 15 percent of the energy
used by incandescent bulbs are actually emitted as light; the other 85 to 90 percent is emitted as
heat [2]. The efficiency of incandescent light bulbs is very low (6 to 20 lumens per watt), which is
much lower than newer more efficient forms of light such as fluorescent lamps or LEDs [2]. The
battery life is also very low in comparison to other newer forms of light emission. The typical
incandescent bulb doesn’t last longer than about 1,000 hours, so they have to be replaced often and
are very inefficient in their power consumption. Standard incandescent bulbs actually are banned
from being used in most because of the Energy Independence and Security Act of 2007 which
raised efficiency standards past what most standard incandescent bulbs can produce. Their main
advantage is their cheap upfront cost, but this does not offset the many disadvantages this light
source has.
2.2 Fluorescent Lights
Fluorescent light sources use the process of fluorescence to produce light in the visible spec-
trum. A fluorescent lamp tube is evacuated and filled with an inert gas to about .3% atmospheric
pressure [3]. A tungsten filament in the tube “coated with a mixture of barium, strontium and
calcium oxides” has a current applied to it and emits electrons through the process of thermionic
emission [4]. A plasma is formed inside the fluorescent tube because these free electrons emitted
from the cathodes (tungsten filaments) ionize some atoms of the inert gas. The plasma increases
the current in the system. As the free electrons ionize the inert gas atoms, these ions and elec-
5

trons cause a snowball effect of collisions and thus the release of electrons, so the plasma would
inevitably increase the current to such high levels the bulbs would fail. This is avoided by adding
a ballast which limits the current in the system so this does not occur.
The fluorescent tube has trace amounts of mercury. On average a four foot long fluorescent
bulb contains about 20 mg of mercury [5]. The plasma vaporizes the mercury and the free electrons
(from both the cathodes and plasma) get into inelastic collisions with the outer electrons of mercury
atoms. Enough kinetic energy is transferred so the electrons jump to higher levels. This is only for a
short time because this higher energy state is unstable and when the mercury electrons regress back
to their lower energy state an ultraviolet (UV) photon is given off. Even excited inert gas atoms
can transfer enough energy to the mercury and ionize it causing this electron jump and regression.
The tube has a phosphor coating which absorbs the UV photons, causing the same excitation to the
phosphor coating electrons. The excited electron quickly reverts back to a lower state of energy
and now gives off a photon in the visible light range. This phosphor coating material is selected to
have photons that are emitted in the visible light range. The extra difference between the energy
absorbed by the UV photon and the energy emitted by the visible light photon goes towards heating
the phosphor coating [4].
Fluorescent lights are a more efficient light source than standard incandescent lights. Straight
tube fluorescents have an efficacy 65-110 lumens per watt while compact fluorescent lamps (CFLs)
can have an efficacy of 33-70 lumens per watt [6]. The lives of fluorescent lights are also much
greater than that of the traditional incandescent lighting. These lifetimes can range from 7,000 to
24,000 hours [6]. Fluorescent lamps also do not give off heat and have a more even light distri-
bution, saving the consumer money in the long run because they use almost 75% less energy than
incandescent lights and can last 10 times longer, so the higher initial cost is offset throughout the
lifetime of the bulb. This type of lighting also has its disadvantages though. One of the most obvi-
ous ones is the use of mercury which is a hazardous material, so fluorescent lights are considered
hazardous waste and need to be disposed of in an appropriate manner. A major drawback is that
if they are used in an area where they will be turned on and off many times the life of the bulb
can be dramatically shortened, which makes them problematic in places like common household
bathrooms where the lights are turned on and off frequently. Fluorescent lights typically don’t
work at temperatures below freezing and can have a delayed start. Lights using magnetic ballasts
6

can flicker at higher frequencies, so most people do not notice, but it can be a problem for people
who are sensitive to light such as epileptics or people with vertigo.
2.3 Light–emitting Diodes
Light-emitting diodes are two lead semiconductors that create light by the process of electro-
luminescence. An LED is manufactured like a typical semiconductor device. There is typically a
sapphire substrate, with a semiconductor layer on top of it. Then epitaxial layers are deposited on
top of this. These layers are doped with p and n impurities, which is vital to the proper functioning
of an LED. Finally a patterned metal layer is deposited on top with two leads. The semiconductor
material is typically a compound material usually including gallium. The material is doped to have
p and n regions. The n regions in the semiconductor material have an excess of electrons and the
p regions have excess holes. When there is no voltage applied to these layers, electrons from the
n region will fill holes in the p region creating a depletion zone where no current can be flown.
Figure 2 gives a more concrete idea of what the depletion zone is.
Figure 2: Graphic of the depletion zone [7].
When a negative voltage is applied to the n region and a positive voltage to the p region the elec-
7

trons in the n region are repelled by the negative voltage, so they move towards the positive bias.
While this happens the holes are repelled by the positive bias and move toward the negative bias.
“When the voltage difference between the electrodes is high enough, the electrons in the depletion
zone are boosted out of their holes and begin moving freely again. The depletion zone disappears,
and charge moves” [7] across the LED. With this current flow the electrons and holes recombine.
When they recombine the electrons which were originally in a higher energy state drop into a
lower energy state and this difference in energy is given off as a photon. This difference in energy
is called the band gap of the material, and the size of this band gap is an intrinsic material property
and determines the wavelength of the photon emitted, thus altering the color of it.
LEDs offer many advantages compared to fluorescent and incandescent lighting sources. The
typical cool white LED produces 60-94 lumens per watt, while the warm white LED produces
27-88 lumens per watt. This is around the same efficiency as fluorescent bulbs, but the efficiency
of LEds is not altered by changes in shape or size. Since the color emitted is a material property,
coatings are needed to achieve different light colors which increases efficiency and cost. Cycling
does not alter the LED life, which is on average 25,000 hours and can be up to 50,000 hours
[6]. Also LED failure is a long process and typically does not suffer from abrupt failures like
incandescent and fluorescent lights. Also LEDs are much more resistant than typical light bulbs.
They are not made of glass, so they do not break as easily and are resistant to external shock
damage because they are solid-state components. The major disadvantage of LEDs, though, is
there high initial cost, but this situation is similar to that of fluorescent lights. With the savings in
electricity the LED will save the consumer in the long run. LEDs also must have a specific current
and voltage applied at a constant rate or life is drastically degraded. Also ambient temperature is
a major problem. Overdriving the LED in a high ambient pressure can cause the device to fail, so
a proper heat sink is needed to avoid this. Also increasing the current in an LED will decrease the
life, which makes it not as useful in high power applications [8].
As science has grown and advanced, light sources have become more efficient, longer lasting,
and less hazardous to the environment. One of the newest and most influential light sources, light
emitting diodes, have many advantages over older more inefficient light sources such as incandes-
cent and fluorescent lights. One of these advantages is the decrease in environmental impact of
these newer technologies. In Figure 3 the spider graph shows the impact of fluorescent and LED
8

lights as compared to incandescent.
Figure 3: Comparison of the Environmental Impacts of Various Light Sources [9].
These three light sources all have more advantageous properties that make them suited for certain
applications. In Table 1 the major properties of incandescent, ﬂuorescent, and LED lights will be
laid out.
Lighting
Type
Efﬁcacy
(lm/watt)
Lifetime
(hrs)
Indoor/
Outdoors
Watts of
electricity used
(equivalent to 60
W bulb)
CO2
Emissions (30
bulbs/yr)
Heat
Emitted
(btu’s/hr)
Compact
Fluorescent
Lamps
65-70 10,000 Both 13-15 1051 lbs./year 30
Incandescent
Standard
“A-19”
10-17 750-2,000 Both 60 4500 lbs/yr 85
LEDs 60-94 25,000-
50,000
Both 6-8 451 lbs./yr 3.4
Table 1: Comparison of Incandescent, Fluorescent, and LED lights [6,10].
9

3 Compound Semiconductors in LED Technology
A semiconductor material has an electrical conductivity value falling between that of a con-
ductor (i.e. copper) and an insulator (i.e. glass) [11]. A semiconductor has a small energy gap
that is a value between an insulator and a conductor [12]. This energy gap permits the electrons
to jump from the valence band into the conduction band when energy is supplied. This action oc-
curs when a semiconductor is heated, thus improving conductivity with an increase in temperature,
which is the opposite condition for a conductor [12]. Semiconductors are important because their
conductivity can be varied simply by doping the material to make it p-type or n-type. When a semi-
conductor is p-type, it is doped with an acceptor impurity atom that has fewer valence electrons
than the atoms they replace. This provides excess holes in the semiconductor. When a semicon-
ductor is n-type, it is doped with a donor impurity atom that has more valence electrons than the
atoms they replace. Donor impurities donate their extra valence electrons to the semiconductor’s
conduction band, providing excess electrons to the semiconductor. Semiconducting materials exist
in two types: elemental materials and compound materials.
A compound semiconductor is composed of elements from two or more different semiconduc-
tor groups of the periodic table [13]. A major class of compound semiconductors are formed from
Group IIIA and Group VA of the periodic table (often referred to as III-V compounds). These
materials can be seen in Figure 4. LEDs are possible through the use of a p-n junction. Electrons
in the n-type material flow into the electron holes in the p-type material. The free flowing electrons
are in the conduction band of the semiconductor material, and when they recombine with the elec-
tron holes in the p-type material they release energy in the form of light. The production of light
by the flow of electrons is called electroluminescence [8].
The energy of the emitted light depends on the energy gap of the semiconductor material. The
reason why compound semiconductors are used in LEDs, and not silicon, is because silicon has an
indirect band gap whereas most compound semiconductors have a direct band gap. The band gap
(another term for energy gap) is considered “direct” if the momentum of electrons and holes is the
same in both the conduction band and the valence band. If the band gap is “direct” an electron can
directly emit a photon. If the band gap is “indirect”, then no photon can be emitted. A circuit and
band gap diagram can be seen in Figure 5.
10

Figure 4: The Periodic Table showing both III–V and II–IV semiconductor compounds materials
[14].
LEDs are extremely versatile because they can be made to emit a variety of wavelengths of
light. The band gap is what determines the wavelength of light emitted, and the semiconductor
material is what determines the energy associated with the band gap. In order to create a specific
color of light from an LED a material needs to be used so that its band gap has the appropriate
energy associated with that specific color of light. Table 2 shows the available colors with wave-
length range for a variety of different compound semiconductors. While looking at Table 2 it is
apparent that the band gap isn’t the only factor that contributes to the final color of a LED. Most
colors are hard to reproduce with a certain materials’ band gap. There are two common ways to
change the color; one that indirectly alters the wavelength emitted and the other directly alters the
band gap.
These two processes involved using special coatings in the LED device and changing the con-
centration of the dopant materials. In order to get certain colors without changing the band gap,
the inside of the LED is coated with a special phosphor. A phosphor is any number of substances
that exhibit luminescence when struck by light of certain wavelengths, such as ultraviolet. The
most common white LED today consists of a blue semiconductor diode combined with lumines-
11

Figure 5: The inner workings of an LED, showing a circuit (top) and band diagram (bottom).
The parallel lines with the ”+” and ”-” represent the battery, and the arrow pointing to the right
represents a light-emitting diode with current ﬂowing through it. The n-type and p-type material is
the compound semiconductor [8].
cent phosphors that partially convert the blue light to yellow and red [16]. The color emitted by
LEDs can also be changed by directly modifying the band gap of the compound semiconductor
being used. By increasing the doping densities the band gap can shrink, causing the energy of the
emitted photons to be less. In other words, by increasing the doping density the emitted light will
be redshifted. LEDs are made with compound semiconductors which are difﬁcult and expensive
to make. The relationship between band gap and doping density can be seen in Figure 6.
Figure 6: A graph showing the effect of doping density on the size of a semiconductor’s band
gap [17].
12

Table 2: A table showing the color and wavelength range for a variety of compound semiconductor
materials. Some colors are only possible through the use of different colored phosphors [18].
13

The most widely used manufacturing process for compound semiconductors is through the use of
a Metal Organic Chemical Vapor Deposition (MOCVD) system.
4 Metal Organic Chemical Vapor Deposition
Metal Organic Chemical Vapor Deposition (MOCVD) is another form of Chemical Vapor
Deposition (CVD) that uses metalorganic compounds. The development of MOCVD has led to
greatly improved uniformities and wider variety of materials, allowing for band gap engineering
to become practical. Particularly III-V and II-VI semiconductors and most of their alloys have
been used in the MOCVD and were successful. This makes the MOCVD one of the most versatile
technique for compound semiconductors which helps greatly for LED fabrication [19].
III-nitride semiconductors such as GaN have been a hot topic for LED applications and laser
diodes. Even though there are a number of products already available on the market for LEDs,
one of the challenges is efficiency. The efficiency of GaN based LEDs are dependent upon light
extraction and internal quantum efficiency, and the conventional thin-film techniques often result
in highly defective optical materials. To solve these technical problems, nanowires (NWs) are
used due to their unique one-dimensionality that remove the problems posed by thin-films. GaN
NWs offer many advantages over traditional thin films such as high aspect ratio and large surface-
to-volume ratio which reduces dislocation density. It also allows for compatibility with silicon
substrates and has a higher light extraction efficiency. Arguably one of the greatest advantages of
these nanostructures is the ability to form axial heterostructures using lattice-mismatch material.
This allows for the avoidance of interfacial defects due to lateral strain relaxation. Along with this,
GaN NWs can be easily doped to form both p and n type materials [20].
Recently, the use of MOCVD and molecular beam epitaxy (MBE) has led to better control of
selective area growth of GaN NWs. The MOCVD method of growing quality III-nitrides offers
advantages such as being inexpensive, reproducible large-scale production, control of precursor
deliver, a high purity, and is a simple reactor process compared to MBE techniques. This makes
the MOCVD the most promising method of growing NWs for nanodevice applications [20].
14

4.1 MOCVD Fabrication Process
Multiple quantum well (MQW) NWs can be grown using the MOCVD system using a hori-
zontal quartz reactor. Figure 7 below shows the diagram for the process flow for growing GaN
MQW NWs LED on a Si(111) substrate.
Figure 7: Growth and fabrication of uniaxial p-GaN/InxGa1−xN/GaN MQW/n-GaN NWs LED
structure on Si(111) substrates [20].
First the growth of Si-doped n-type GaN NWs on the Si substrate with gold and gallium nan-
odroplets at 950°C was done. The gallium and nitrogen were supplied by trimethylgallium (TMGa)
and ammonia (NH3). Then to grow the InxGa1−xN/GaN MQW NW, TMGa, trimethylindium
(TMIn) and NH3, sources were used to supply the gallium, indium and nitrogen. A pulsed flow
precursor method was used to grow the MQW and GaN NWs at 630 and 710°C respectively. By
varying the number of pulses, the period and thickness of the MQW structures can be tuned. The
gases were allowed into the chamber for 3 minutes each and the pressure was maintained at 600
torr. In the final stage, the Mg-doped GaN NWs were grown at a temperature of 950°C [20].
4.2 Gallium Nitride Nanowire Properties
Figure 8 shows the tilt-view FESEM image of the NWs grown on the Si(111) substrates. Figure
2b shows the defined hexagonal shaped pattern of the NWs, and 2c is a NW that is vertically aligned
to the silicon substrate. This shows that the GaN NWs were indeed hexagonal faceted. Figure
2d shows the X-ray diffraction (XRD) analysis and it reveals that the peak of the wurtzite-type
hexagonal GaN NWs formed. It preferred the c-axis direction in their orientation.
15

Figure 8: (a) A schematic of InxGa1−xN/GaN NWs and (b) a tilt-view FESEM image of the NWs
grown on an Si(111) substrates. (c) A vertically aligned single NW and (d) is the XRD pattern of
the NWs display diffraction predominantly from the wurtzite peak [20].
Then a polyimide resist layer was spin coated over the NWs covering them completely. A 100
nm indium tin oxide (ITO) layer was then applied to the top surface. This serves as a transparent
electrode and current spreading layer. Thin Au/Ni layers were then deposited to serve as the p
and n electrode contacts. Then a photoluminescence (PL) measurement can be performed and the
results are shown in Figure 9.
Figure 9: PL spectra of GaN NWs (a) with different pairs of MQW and (b) with increasing indium
concentration [20].
As the MQW pairs increased, the intensity of the LED increased as seen in Figure 9a. This is
caused by the fact that the increased pairs increase the amount of incident light absorption. Figure
16

9b shows an increase in indium content which determines the band gap properties of the devices.
The increase of indium content causes the band gap emission to increase in width. So varying
the indium concentration would be an easy way to tune the emission wavelength of the MQW
NWs [20].
5 RGB LEDs and Applications
A majority of LED systems commonly used today are based on RGB (Red, Green, Blue)
LEDs. These systems employ diodes that radiate red, green, and blue light all housed in one
casing. Nearly any color within the visible light spectrum can be achieved by applying varying
currents to each individual diode. Color emissions of the individual LED diodes differ based
on the semiconducting material composing the diode as well as the processing or doping of that
material as mentioned earlier in the paper and portrayed in Table 2. The change in material and/or
doping affects the band gap between conducting and valence bands, which in turn affects the
amount of energy released during recombination; that change in energy is expressed as different
wavelength of light. For example, a Gallium Arsenide LED system emits infrared light. In terms of
doping, when phosphorus is added to the material (GaAs1−xPx), the band gap increases. Increasing
the band gap increases the potential energy of recombination and subsequently decreasing the
wavelength emitted during recombination, thereby changing the color [21].
There are two methods of creating white LED light. The first method uses blue light that gets
absorbed by a phosphor and reemitted as what is perceived as white light; the second method is to
combine different colors of light [22]. Creating white light utilizing phosphors, a combination of
host material and activator, is the most common method in white LED production. An example of
a host material is silicon-based nitride or silicon-based oxynitride and an example of an activator is
zinc or copper [23]. Phosphors are defined as a synthetic fluorescent or phosphorescent substance
and can be engineered to absorb a specific wavelength of light. Blue light absorbing phosphors
are used because blue light contains the most “information” due to having the most energy within
the visible light spectrum. Humans rely on light starting from around 450 nanometers to around
750 nanometers, meaning that an approximation of white light will need to encompass a major-
ity of that spectrum. Blue light, having the shortest wavelength and most energy can be broken
17

down into shorter wavelengths that can compose that larger range. An individual phosphor can
only broaden a spectrum of light so much so to compensate, bi-band or tri-band phosphors are
employed to cover a larger range of wavelengths; however, there are white LEDs that use only
yellow emitting phosphors that mix with the blue light and that combination is perceived as white
light. For white LED production, phosphors can be directly applied on top of the semiconducting
material or mixed in with the clear silicon used to encase the LED. Layering the phosphor on the
semiconducting material can increase the phosphor’s light emission, but it also makes the phosphor
the same temperature as the semiconducting material, leading to a shortened life span.
The second method of white light production is simply combining different colors to manu-
ally construct white light, essentially doing the opposite of what happens when white light shines
through a prism. As for infrared, UV, and the remaining colors in the visible light spectrum, the
semiconducting material can be changed or the processing of an existing material can be changed
as expressed in Table 2. But rather than having an individual diode for every color, an RGB system
can also be used to create a majority of the colors within the visible light spectrum similarly to
white light. In an RGB system, every color has a percentage of red, green, and blue light and that
percentage can be achieved by adjusting the current running through the system. For example, if
the system had 100% red, 100% green, and 100% blue white light is achieved. If the system had
0% red, 100% green, and 100% blue, then cyan is achieved.
In a study of phototherapy treatment for psoriasis, ultraviolet B light was used to curb skin
growth using localized irradiation [24]. Psoriasis is a chronic skin disease that causes excessive
and fast build up of skin cells and is characterized by red, itchy patches. Ultraviolet B light ranges
from 280 nm to 320 nm and has a therapeutic effect due to its immunosuppressive characteristic
and its status as an immunomodulator. The most important mechanism as an immunosuppressant
is apoptosis induction. Along with apoptosis induction, UV light has been proven to deplete T
lymphocytes, decrease antigen presentation, and modulate the synthesis, release, and activity of
inﬂammatory mediators and cytokines. UV radiation produces psoralens, chemicals that inhibit
the production of DNA in cells [25]. By inhibiting the DNA synthesis in epidermal cells, the
cells undergo apoptosis. The UVB Phototherapy System is composed of two main components:
a patient attached unit as seen in Figure 10, that delivers the therapeutic radiation and a medical-
grade DC power supply along with the required wires.
18

Figure 10: The PAU component of the UVB Phototherapy System depicted is composed of 72 UV
LED units with independent currents [24].
The PAU holds an array of 72 UV LEDs, each with their own individual current to compensate
for any light output deviation up to 10% in order to achieve uniform radiation. Though UV-B
radiation ranges from 280 nm to 320 nm, the device uses a wavelength of 311±2 nm because the
most effective range was 304 nm to 313 nm and the range 290 nm to 300 nm caused sunburn–
like effects. The study was conducted on 20 people split into two groups: one group received
aggressive dose appliance while the other received gradually increased dosage appliance. The
study was a success with the ﬁrst group showing a 93% improvement by the end of the study and
the second group showing an 84% improvement. The device’s power output over the continuous
600 experimental hours showed less than a 10% decrease.
Blue LEDs are also used for skin treatment, utilizing a wavelength of 415 nm and an output of
40 mW/cm2 [26]. The Omnilux Blue System utilizes ﬁve arrays of LEDs that directly irradiate the
skin, similar to the UVB Phototherapy System. The light generates atomic oxygen that kills and
prevents excessive bacteria on the skin as well as preventing pH gradients. A common application
of this device is the treatment of acne with 20 minute treatments performed two times a week for
eight weeks.
In an article published in a 2013 issue of Lasers in Medical Science, the goal of a study was
to determine the effect of red irradiation (630-660 nm) on the muscle recovery immediately after
high-intensity physical exercise [27]. The experiment had already undergone animal testing and
proved successful, so the predictions were positive. There has been human testing regarding a sim-
19

ilar experiment using near-infrared light and combined wavelength irradiation (ex. using infrared
light and red light) that have proven to also be successful, further backing this study. The proposed
properties of red irradiation include anti-inflammatory, analgesic, and reparative properties. The
light works by spurring the cellular metabolism through the photostimulation of the elements in-
volved in the electron-transport chain [28]. 17 subjects were split into two groups with the first
being administered the light-emitting diode phototherapy (LEDT) and the second was adminis-
tered a placebo. The first group underwent phototherapy using a BIOS Therapy II device set to
630 nm and 20.4 J/cm2 (utilizing a single diode) immediately after strenuous arm exercise. The
isometric muscle strength, muscle soreness, and elbow range of motion (ROM) was measured be-
fore the exercise, 24 hours, 48 hours, 72 hours, and 96 hours after the administration of the LEDT.
The experiment was successful, finding greater reduction in strength loss, muscle soreness, and
elbow ROM impairments in the LEDT group than in the placebo group as seen in Figure 11.
Figure 11: The graphs above show the effect of red irradiation on normalized force (left) and
normalized range of motion (ROM) for the elbow (right) before undergoing strenuous physical
exercise and 24, 48, 72, and 96 hours after immediate administration of LEDT. The group receiving
the LEDT treatment showed significant difference in force and ROM, outperforming the placebo
group in both areas [29].
Infrared signaling has been present in everyday life for decades as it was first developed as a
way to change the channels on black and white televisions. The technology now has numerous
applications in free-space communication, applicable in everything from garage door remotes to
the automatic hand dryers in bathrooms [30].
There are five main types of IR devices: emitters, detectors, photo-interrupters, photo-reflectors,
20

and transceivers. Emitters only transmit IR signals and detectors only receive them. A common
use of a simple IR emitter and detector system in free-space communication is a television remote.
The actual remote utilizes an LED light that emits an IR signal in ASK (amplitude shift keying)
form, which is received and translated into binary code by the detector on the television or cable
box. Photo-interrupters are a combination of an emitter and detector in one house each facing one
another and constantly relaying IR signal. It’s when the “beam” between the two is broken that the
tool knows to respond. Printers, fax machines, copy machines, cameras, and industrial machinery
all utilize this device. Photo-reflectors also house both an emitter and detector, but rather than
being pointed at one another, they’re on the same plane. The emitter constantly transmits a signal
so that when an object moves in front of the signal, it rebounds off the object and is received by the
detector and the tool knows to respond. Automatic hand dryers in the bathroom use this device so
that when your hands move underneath, the IR beam is reflected off your hands to the detector and
the dryer turns on. A transceiver is an emitter and a sensor housed with either one or two lens that
increase the form factor of the device and the emitter is typically vertically above the sensor [30].
Nanowire LEDs are moving into the scientific spotlight due to their use in Single Photon
Lithography (SPL) [29]. Single Photon Emission elements (SPEs) are fabricated from GaN nanowires
and are formed on a chip. They are then connected to a current and used to expose trace amounts of
photoresist, which contains a photoactive compound called DiazoNaphtoQuinone-(DNQ)-sulfonate,
that is coated on a sample as seen in Figure 12.
Figure 12: The figure shows the basic assembly of SPL system; SPEs are connected to a chip and
when a current is applied, emission exposes the DNQ-sulfonate photoresist on a sample [29].
21

By improving the resolution to such an extent, this technology can result in the production of
sub-nanometer systems, even on the molecular level, as well as more efficient mass production
of nano-devices. Though the system is currently not commercially available, the device shows
promise due to its resolution and speed.
6 Limitations of LEDs
Despite the numerous advantages to the incorporations of LEDs and Solid Slate Lighting sys-
tems, there are various limitations to their efficacy and use. Some major issues include thermal
management of LED devices, efficiency droop, and the phosphor coatings used to filter light.
Heat is a major concern with LED systems. Often LED panels have many fixtures per area,
which can cause emitted heat to be concentrated and thus make them more difficult to cool. If even
a small amount of heat is not properly dissipated, device failure can be a result. During operation,
as much as 70-80% of the applied electrical energy is converted into heat. The retention of this
heat within the device can cause decreased lifetime. For every 10°C increase above the maximum
operating limit, a decrease of up to 50% in operating life can result [31]. Figure 13 illustrates
the relation between junction temperature, useful lifetime, and lumen maintenance for a particular
brand of LED.
Figure 13: How changes in junction temperature affect the useful lifetime of an LED [32].
22

At the lowest temperature (115°C), the LED reaches its useful lifetime of 100,000 hours with
a lumen maintenance value of 20%, whereas the highest temperature (135°C) reaches a useful
lifetime of 13,000 hours with 0% lumen maintenance. Heat sink technology within the bulb can
dissipate this generated heat, this can by extension, increase the total costs for air conditioning
in commercial applications [33]. Heat sinks can include thermally conductive adhesives or solid
copper or aluminum sinks designed with many finely shaped features called fins which serve to
increase surface area for dissipation. Changes in temperature can shift the wavelength of light
emitted from the device, which can be a problem especially for spectrum sensitive devices.
The phenomena known as “droop” occurs when internal quantum efficiency declines at the
higher current density needed for general lighting applications. This is caused by Auger recombi-
nation in which the joining of an electron and a hole do not produce a photon, but instead kinetic
energy is transferred to a tertiary carrier, another electron, which is later dissipated as heat. This
results in a decrease in conversion efficiency at higher current densities, thus increasing the number
of LED chips necessary for a particular lumen output [34]. Figure 14 illustrates this effect.
Figure 14: (a) Schematic of an LED under current injection. (b) Electrons and holes recombine
radiatively in the quantum wells (QWs) by emitting photons. (c) In the Auger effect, an electron-
hole pair recombines without emitting a photon by exciting another electron to a high kinetic
energy. In: Indium. GaN: Gallium nitride [34].
The placement and arrangement of the phosphor, as the well as the dimensions of the reflector
cup are vital to the efficiency of white LEDs. In the case of a conformal and phosphor-in-cup
layout shown in Figure 15 (a and b, respectively), this is due to the fact that phosphors emit light
isotropically, and the emitted light can directly impinge upon the LED chip where the light can be
reabsorbed, therefore limiting the light extraction efficiency.
23

Figure 15: Arrangements of the phosphor in white LED: (a) Conformal distribution directly on
the LED chip. (b) Uniform distribution in reflector cup (phosphor-in-cup) (c) Uniform distribution
thin layer above LED chip (remote phosphor) (d) Remote phosphor distribution in diffuse reflector
cup [35].
If the phosphor were to be placed a relatively large distance from the chip (remote phosphor),
shown in Figure 3c, the chance of a light ray directly hitting the chip is much smaller than before.
This also reduces the operating temperature of the phosphor, which can lead to more reliable white
LED systems. However, there is still a great chance of the light being reflected from the reflector
cup onto the LED chip and being absorbed. To prevent this, a diffuse reflector cup is used in
accordance with the remote design as shown in Figure 3d. Diffuse reflectors have an angular
distribution of the reflected intensity, I, given by equation 1 below:
I ∝ cos(θ) (1)
where θ is the angle of reflection irrespective of the angle of incidence [35]. This allows the light to
be reflected upward and away from the LED chip, minimizing the amount that is reabsorbed. In an
experiment by Kim et al. it was found that using the remote phosphor with the diffuse reflector cup
showed a 15.4% increase in light extraction efficiency as compared to traditional phosphor-in-a
24

specular cup arrangements [35].
Limitations such as thermal management, efficiency droop, and the layout of the phosphors
within LED systems need to be researched and improved upon before LED light sources become
prevalent in industrial, commercial and residential use.
Even though LEDs and LED systems are more energy efficient than either incandescent or
fluorescent light sources, there exists a significantly larger upfront cost to current LED technology
that is a deterrent for many consumers today. The cost savings is only seen in the long term use of
the LEDs. At present, LEDs are about 50 times as expensive as incandescent light sources and 7
times greater than CFLs [35], though it will improve significantly with time. If the cost of current
LED technology were to be averaged over the total lifetime of the product, it would be one seventh
the cost of incandescent sources and two thirds the cost of fluorescent sources. According to a
prediction known as Haitz’s law, it is forecasted that every 10 years the amount of light generated
by an LED increases by at least a factor of 20, while the cost per lumen drops by at least a factor
of 10 [36]. For example, if all white light sources were to be converted to energy-efficient LED
sources, consumption of energy worldwide could be reduced by approximately 1,000 TWh/yr−1,
which equates to the same amount of energy as 230 typical 500 MW coal plants; this would serve
to reduce greenhouse emissions by about 200 million tons [37]. According to the US Energy
Information Administration, the commercial and residential sectors of the US were estimated to
use about 412 billion kilowatt hours for lighting in 2014, which equates to 11% of the total energy
consumption of the country [37]. Widespread application of LEDs could greatly reduce both the
cost efficiency of LEDs as well as being more environmentally friendly.
7 Fabrication Process
7.1 Substrate Selection and Clean
The device being fabricated is a blue light emitting LED. It uses “InGaN/GaN multiple quan-
tum well (MQW) on n-GaN nanowires on Si (111) substrate” [38]. The epoxy dome lens will have
a phosphor coating that will convert the blue light to white light. One of the most important steps
in any nano-device fabrication process is the selection of the substrate. The substrate works as the
25

foundation for the entire device, so if an incorrect or faulty foundation is used to build the whole
device it will likely result in an ineffective device. Typically one would buy a wafer with the ap-
propriate characteristics from a wafer provider. An example of one of these providers is Sil’tronix
Silicon Technologies. They specialize in silicon wafer production and distribution. To obtain a
wafer, a wafer from their stock could be bought or a specialized order, a request for quotation, can
be placed specifying exact characteristics that the Si wafer should have. The second method will
be used in our fabrication process.
A silicon wafer created from the Czochralski process with a crystal lattice structure 111 will be
used. This is because from the research conducted both the Czochralski process of the float zone
process can be used for the silicon wafer, but the Czochralski process is cheaper. The crystal lattice
structure of the wafer is 111 because Si 111 is compatible with the aluminum nitride (AlN) buffer
layer that will be used to grow the nanowires on. The diameter size of the wafer will be 6 inches
and it will be doped with phosphorus to have an n-type dopant. No specific diameter size or range
has been specified in the research conducted, so an arbitrary standard size for industry was chosen.
The doping is important for the wafer, but the specific dopant is not so important. The Si needs
to be n-doped but what material is used for this is not crucial. The Si wafer needs to be n-doped
because in the construction of our device the negative metal contact will be placed at the bottom of
the wafer, so the wafer must be conductive enough to allow electrons to flow through it relatively
easily to reach the nanowires where the recombination of the electrons and holes will take place.
The next step after purchasing a wafer is to clean it to make sure that the device fabrication
begins on an extremely clean wafer with the slightest to no contamination. The process used to
clean the wafer is the typical RCA cleaning process. It starts with standard clean 1. The param-
eters for standard clean 1 are as follows: 100ml of 28% Ammonium Hydroxide, 100ml of 30%
Hydrogen Peroxide, and 500ml of deionized water (DI) in the first immersion chemical reaction
tank. The solution will be heated to 75°- 80°C and the temperature will be maintained. The wafer
then will be immersed into the tank for 5 minutes. This part of the clean is to chemically remove
organic contaminants. Immediately after the 5 minutes the wafer will be placed into a solution
whose purpose is to remove the native oxide on the silicon wafer. 30 parts DI water will be placed
into a propylene beaker. Then 1 part 49% Hydrofluoric (HF) acid will be added to the beaker and
mixed. A propylene beaker is used because a glass beaker cannot be used with HF acid. The wafer
26

will be placed in this solution for 15 seconds, and then will be rinsed in a DI water bath for 30
seconds. The wafer then will be immediately transferred to the next immersion tank. This tank
has 1 part 37% hydrochloric acid (HCl), 1 part 30% hydrogen peroxide, and 6 parts DI water. The
water is added first then the acid and peroxide. This solution is heated and kept at a temperature
in the range of 75°- 80°C. The wafer is placed in this tank for 10 minutes. Then the wafer will be
placed in the same DI bath used in the step before for 5 minutes. After that the wafer is placed
in a new clean DI bath for 30 seconds and then dried with a nitrogen gun. This last clean is to
chemically remove metallic contaminants. After this step the cleaning process is done and cleanup
of the chemical waste needs to be done. All the tanks need to have their chemical waste disposed
of in an appropriate manner. After the disposal of the waste then the next step in the fabrication
process can be started.
7.2 Substrate Preparation for GaN Nanowire Growth
The next step in the process is the growth of an AlN buffer layer on the Si wafer. This layer
is deposited by plasma assisted molecular beam epitaxy. Before deposition though, the chamber
should be cleaned and primed for the deposition. 10 nanometers of AlN is deposited “at a tempera-
ture of 530 °C using an Al flux of about 1.3x107 mbar beam equivalent pressure (BEP), a RF power
of 450 W and a nitrogen flow rate of 2.7 sccm” [39]. Then a layer of hydrogen silsesquioxane is
spin coated onto the substrate. Then “after bakings at 160 °C for 5 min and subsequently at 300
°C for 20 min is transformed into a 30 nm thick silicon oxide layer” [39].
This oxide layer is now patterned through electron beam lithography. A polymethyl methacry-
late (PMMA) resist is deposited onto the oxide. After depositing the positive PMMA resist it is
then tempered on a hot plate for 3 minutes at 150 °C [40]. Then the e-beam will expose holes in
the resist with the dimensions and pattern wanted for the nanowires. The resist is then developed
in the solvent dichlorobenzene, the typical e-beam solvent for PMMA. The wafer is submerged in
the solvent for 1 minute then placed into a DI bath for about 30 seconds, then dried with a nitrogen
gun [40].
To transfer the pattern from the resist to the oxide layer the wafer will be placed in an RIE and
the SiOx will be etched. Before this, however, the chamber should again be cleaned and primed for
27

the etching about to occur. The process conditions are a 100 mT pressure, DC bias of 190 V, 300
W power, a gas ﬂow of 45 sccm CF4 and 5 sccm O2, for an etching time of 30 seconds based on
the assumption the SiOx etch rate is about 10.01 ˚A/s. This value was obtained experimentally in
the Reactive Ion etching lab conducted earlier this summer. To remove the resist an ashing will be
done. A tool that can be used to do this is the Technics PE II-A plasma system. Figure 16 shows a
picture of this tool.
Figure 16: Technics PE II-A Plasma System [WEBSITE].
The chamber will be cleaned before the ashing, and then the wafer will be placed in the chamber
and the PMMA resist will be removed. The conditions for this are a 280 mT pressure, 300 W
power, O2 ﬂown into the chamber, and the etch time will depend upon the amount of resist needed
to be etched [41]. After the resist is removed the wafer is now ready to move on to the next
fabrication step, the growth of the nanowires in the mask holes. Figure 17 shows a schematic of
what the wafer will look like post ash, and an SEM image of the mask with the holes in it.
Figure 17: (a) Schematic view of the patterned mask and (b) SEM micrograph of the mask [39].
28

7.3 Nanowire Growth using MOCVD Fabrication
The GaN NWs were grown at a pressure of 200 Torr and at 1000 °C. The temperature is a
crucial factor in the growth of GaN as it determines the effects of the Ga and N kinetics and the
precursor diffusion behavior. It also affects how the nanowires are grown, and the trends of the
effect of temperature can be seen in Figure 18.
Figure 18: (a) graph shows the height (c-plane) and width (m-plane) of GaN nanowires as a func-
tion of the growth temperature. (b) Shows the PL spectra of GaN NWs intensities at different
growth temperatures. (c,d,e) Shows the morphology of the GaN NWs at different growth temper-
atures [42].
29

Since the GaN NWs were desired to grow in the c-plane, a temperature of 1000 °C was chosen
based on the Figure 18(a) above. This also will give the NWs a high intensity and very deﬁned
high aspect ratio as seen above in Figures 18(b) and (e). The NWs depicted in Figure 18 were
grown catalyst free, this causes the tip of the NWs to create a point under most conditions [42].
The precursors TMGa and NH3 will be used for the III-V GaN nanowires and the carrier gas,
hydrogen, will be used. The nanowire deposition is performed using the pulsed mode, allowing
for greater control of the nanowire dimensions. Flow rates of 15 sccm of TMGa and 5 sccm of
NH3 were chosen, though the ratio of the gases here does not matter as they are being put in the
chamber separately. An injection and interruption step will be performed with each gas, thus Ga
will be injected, then performing an interruption with only H2 in the chamber, then an injection
of the N2 gas then another interruption. The injection and interruption time ratio determines the
amount of material deposited at one time, and desorption of the material respectively [42].
As the injection time of the TMGa increases, the amount of material deposited laterally in-
creases, while a longer interruption time increased the amount of material desorption laterally, this
can be seen in Figure 19.
Figure 19: SEM images of GaN NWs with various injection and interruption times of Ga that
affect the morphology of the NWs [42].
Along with this, as the NH3 injection time increases the lateral width of the nanowires increase,
30

and as the interruption time increases, the vertical length increases, as seen in Figure 20.
Figure 20: SEM images of GaN NWs with various injection and interruption times of N that affect
the morphology of the NWs [42].
With this in mind, we chose an injection time of 5 and 10s for the TMGa and NH3 respectively,
and an interruption time of 1s for both gases. Since each cycle produces a growth of approximately
4.2 nm, the process will run for 300 cycles, producing 1250 nm GaN NWs [42].
This wire alone will produce roughly a 367 nm wavelength, since a blue emitting LED is
desired, a MQW structure can be deposited on top of these wires, then a p-type GaN NW is grown
to change the wavelength emitted, this is discussed more closely earlier in the paper. As the exact
flow rate was not given in detail to, the TMIn gas flow rate is speculated in this recipe. InGaN is
usually grown at a lower temperature than the GaN NWs, and a temperature of 750 °C will be used
to deposit the InGaN layers. Looking at Figure 9 we established that 10 pair QWs and an Indium
concentration of 18% would be needed in order to produce the desired blue LED [20].
Then the Mg doped GaN layer of the NW will be grown by a non-pulsed MOCVD step. 0.2
sccm of TMGa and NH3 will be flown into the chamber with 1 sccm of hydrogren. The gas Cp2Mg
as the precursor and had 5 sccm flown in the chamber with a temperature of 950 °C and a pressure
of 600 torr. The flow rate was picked due to a study done varying the flow rate from 5 and 7 sccm,
this can be seen in Figure 21 [43]. This shows that the red curve or the 5 sccm flow rate has nice
linear behavior which is why it was chosen. The temperature of 950 °C and a pressure of 600
31

torr were chosen as they appeared to be a common temperature and pressure that was described in
several different papers.
Figure 21: The I-V curves of the GaN p-n junction at different Cp2Mg ﬂow rates [43].
7.4 SEM Imaging After Nanowire Growth
After the InGaN/GaN nanowires have been grown via the MOCVD process, the nanowires will
be imaged under a scanning electron microscope. Figure 22 shows an SEM image taken of GaN
nanowires. Height measurements taken from the SEM images can be used to calculate an average
height of the nanowire array. This information can be used to estimate the relative thickness of
the polyimide layer applied for planarization, and thus the spin needed to acquire that thickness as
well. With the processing parameters being used, the estimated height of the nanowires should be
around 2 microns. Also, using the BSE detector, the compositional contrast between the InGaN
QWs and the p-type GaN portion of the nanowires can be used to estimate the heights of those
particular areas. This data can be used to determine how much of the polyimide layer needs to be
etched, so as to leave only the p-type heads of the nanowires exposed prior to the application of
the ITO layer.
32

Figure 22: SEM images of a GaN ensemble take from 3 views: (a) side view, (b) bird’s view, (c)
top view [44].
33

7.5 X-Ray Photoelectron Spectroscopy
X-Ray Photoelectron Spectroscopy (XPS) will be used after the nanowires have been grown
in order to characterize the elemental composition and check for potential sources of contamina-
tion within the wires that could otherwise decrease the efﬁciency of the device. XPS works using
a k-alpha x-ray source that is directed at the sample under ultra high vacuum conditions. When
the x-rays strike the surface, their absorption causes the ejection of electrons, which each have a
unique XPS spectra. A detector reads the energy of the ejected electrons in order to determine ele-
mental composition. The peaks generated by a typical XPS graph can be correlated to a particular
element/atom by means of a lookup table which gives the binding energies of those atoms in their
respective locations within the electron shell.
Figure 23 shows an XPS spectra for GaN nanowires. (a) shows a general spectra of the binding
energies, with the main components being Ga, N, and O. The main XPS peaks are at the location
of Ga3d (19.57 eV), Ga2p1/2 (1144.5 eV), Ga2p3/2 (1117.7 eV), N1s (397.5 eV), and O1s (531.2
eV), respectively [45]. The core level of Ga has a positive shift from that of elemental Ga. This is
caused by subtle changes in inner electron binding energy, which are due to different chemical en-
vironments [45]. When this positive shift is seen, it is indicated that the Ga atoms in the nanowires
are in the compound state (GaN) [46], conﬁrming the bonding of Ga and N, and the absence of
elemental gallium [45].
34

Figure 23: Example of an X-Ray Photoelectron Spectroscopy graph for GaN nanowires [45].
35

7.6 Polyimide Application
Once the nanowires are formed a polyimide layer is applied on top of the nanowires, cover-
ing them entirely. Polyimides are thermally stable polymers with high softening temperatures and
are used extensively in fabrication, arguably one of the most well known polyimides being Kap-
ton [47]. Kapton is used for electrical insulation, as magnetic and pressure-sensitive tape, and in
tubing. Reliant on a stiff aromatic backbone, polyimides serve as the structural component of the
nanowire LED: supporting the nanowires by filling in the space between them and providing a
flat surface for the transparent conductive oxide (whether AZO, ITO, etc.) to be deposited after-
wards. Polyimides are coated onto samples by applying a precursor poly(amic acid) and curing it
to become the polymer, expressed by the second step in Figure 24. There was no indication that
a specific polyimide was required for this step found in the research, but the poly(amic acid) used
will be the 155A series from Fujifilm due to recipe availability [48].
Figure 24: Synthesis of Kapton polyimide [47].
36

Despite the fact that spin coating was the recommended method of polyimide application, there
were not any specific parameters for said application. The chosen tool for the spin coating is the
Headway spin coating tool due to its use in the spin coating of small samples rather than industrial
sized, which can be seen in Figure 25. The parameters for the spin coating are 15 s at 350 rpm
and 45 s at 1000 rpm and softbaked at 120 °C for 6 min to produce about a 15 µm thickness. The
polyimide then needs to be cured and the chosen tool for this is a YES-PB-HV series high vacuum
oven, capable of pressures down to 10-5 torr and maintaining max temperatures of 450 °C. The
parameters for this cure are 400 °C for 2 hours in a 150 torr nitrogen environment, which reduces
the height of the polyimide to 10 µm [48].
Figure 25: YES-PB-HV Series High Vacuum Oven [49].
The nanowires have an average height of 2 µm, so applying 10 µm of polyimide wouldn’t
typically be done for efficiency and finances’ sake, but recipe availability for this step was not
found; that being said, experimenting with different spin speeds, temperatures, and other parame-
ters would not be difficult. In order to find the optimal spin speed for the spin coating, poly(amic
acid) would be applied to four silicon wafers with each wafer assigned a different spin speed (2000,
3000, 4000, and 5000 rpm) and after each were coated with those parameters they would undergo
37

curing with the goal to achieve a thickness of about 5 µm. If the thickness exceeds 5 µm even
after 5000 rpm, then the spin speed can be increased again to get as close as possible. The experi-
ment could not be conducted for this paper because of time and resource restrictions, so the recipe
above should be used for deﬁnite results and the etch recipe should reﬂect the spin coating recipe
as required.
7.7 Polyimide Etch
As previously stated, the average height of the nanowires is 2 µm and the polyimide needs
to be etched below even that as the ends of the nanowires need to be exposed for the transparent
conducting oxide deposition. Plasma etch is the proposed method of etching due to the ease of
control and for recipe’s sake. The tool used for the process is the Trikon Omega 201, an inductively
coupled plasma reactive ion etch tool shown in Figure 26.
Figure 26: The inner workings of an Trikon Omega 201 asher and etch tool [50].
The parameters for an oxygen plasma etch are 1000 W and 130 °C and when run for an hour,
the etch will produce an etch rate of 200 nm/minute. The polyimide layer needs to be etched
enough so that only the p-type GaN is exposed to the transparent conducting oxide deposited
afterwards. In order to determine how much to etch, the nanowires total height and height of the
p-type would be measured using FESEM imaging. For this paper, working under the assumption
38

that the nanowires are an average of 2 µm and that the p-type is roughly 400 nm as well as the 10
µm polyimide layer, the etch should be calculated to remove roughly 8.1 µm [20]. It shouldn’t
be 8.4 µm because if too much of the polyimide is etched away and the indium gallium nitride
is exposed, the LED won’t function due to lost recombination sites. And because the transparent
conducting oxide - p-type GaN relationship is more so dependent on contact than surface area,
etching under the maximum height of the p-type is a safer approach. In addition to that, a thicker
layer of the transparent conducting oxide would decrease conductivity. By this logic, the optimal
etching time with the previously stated parameters should be ∼40.5 minutes. After etching, the
polyimide would be sonicated in a bath of deionized water in order to prevent contaminants from
being trapped under the transparent conductive oxide layer.
7.8 Transparent Conducting Oxide Deposition
Once the polyimide layer is etched ∼100 nm, in order to expose the p-type GaN nanowire
heads, the polyimide surface needs to be coated with a TCO layer. In this process indium tin oxide
(ITO) was used as the TCO layer because of its high transparency and great conductivity. The ITO
film is introduced as the current spreading layer and the light anti-reflecting layer on the p-GaN
nanowires [51]. Since GaN nanowires are being used, the ITO layer is also used as a planarization
layer in order to make it easier to deposit the metal contacts. The following fabrication steps get
more complicated because of the polyimide substrate each subsequent layer is being deposited on.
Polyimide begins to degrade at temperatures above 450 °C making it imperative that all fabrication
steps beyond this point operate below this temperature. Special consideration needs to be taken
when choosing the optimal substrate temperature during deposition.
The ITO layer needs to meet specific requirements if it is to be used in an LED. The ITO
layer needs to have sufficient electrical and optical properties in order to meet the demands of the
device. Therefore the deposition parameters of the ITO are extremely important. A DC magnetron
sputtering process will be used to deposit the ITO layer, so the primary parameters that need to be
considered are the substrate temperature, the processing power, the processing pressure, and the
oxygen and argon flow rates.
The substrate temperature does not become a huge problem during the deposition of ITO be-
39

cause DC magnetron sputtering will be the primary method of depositing the ITO layer. Typical
sputtering substrate temperatures are well below 450 °C. When depositing crystalline materials
using magnetron sputtering the substrate temperature is a key component in the final crystalline
structure of the film. In a study observing the effects of substrate temperature versus the electrical
properties of ITO, it showed that an increased substrate temperature produced a drastic decrease
in the resistivity of the ITO up until 250 °C [52]. Figure 27 shows the relationship between the
resistivity and the substrate temperature.
Figure 27: Graph of the resistivity of the ITO substrate vs. substrate temperature (°C) [52].
One reason for the sudden drop in resistivity is that the increase in substrate temperature may have
led to oxygen-deficient films, resulting in an increase in charge carrier density [52]. It may not be
a good idea to increase the substrate temperature too high for fear of compromising the crystalline
structure of the film. For this reason a substrate temperature of 170 °C was chosen.
The next parameter to choose is the processing parameters. A higher power will create a more
energetic plasma in the sputtering chamber, which will result in a faster deposition rate. During
the DC magnetron sputtering lab for the Penn State Nanofabrication Manufacturing Technology
program, estimate values for the deposition rate at different processing powers were given. These
estimated values were ∼5 ˚A/s for 200 W and ∼2 ˚A/s for 100 W. For this fabrication process
a slower deposition rate is required in order to produce a better quality film. This means that
40

a relatively low processing power of 80 W will be used for this fabrication process. This will
roughly produce a deposition rate of ∼1.6 ˚A/s, and will require a deposition time of 15 minutes.
The pressure in the process chamber is an important parameter to consider, but the exact work-
ing pressure is difficult to calculate without the proper calculations. The sputtering machine being
used will have similar base and working pressures as the Kurt J. Lesker CMS4 Sputter System.
The base pressure will be ∼ 5x10−6 Torr, while the working pressure will be a few mTorr. The
high pressure during the deposition step is necessary to create a plasma in the process chamber.
The final parameters to consider are the gas flow rates, and how changing them will affect the
electrical and optical properties of the ITO. The only two flow rates to consider will be the flow
rate of argon and oxygen into the process chamber. Flow rate is significant because certain flow
rates can result in bad results for the electrical and optical properties of the deposited ITO. Studies
done on the flow rate of argon when deposited ITO showed that an increased flow rate drastically
increases the resistance of the ITO layer. The higher argon flow rate is due to an enhanced ionic
impurity scattering that caused a great number of native defects in the ITO layer [53]. Studies have
shown that for an amorphous TCO film, the electrical properties are governed by the ionic impu-
rity scattering rather than the grain boundaries or structural disorder [53]. The higher argon flow
rate increases the working pressure near the reaction zone, increasing the frequency of collisions
between ionized atoms and the residual gas. As a result, the oxidation state deteriorates under the
higher argon flow rates and leads to an increased number of ionic defects [53]. This relationship
between argon flow rate and the resistivity of the ITO can be seen in Figure 28.
Figure 28: Variation of resistivity with argon flow rate given a constant deposition time of 15 min.
and a target-substrate distance of 17 cm and 22 cm, respectively [53].
41

Based on this data, an argon flow rate of 25 sccm was chosen. Oxygen flow rate is just as
important as the argon flow rate because of oxygen’s effect on the electrical and optical properties
of ITO. Variations in oxygen flow rate can directly influence the amount of oxygen vacancies in
the ITO film. By using X-Ray Photoelectron Spectroscopy (XPS) the concentration of oxygen
vacancies can be determined within the ITO film. Based on an analysis of various ITO films made
by altering the oxygen flow rate, it can be concluded that the increase of oxygen flow rate gives rise
to more oxygen vacancies and, correspondingly, there is a decrease in optical absorption as well
as a blueshift of the optical absorption edge [54]. The influence of oxygen flow rates on optical
transmission can easily be seen in Figure 29.
Figure 29: The absorption and transmittance of deposited ITO as a function of wavelength [54].
In order to understand how the concentration of oxygen vacancies can affect the absorbance of
light refer to Figure 30. A higher oxygen flow rate will increase the optical transmission in the blue
spectrum of light, which is ideal for our specific device, which will emit blue light. Therefore, an
oxygen flow rate of 30 sccm will be chosen for this fabrication process. A summary of the entire
deposition recipe can be seen in Table 3.
42

Figure 30: The calculated absorption coefficient of ITO with different oxygen vacancy concentra-
tions [54].
Table 3: DC Magnetron Sputtering recipe for the deposition of ITO.
Once the deposition is finished the ITO layer must be annealed in order to increase the conductivity
of the film. Plenty of research supports that increasing the annealing temperature of ITO increases
the crystallinity. In one study, the ITO was annealed at temperatures ranging from 25-250 °C, and
it showed that as the annealing temperature increased the sheet resistance decreased, the average
transmission increased, and the average grain size increased [55]. The use of polyimide makes
it possible to anneal the ITO at such high temperatures because polyimide has a very high glass
transition temperature compared to other polymers. Therefore, after the deposition of the ITO, it
was annealed in an argon environment at 250 °C.
After the annealing of the ITO layer, the characterization of the ITO layer is important. The
characterization of this layer will determine whether or not the above parameters in Table 3 will
43

provide desirable electrical and optical properties. A specific area of research is the study of
aluminum doped zinc oxide (AZO) as a replacement for ITO for the use as a TCO. ITO has rare
earth elements, such as indium that are expensive and hard to find. The materials in AZO are much
cheaper and far more abundant. This is why so much research is being done to incorporate AZO
in the fabrication of LEDs and solar cells. The only downside is that AZO is much less conductive
and less transparent than ITO. An extensive characterization of AZO was done in order to test the
likelihood of AZO being a good replacement for ITO for future fabrication processes.
7.9 ITO/Polyimide Transmittance Test
After the polyimide film has been spun on, a transmittance test will be carried out to see how
much light at the expected peak wavelength will be able to pass through the layer. This same test
will be performed after the ITO layer has been applied. Figure 31 shown below shows the optical
transmittance of a 70 micron polyimide layer. The average transmittance within the visible light
range (400-700 nm) was about 86% [55].
Figure 31: Optical transmittance of polyimide film as compared to Kapton film [55].
After the ITO layer was applied, the transmittance of both it and the polyimide layer will be
tested. Figure 32 shows the transmittance of ITO/polyimide at different annealing temperatures. As
44

the annealing temperature increased, the optical transmittance also increased. The transmittance
was shown in this study to reach maximum of 83.5% in the visible range after being annealed at
250 °C [55].
Figure 32: Transmittance of the ITO ﬁlms under different annealing temperatures [55].
7.10 AZO Study
To examine the potential for AZO to replace ITO as a transparent conducting oxide several
AZO recipes were deposited onto glass substrates using atomic layer deposition. To characterize
their transmittance, the sample was loaded into a UV-Vis spectrophotometer. Figure 33 shows be-
low that the AZO sample had an approximate transmittance of 81-83% at the expected wavelength
of light (450-500 nm) to be emitted by the InGaN/GaN nanowire LED fabricated in this process.
The samples next underwent the Shipley 1813 photolithography process to transfer a cross
pattern consisting of four separate bars with a length of 10.5 mm and a width of 3 mm. The
samples were then hard-baked at 120 °C for 2 minutes. Using a Digital Multi-Meter the resistance
of the AZO was measured for all 4 bars of each sample. The Tencor P16+ Proﬁlometer was used
to measure the thickness of each bar. These values, along with the resistance measurements and
photomask dimensions were used to calculate the resistivity using the formula below, where ρ is
45

Figure 33: Graph showing transmittance of 24:1 Zn:Al AZO sample deposited on a glass substrate
using a spectrophotometer.
the resistivity, R is the resistance, A is the cross sectional area, and L is the length.
ρ =
RA
L
(2)
The reciprocal of the calculated resistivity values was subsequently used to determine the conduc-
tivity of the AZO with different Zn to Al ratios. Table 4 shows a summation of the results obtained
from this test.
Table 4: Table showing the results of the AZO resistivity tests.
46

The most conductive AZO layers were found at a Zn to Al ratio of 20:1 and 18:1, with conduc-
tivity values of 3.29x102 ohm cm−1 and 3.01x102 ohm cm−1, respectively.
7.11 Deposition of Metal Contact Electrodes
The most common metal used in contacts is gold because of its relatively good conductivity.
However, gold does not adhere well to most surfaces unless it is another metal. Therefore another
thin film will be deposited before the gold layer in order to increase the adhesion of gold to the
LED. Nickel will be the adhesive layer used for the gold metal contact, as well as the diffusion
barrier. Au/Ni metal contacts are common in semiconductor manufacturing because of their overall
good conductivity. An E-Beam evaporator will be used to deposit both materials on the backside
and top of the LED. To be more specific, there will be a thin film coating the bottom of the Si(111)
substrate that is used for the n-type metal contact and there will be a another thin film partially
coating the ITO layer, which will be used as the p-type metal contact.
First the bottom side of the substrate will be coated with the metal contact. No shadow mask
will be used because the entire bottom side of the wafer will be covered. Nickel will be evaporated
first, and only 13 nm of nickel will be deposited. Once the nickel is deposited, a new deposition
will begin for the gold layer. The gold layer will be deposited the same way, but this time 30 nm
of gold will be deposited over top of the nickel. Once the bottom side of the device is coated with
the n-type metal contact, the same process will be done for the p-type metal contact on top of the
ITO layer. Instead of depositing over the entire surface of the ITO, a shadow mask will be used
so that the Au/Ni metal contact will only partially cover the ITO layer. The same thicknesses will
be used for the Au and Ni film layers. Once both metal contacts have been deposited onto the the
device, the metal contacts must be put through an annealing process.
The annealing process is primarily used to increase the electrical and optical properties of the
metal contacts. The main reason why annealing is able to better these properties is because it can
help get rid of the Schottky barriers that are formed between the metal contact and the underlying
layer. In this process, the device was only annealed for 15 minutes in air at a temperature of 400
°C in order to preserve the structural integrity of the polyimide layer. Once annealed the device is
nearly finished, it just needs the wires to be welded to the metal contacts, and encapsulated in the
47

epoxy cap that protects the entire device when it is sent into the market. In order to convert the
blue light emitted from this LED, a phosphor coating needs to be applied to the device. The typical
coating applied to such a device is a yellow Yttrium Aluminum Garnet phosphor. A complete ﬂow
diagram can be seen in Figure 34.
Figure 34: A schematic of the entire fabrication process of the InGaN/GaN nanowire LED.
7.12 Photoluminescence Spectroscopy Test of Completed Device
Photoluminescence spectroscopy (PLS) will be performed after the device has been completed
to evaluate the band gap of the device as well as the peak wavelength of the emitted device. This
wavelength should be in the blue range of the visible spectrum (450-500 nm). This will deter-
mine if the LED is capable of producing white light once the phosphor is applied. PLS is a non-
destructive and non-contact method of characterization. Light is directed onto a sample, where
48

upon absorption induces a process called photo-excitation. This causes the material to jump to a
higher electronic state and release energy in the form of photons as the material relaxes and returns
back to a lower energy level. This emission of light is known as photoluminescence [56].
8 Conclusion
A comparison of incandescent, fluorescent, and LED light sources was conducted to show the
limitations of current light sources and the need for the transition to more efficient light sources,
such as LEDs. Then a brief explanation of the operational physics that makes LEDs possible was
examined. This includes the mechanics of how compound semiconductors are made and used in
the fabrication of LEDs. Afterwards an overview of RGB LEDs and their impact in current LED
technology was given, as well as current and future applications of LEDs. Finally the costs and
limitations of LEDs were evaluated.
A unique fabrication process was created in order to grow InGaN/GaN nanowires on a Si(111)
substrate to be used as the p-n junctions for an LED. A patterned SiO2 layer was used to control the
boundaries of the InGaN/GaN on the Si(111) substrate. After the nanowires were finished growing
a polyimide layer was spin coated over the nanowire array. This served as a structural support,
as well as a planarization layer for subsequent layers. The polyimide layer was etched in order to
expose the p-type GaN head of the nanowires, in order to provide a contact for the following ITO
layer. After the deposition of the ITO layer metal p and n contact electrodes were applied to the
device. The p-type contact electrode was applied on top of the ITO layer and the n-type contact
electrode was applied to the underside of the Si(111) substrate.
A major limit to the success of this device is the needed use of a phosphor coating on the inside
of the epoxy dome surrounding the LED. The only ways to produce a white light is through the
use of multiple different colored LEDs or through the use of a phosphor coating on a blue or UV
emitting device such as the one fabricated in this paper. Another limitation of our device is the
non-uniform height of the nanowires. This will lead to not all of the nanowires making contact
with the ITO layer, reducing the overall conductivity of the device. Further research should be
conducted in order to improve the efficiency and the overall cost of this device. Areas of this that
can be changed is the incorporation of AZO as the TCO layer instead of using ITO. Even though
49

ITO is currently more conductive and transparent, it is much more expensive and less abundant
than the materials used in AZO. More efficient fabrication processes can be explored in order to
grow higher quality nanowires for further use in LEDs.
Group Statement
“A group is like a cookie recipe. We have our flour, butter, eggs, sugar, and chocolate, but cooper-
ation is the walnuts that make the cookie great. The culmination of all these ingredients working
together, and communicating on a continuous basis during the baking process helps to form a
coherent mold. Teamwork is the ammonia in fine china that makes washing down cookies with
tea–the ultimate form of satisfaction.”
50

Glossary
AZO Aluminum-doped Zinc Oxide. 35
band gap Refers to the energy difference between the top of the valence band and the bottom of
the conduction band in insulators and conductors. 7
blueshift Any decrease in wavelength, with a corresponding increase in frequency, of electromag-
netic waves. 41
charge carrier density Denotes the number of charge carriers per volume. 40
conduction band The lowest range of vacant electronic states. 9
current density The amount of electric current flowing per unit cross-sectional area of a material.
23
diffuse (Of reflected light) scattered, as from a rough surface (opposed to specular). 23
dislocation density The measure of the number of dislocations in a unit volume of a crystalline
material. 14
efficiency droop A decrease in the conversion efficiency of LEDs at high current densities. 22
electron binding energy A measure of the energy required to free electrons from their atomic
orbits. This is more commonly known as ionization energy. 34
electron holes The lack of an electron at a position where one could exist in an atom or atomic
lattice. 10
epitaxial An oriented overgrowth of crystalline material upon the surface of another crystal of
different chemical composition but similar structure. 7
filament 1. (In a light bulb or other incandescent lamp) the threadlike conductor, often of tung-
sten, in the bulb that is heated to incandescence by the passage of current. 2. Electronics.
The heating element (sometimes also acting as a cathode)of a vacuum tube, resembling the
filament in an incandescent bulb. 4
51

fluorescence The emission of radiation, especially of visible light, by a substance during exposure
to external radiation, as light or x-rays. 5
incandescence The emission of visible light by a body, caused by its high temperature. 4
internal quantum efficiency The ratio of the radiative electron-hole recombination coefficient to
the total (radiative and nonradiative) recombination coefficient. 14
ionic impurity scattering The scattering of charge carriers by ionization in the lattice. 41
ITO Indium Tin Oxide. 16
lumen maintenance Compares the amount of light produced from a light source or from a lu-
minaire when it is brand new to the amount of light output at a specific time in the future.
22
lumens The unit of luminous flux, equal to the luminous flux emitted in a unit solid angle by a
point source of one candle intensity. 5
p-n junction They are formed by joining n-type and p-type semiconductor materials. The n-type
has a high electron concentration and the p-type a high hole concentration, electrons diffuse
from the n-type side to the p-type side. 10
phosphor A substance that exhibits the phenomenon of luminescence. 6
polyimide Polymer composed of monomer imides. 16
quantum wells A potential well with a discrete energy value that quantizes the electron motion.
4
valence band The highest range of electron energies in which electrons are normally present at
absolute zero. 9
wurtzite-type A hexagonal crystal structure where the atoms are stacked in an ABBABBABB
pattern. 15
52

Index
A
acceptor impurity atom, 10
annealing, 43–45, 47
ashing, 28
asher, 38
aspect ratio, 14, 30
C
cathode, 5, 6
conduction band, 10
conductor, 10
cost of ownership, 4
Czochralski process, 26
D
depletion zone, 7, 8
direct band gap, 10
dislocation density, 14
donor impurity atom, 10
E
e-beam evaporator, 47
efficacy, 6, 9, 22
efficiency, 4, 5, 8, 14, 22–25, 34, 37
efficiency droop, 22, 25
electroluminescence, 7, 10
Energy Independence and Security Act of 2007,
5
epileptics, 6
epitaxial, 7
F
FESEM, 15, 16, 38
fluorescent, 4–6, 8, 17, 25
H
hazardous, 4, 6, 8
I
incandescent, 4–6, 8, 9, 25
indirect band gap, 10
insulator, 10
intrinsic, 8
irradiation, 18, 19
M
magnetron sputter, 40
P
periodic table, 10, 11
phototherapy, 18–20
plasma, 5, 6, 27, 28, 38, 40, 41
PMMA, 27, 28
psoriasis, 18
Q
quantum wells, 4, 15, 23
R
RCA, 26
Reactive Ion Etch, 28, 38
recombination, 17, 26, 39
Auger recombination, 23
redshifted, 12
S
scanning electron microscope, 32
semiconductor, 7, 10
compound semiconductor, 10–12, 14
solid-state, 8
specular, 25
Sprengel pump, 5
T
thermionic emission, 5
TMGa, 15, 30, 31
transceivers, 20
tungsten, 5
U
ultraviolet, 6, 11, 18
V
valence band, 10, 17
valence electrons, 10
W
wurtzite, 15, 16
X
X-Ray diffraction, 15
53

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57

Feasibility Report
1. Introduction
(a) Discuss the need to improve the efficiency of current light sources as well as the need
to develop more advanced light sources.
(b) Introduce LEDs as a viable alternative to current lighting sources.
(c) Objective.
2. Comparison of Incandescent, Fluorescent, and LED Light Sources
(a) An introduction to the 3 main forms of lighting and the physics of their operation.
(b) A comparison of the 3 main lighting forms.
3. Compound Semiconductors in LED Technology
(a) A brief introduction of what semiconductors are and the physics behind them.
i. Discussed how they are doped.
(b) Define what compound semiconductors are.
i. Discuss their connections to LEDs.
(c) Went into detail about how the band gap influences the wavelength of light emitted, and
how the band gap works.
i. How to change the band gap of the semiconductor.
(d) Talked about what colors of light each type of compound semiconductor emits.
4. Metal Organic Vapor Deposition
(a) A brief introduction of the MOCVD tool and its uses for the deposition of materials for
LEDs.
(b) An example of a MOCVD process for growing GaN nanowires with InGaN MQWs.
(c) Show how some of the process parameters can be changed to get different outcomes for
the LED such as:
i. Indium concentrations.
ii. MQW pairs.
5. RGB LEDs and Applications
6. Limitations of LEDs/Cost
(a) Discuss some common problems faced with LED manufacturing.
i. Discussed issues due to heat, efficiency droop, and the phosphor application to the
LED chip.
ii. Discussed some general problems related to current costs of LEDs and how their
efficiency can reduce total energy consumption.
58

LED_Final_Paper

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